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Technical groups, follow aiche, you are here, ftir: a flexible tool for industrial gas analysis.

Fourier transform infrared (FTIR) spectroscopy identifies and quantifies gas and vapor samples. This article outlines how FTIR analyzers work and how they are commonly used in field applications.

images

▲ Figure 1. FTIR spectroscopy is the most widespread analytical technology for industrial applications that require the continuous and simultaneous measurement of multiple species.

Fourier transform infrared (FTIR) measurement techniques employ an optical modulator known as an interferometer. Collection of a spectrum using a prism or series of infrared (IR) filters allows measurement of only a single wavelength band at any given moment. On the other hand, the Fourier transform of interferometer data allows users to obtain absorbance information across all spectral wavelengths simultaneously. The ability to collect a wide range of wavelength information simultaneously enables an FTIR instrument ( Figure 1 ) to measure a spectrum much faster than a dispersion or filter instrument. This article discusses the basics of FTIR technology and how it is commonly used today.

FTIR development

For many years, the main factor preventing FTIR analysis from becoming more commonplace was the long computation time required for the Fourier transformation, a mathematical tool that converts a time-domain signal to a frequency-domain signal. However, the invention of a fast Fourier transform algorithm and advances in digital signal processing have made it practical to build small, low-cost FTIR spectrometers capable of rapidly measuring a complete mid-IR spectrum with moderate to high resolution and very good signal-to-noise ratio. Coupled with advanced algorithms for identification and quantification of the chemical species represented in the spectrum, modern gas and vapor FTIR analyzers can identify multiple unknowns and can quantify components of complex gas-phase mixtures containing dozens of volatile organic compounds (VOCs).

The introduction of cube corner mirrors was a major improvement to FTIR spectrometers. It helped to relax the precision requirements for moving optical alignment components, making FTIR suitable for industrial and field measurement applications. Regulations limiting the emission of pollutants into the air have created a market for monitoring equipment that can demonstrate compliance with regulatory emission limits. Emissions monitoring has become a widespread application for FTIR gas analysis.

Other measurement technologies have been approved for emissions monitoring, but FTIR spectroscopy has become the most popular analytical technology for industrial applications requiring the continuous simultaneous measurement of multiple parameters. This ability to measure almost any gas, combined with the robustness and reliability of the technology, enable FTIR to be employed in a wide variety of applications.

images

▲ Figure 2. An FTIR spectrometer consists of (a) an IR source, (b) moving or stationary cube corner mirrors, (c) a reference laser source, (d) a focusing optic, (e) a sample cell, (f) the sample gas inlet and outlet, and (g) an IR detector.

Several components work together in an FTIR spectrometer to generate the desired data ( Figure 2 ). These elements include:

  • a broadband IR source that emits at all recorded wavelengths simultaneously
  • an assembly of mirrors that can be moved continuously to vary the distance traveled by the two beams
  • a beamsplitter (not visible in Figure 2 ) that separates the IR beam into two parts
  • a reference laser source, which is used to track the position of the moving mirror(s)
  • focusing optics that transfer the beam to the sample cell and then to the detector
  • a sample cell filled with a sample gas or test gas
  • a gas inlet and outlet
  • an IR detector that responds to the entire wavelength range of the spectrometer
  • a laser detector (not visible in Figure 2 ) that responds to the wavelength of the reference laser.

The beamsplitter and the mirror assembly are collectively known as the interferometer — the heart of an FTIR spectrometer. The motion of the mirrors causes the two beams produced by the beamsplitter to be out of phase; when they recombine, the IR intensity of the resulting beam varies according to the positions of the mirrors. The interferometer can be considered an optical modulator and the modulation of the beam is the key to calculating intensity at each frequency from the signal recorded by the IR detector.

Data measurement

FTIR gas analyzers identify and measure gaseous compounds by their absorbance of IR...

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Gas Analysis

Real-time. quantitative. calibration-free..

MATRIX-MG 5

MATRIX-MG Series: Configurable High Performance

Gas cells with different optical lengths are used to cover mixing ratios in the ppb (MG26), ppm (MG2 and MG5), and percentage level (MG01). Complex gas mixtures can be analyzed thanks to the spectral resolution of up to 0.5 cm -1 .

Versatile Gas Analyzer OMEGA 5

OMEGA 5: The Versatile Gas Analyzer

The OMEGA 5 is perfectly suited for industrial applications due to its operational simplicity. The optical path length of 5 m allows detection limits in the ppb range.

Gas Analysis Software OPUS GA

OPUS GA: Powerful Gas Analysis Software

The comprehensive software OPUS GA allows the continuous and fully automated quantification without the need for calibration measurements or expert knowledge.

MGA Gas Analyzers

Powerful Gas Analyzers to Monitor and Report on Air Pollution and Greenhouse Gases.

MGA Gas Analysis Control Software

MGA Gas Analysis Control Software

The instrument’s software controls the measurement process, retrieves the results from spectral data and provides online access through several interfaces. All analyzers are equipped with a touch screen interface and can be controlled remotely.

Analysis of Industrial Gases and Process Control

The gas analyzers are suited for industrial applications due to the easy-to-use software OPUS GA and operational simplicity. Continuous data acquisition is enabled through detector types that do not require liquid nitrogen. Retrieved analysis results can be transferred and it is possible to control the gas analysis software OPUS GA via various interfaces.

ir gas analysis

Emission Monitoring and Quantification of Greenhouse Gases

ir gas analysis

The spectral resolution of 1.0 cm -1 (OMEGA 5) or 0.5 cm -1 (MATRIX-MG) makes unambiguous identification of gas species even in complex gas mixtures possible, for instance when monitoring exhaust gas emissions (e.g., analysis of NO x in H 2 O) or when investigating gas mixtures with highly potent greenhouse gases such as SF 6 .

Scientific Research

The gas analyzers are perfectly suited for the analysis of varying gas compositions, i.e., in scientific research or for the investigation of catalytic reactions, since no calibration measurements are required to define quantification methods. If an additional compound needs to be analyzed, the corresponding quantification method is added by a few clicks in OPUS GA .

ir gas analysis

Trace Gas Analysis and Purity Control

ir gas analysis

Reference spectra of excellent quality, high wavenumber accuracy, outstanding sensitivity, gas cells with high optical throughput, and efficient consideration of spectral interference, allow the analysis of trace gases and purity control even of IR-active matrix gases.

Analysis of Battery Gases

The analysis of battery gases is achieved through high-quality reference spectra, the flexible gas analysis software OPUS GA and the spectral resolution of 1.0 cm -1 or 0.5 cm- 1 .

ir gas analysis

Our Gas Analyzers in Scientific Research (selected publications)

Plasma-generated nitric oxide water: A promising strategy to combat bacterial dormancy (VBNC state) in environmental contaminant Micrococcus luteus, Journal of Hazardous Materials, 2023

Enhancement and limits of the selective oxidation of methane to formaldehyde over V-SBA-15: Influence of water cofeed and product decomposition, Catalysis Communications, 2021

Catalytic decomposition of NO 2 over a copper-decorated metal–organic framework by non-thermal plasma, Cell Reports Physical Science, 2021

Comparing Different Thermal Runaway Triggers for Two Automotive Lithium-Ion Battery Cell Types, Journal of the Electrochemical Society, 2020  

Cu-Al Spinel as a Highly Active and Stable Catalyst fort he Reverse Water Gas Shift Reaction, ACS Catalysis, 2019  

Combination of Chemo- and Biocatalysis: Conversion of Biomethane to Methanol and Formic Acid, Applied Sciences, 2019

Nitric-oxide enriched plasma-activated water inactivates 229E coronavirus and alters antiviral response genes in human lung host cells, Bioactive Materials, 2023

Transient Redox Behavior of a NH3-SCR Cu-CHA SCR Catalyst: Effect of O2 Feed Content Variation, Topics in Catalysis, 2022

The deactivation of an NH 3 -SCR Cu-SAPO catalyst upon exposure to non-oxidizing conditions. Applied Catalysis A: General, 2019

Effect of the NH 4 NO 3 Addition on the Low-T NH 3 -SCR Performances of Individual and Combined Fe- and Cu-Zeolite Catalysts, Emission Control Science and Technology, 2019

Selective synthesis of dimethyl ether on eco-friendly K10 montmorillonite clay Applied Catalysis A: General, 2018  

The Effect of CH 4 on NH 3 -SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas Vehicles, Topics in Catalysis, 2018

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MAX-iR FTIR Gas Analyzer

ir gas analysis

The Thermo Scientific MAX-iR FTIR Gas Analyzer is designed to meet the challenges of inline process monitoring, batch sampling, gas purity/certification, and more. Combined with Thermo Scientific StarBoost Enhanced Optical Technology, the MAX-iR analyzer enables users to achieve single-digit ppb detection limits for many applications. For ultra-high purity bulk gas applications, the detection limits can be further reduced to mid ppt.

Thermo Scientific MAX-iR FTIR Gas Analyzer

MAX-iR FTIR Gas Analyzer features

Perform sensitive real-time analysis with Thermo Scientific MAX-Acquisition Automation Software—high-performance analytical software specifically developed for real-time process and CEM environments. For maximum productivity without human interaction, the software features fully automated method control, data publishing, configurable alarms, remote control, and reporting tools. Other features of the MAX-iR FTIR Gas Analyzer include:

  • 24-bit analog to digital (ADC) integrated detector modules for excellent signal-to-noise without the use of liquid nitrogen
  • A long-life vertical cavity surface-emitting laser (VCSEL) diode and a silicon carbide (SiC) IR source for increased longevity and dependability
  • Integrated temperature and pressure sensors for increased precision in demanding certification applications
  • Dual-level vibrational dampening for operation in challenging field environments

ir gas analysis

Applications

Ftir gas analysis for industrial applications.

Industrial process applications from in-process monitoring to end-product quality assurance can benefit from the MAX-iR analyzer’s ability to detect down to single-digit ppb while simultaneously measuring for impurities in real-time. Use the MAX-iR FTIR Gas Analyzer for chemical manufacturing, air separation, semiconductor, and beverage applications.

FTIR gas analysis for ambient air applications

Measuring challenging compounds like ethylene oxide requires technology that avoids false alarms for benign interferences and meets standards from regulatory bodies such as the EPA and US Occupational Safety and Health Administration (OSHA). Successful ambient air monitoring requires the ability to discriminate target compounds down to single-digit ppb even in the presence of high concentrations of interferences, such as water, solvents, and hydrocarbons. The MAX-iR Gas Analyzer is suitable for a wide range of continuous ambient air monitoring applications, whether in the factory or the lab, including medical sterilization, chemical manufacturing, semiconductor fabrication, and automotive production.

FTIR gas analysis of source emissions

The fully automated Thermo Scientific EMS-10 platform provides fast, reliable continuous emissions monitoring without the need for recalibration or liquid nitrogen. Our source emission systems all accept StarBoost Optical Enhancement Technology, providing single-digit ppb detection of difficult-to-measure compounds such as formaldehyde and ethylene oxide. Use the MAX-iR FTIR Gas Analyzer for key source emissions analyses including chemical manufacturing, source testing, gas-fired turbines, and cement manufacturing.

ir gas analysis

FTIR industrial gas analysis resources

Find FTIR-based techniques, webinars, and applications describing how to analyze exhaust gases, monitor continuous emissions gases (CEM), and evaluate high-purity gases.

For Research Use Only. Not for use in diagnostic procedures.

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  • Published: 16 October 2020

Non-dispersive infrared multi-gas sensing via nanoantenna integrated narrowband detectors

  • Xiaochao Tan 1 ,
  • Heng Zhang 1 ,
  • Junyu Li 1 ,
  • Haowei Wan 1 ,
  • Qiushi Guo 2 ,
  • Houbin Zhu 3 ,
  • Huan Liu 1 &
  • Fei Yi   ORCID: orcid.org/0000-0001-6726-9627 1  

Nature Communications volume  11 , Article number:  5245 ( 2020 ) Cite this article

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  • Imaging and sensing
  • Nanophotonics and plasmonics
  • Nanosensors

Non-dispersive infrared (NDIR) spectroscopy analyzes the concentration of target gases based on their characteristic infrared absorption. In conventional NDIR gas sensors, an infrared detector has to pair with a bandpass filter to select the target gas. However, multiplexed NDIR gas sensing requires multiple pairs of bandpass filters and detectors, which makes the sensor bulky and expensive. Here, we propose a multiplexed NDIR gas sensing platform consisting of a narrowband infrared detector array as read-out. By integrating plasmonic metamaterial absorbers with pyroelectric detectors at the pixel level, the detectors exhibit spectrally tunable and narrowband photoresponses, circumventing the need for separate bandpass filter arrays. We demonstrate the sensing of H 2 S, CH 4 , CO 2 , CO, NO, CH 2 O, NO 2 , SO 2 . The detection limits of common gases such as CH 4 , CO 2 , and CO are 63 ppm, 2 ppm, and 11 ppm, respectively. We also demonstrate the deduction of the concentrations of two target gases in a mixture.

Introduction

The mid-infrared (mid-IR) spectral range (wavelength λ  ~ 2 to 20 µm) is known as the “molecular fingerprint” region, where a wide variety of gas molecules exhibit highly characteristic rotational or vibrational transition bands 1 , 2 , 3 , 4 , 5 . Notably, in the mid-IR region, the absorption strengths of molecular transitions are typically 10–1000 times greater than those in the visible or near-IR region 6 . As such, mid-IR spectroscopic gas sensors can be employed to uniquely identify and quantify the presence of substances with high sensitivity and selectivity. Non-dispersive infrared (NDIR) spectroscopy is one of mid-IR spectroscopic gas sensors that analyzes gases based on their characteristic absorption wavelengths in the mid-IR caused by their molecular vibrations 1 , 7 , which can find profound applications in traced gas sensing 8 , 9 , breadth analysis 10 , 11 , environmental monitoring 12 , 13 , to name a few.

In conventional NDIR gas sensor, the light source is broadband and not pre-filtered. When the light beam containing a wide range of wavelengths passes through and interacts with the sample gases in a chamber, only a portion of the optical energy is absorbed by the gases at their characteristic absorption wavelengths 14 . To analyze the concentration of a target gas, a bandpass optical filter is typically added before the detectors to eliminate all unwanted wavelengths in the light beam and only allow the characteristic absorption wavelengths of the gas to reach the detector. In other words, the spectral selectivity in conventional NDIR architecture is enabled by the added filters, rather than the detectors (see Supplementary Note  1 ).

In order to analyze several target gases in a mixture at the same time, one can simply implement multiple pairs of “bandpass filter + optical detector” in the NDIR gas sensor 15 , 16 . However, this scheme greatly increases the cost, the system complexity as well as the operating time, especially when the number of target gases is large 17 , 18 . Such challenge is fundamentally rooted in the lack of spectral selectivity of most commercially available mid-IR detectors. One way to avoid the need for separate optical filters is to introduce pixel-level spectral selectivity to mid-IR detectors by integrating plasmonic metamaterial absorbers (PMAs) onto the detector pixel. PMA is composed of arrays of metallic plasmonic resonators that can selectively absorb a certain spectral band of light and, therefore, can be regarded as nanoscale absorption filters 19 , 20 , 21 , 22 , 23 .

Following this idea, we propose a new NDIR architecture in which an array of narrowband PMA integrated pyroelectric elements are used to spectrally resolve the absorption of multiple gases at the same time 24 . By tuning the geometry of metallic plasmonic resonators, the central detection wavelength of each element can be independently adjusted to match the characteristic absorption bands of different target gases. The multiplexed sensing platform can thus be used to analyze multiple target gases in a mixture with significantly reduced device footprint and operating time. Leveraging the proposed gas sensing platform, we demonstrate the sensing of 8 different gases: H 2 S, CH 4 , CO 2 , CO, NO, CH 2 O, NO 2 , SO 2 , with the detection limits of 489, 63, 2, 11, 17, 27, 54, 104 ppm, respectively. We also demonstrate that the concentrations of two target gases in a mixture can be deduced from the voltage responses of two narrowband detectors. Although the sensing platform in its current form is still bulkier than the commercial NDIR sensors, we believe that by reducing the thickness of the pyroelectric elements and improving the quality factors of the narrowband PMA, integrated multiplexed gas sensors with centimeter long sizes can be achieved.

Principle of operation

The schematic diagram of the proposed NDIR multiplexed gas sensing platform, which is composed of three parts: the broadband light source, the gas cell and the multiplexed sensor with necessary focusing optics is plotted in Fig.  1a . An example of packaged multiplexed pyroelectric sensor with different detection wavelengths for spectral sensing is shown in Fig.  1b . The absorption filter is essentially a metal-insulator-metal (MIM)-based metamaterial absorber that consists a top layer of gold nanodisk antennas, a SiO 2 spacer, and a gold backplate. The absorber is directly fabricated on top of a commercially available thin lithium tantalate (LT) substrate with pre-deposited gold electrodes on the top and bottom surfaces. The top gold electrode of the LT substrate is also used as the gold backplate of the MIM absorbers for simplicity. LT as the sensing material offers a very broadband infrared response, enough to cover the characteristic absorption bands of typical gases, and a high pyroelectric coefficient 25 . For simplicity, here we cut the LT substrate with integrated MIM absorbers into separate elements with different spectral responsivity. Apparently, one can increase the number of elements in the package to monitor more gases with different spectral response. Figure  1c illustrates the operating principle of one narrowband detection element. The gold nanodisk antennas serve to resonantly absorb the mid-IR radiation in a narrowband fashion and convert absorbed optical energy into heat 26 , which elevates the temperature of the LT substrate. The resulting temperature increase ∆ T in turn causes the LT layer to generate the pyroelectric readout current ∆ I out 27 , 28 , which is then converted to readout voltage ∆ V out by the readout electronics.

figure 1

a A schematic diagram of the gas sensing system based on the proposed NDIR architecture with an array of narrowband PMA integrated pyroelectric elements used as the spectral sensor. The system is composed of three parts: the broadband light source, the gas cell, and the multi-element sensor together with necessary focusing optics. b The joint package of the multiple pyroelectric elements with different detection wavelengths. c The device geometry of the narrowband detection element. From the top to the bottom are: the Au nanodisk antenna, the silicon dioxide spacer, the gold backplate that is also used as the top electrode of the pyroelectric element, the lithium tantalate (LT) substrate, and the gold bottom electrode. The length of the smallest repeatable unit is the periodicity P and the radius of the nanodisk is R . The area size of each absorber is 1 × 1 mm and the thickness t p of the LT substrate is 75 μm. To provide electrical access to the gold backplate buried underneath the silicon dioxide spacer, a window area beside each absorber is opened by removing the spacer.

Design of plasmonic metamaterial absorbers

To sense multiple target gases, the spectral absorption of the narrowband detectors should be designed to match the characteristic absorption bands of different target gases in the mid-IR. Figure  2a shows the scanning electron microscope (SEM) image of the fabricated gold nanodisk antenna array (see Supplementary Note  2 for the details about the design of the MIM absorbers for 8 target gases, and Supplementary Note  3 about the fabrication and package of the narrowband detectors). To reveal the microscopic picture of resonant light trapping and dissipation of the optical energy, we need to look at the distribution of the optically induced currents in the absorber. Figure  2b plots the local distribution of the light-induced current density magnitude | J |and current density vector J in an MIM absorber at its resonant wavelength λ peak  = 5.73 μm. Under the excitation of the y-polarized (Cartesian coordinate system in Fig.  1c ) plane wave, the induced local currents oscillating along the y-direction are maximized in the center region of the nanodisk antenna and decrease in magnitude towards the edges of the antenna 29 , resulting in net electric charges and enhanced local electric fields near the antenna edges. The ohmic loss caused by the oscillating currents is mainly distributed at the lower surface of the antenna and the upper surface of the gold backplate, which serves to heat up the LT underneath. Figure  2c shows the measured absorption spectra of the 8 fabricated MIM absorbers and the characteristic IR absorption spectra of their target gases, whose absorption bands are far away from each other. It ought to be noted that by optimizing the antenna structure and the pattern of the array 30 , or replacing the metallic antennas with dielectric antennas 31 , the line-widths of the narrowband detectors can be further reduced, which in turn, improves the sensor selectivity.

figure 2

a The scanning electron microscope (SEM) image of the gold nanodisk antenna array. b The distribution of light-induced current density magnitude | J  | and current density vector J in the YZ cut-plane of the MIM absorber at λ peak  = 5.73 μm, obtained by COMSOL, a finite element method-based solver. c The measured absorption spectra of 8 fabricated MIM absorbers compared to the infrared absorption bands of eight target gases: H 2 S, CH 4 , CO 2 , CO, NO, CH 2 O, NO 2 , SO 2 . See Supplementary Note  7 for the absorption spectra in the full wavelength range. Source data are provided as a Source Data file.

Heat generation in the detector

Since the sensitivity of the NDIR sensor to the gases is strongly correlated to the mid-IR responsivity of each narrowband detector element, it is necessary to accurately model the photothermal and the temporal response of the detector element. We used the heat transfer module in COMSOL Multiphysics, a finite element method based solver, to calculate the steady-state distribution of the temperature increase Δ T in the PMA integrated LT detector caused by the dissipated electromagnetic energy. Figure  3a shows the heat transfer model of the suspended LT substrate. The four corners of the LT substrate are suspended by silicon posts. The simulated steady-state distribution of the temperature across the upper surface of the LT layer is shown in Fig.  3b . We also plot the steady-state temperature distribution along the cut line A-A’ in Fig.  3c . The maximum steady-state temperature at the center point B is found to be T center  = 297.32 K. The average value of the steady-state temperature T average , defined as the steady-state temperature distribution integrated over the upper surface of the LT substrate and divided by the area size of the upper surface of the LT substrate, is then calculated be 295.68 K, as shown by the red dotted line in Fig.  3c . We also plot the increase of the average temperature T average (t) in the time-domain in Fig.  3d . The thermal time constant τ T is found to be 0.66 s. (The temperature change as a function of the modulation frequency of the optical chopper is shown in Supplementary Note  4 ).

figure 3

a The schematic diagram of the suspended LT substrate used for calculating the steady-state temperature distribution and the dynamic temperature change in the time-domain. b The steady-state temperature distribution at the surface of the LT layer. c The steady-state temperature distribution along the cut line A-A’. The red dotted line indicates that the average temperature is 295.68 K. d The time-domain average temperature at the upper surface of the LT substrate assuming that the power is modulated by a square wave with the frequency of 0.1 Hz. Source data are provided as a Source Data file.

Photoresponse of the narrowband detectors

In order to assess the spectral response of the narrowband detectors, we measured the IR absorption spectra (Fig.  4a black curve) and the wavelength-dependent voltage responses (Fig.  4a red curve) by using a frequency-tunable quantum cascade laser (QCL). When the radius of Au nanodisk is 0.94 μm and the periodicity is 3 μm, the detector has a narrowband absorption spectrum peaked at 5.52 μm with a full width at half maximum (FWHM) of 670 nm. Importantly, the wavelength-dependent voltage responses of the detectors reproduce the IR absorption spectra of the PMAs very well. Figure  4b shows the voltage response of the detector as a function of the modulation frequency. It is found that when the modulation frequency is 7 Hz, the output voltage of the detector drops to 70.7% (3 dB) relative to the output voltage at the frequency of 4 Hz. Therefore, the modulation frequency should be set below 7 Hz. Moreover, the inset is the dynamic response of the narrowband detector (5.52 μm) in the time-domain at the modulation frequency of 5 Hz recorded by an oscilloscope (Tektronix, DPO2024B). The average measured voltage response at the modulation frequency of 5 Hz is 90 V W −1 . Factors that cause the measured responsivity to be lower than the calculated responsivity include: (1) The heat conduction between the LT element and the printed circuit board containing the impedance matching circuit is more significant than the calculated case. (2) The actual spot size of the optical beam arriving at the LT element can be smaller than the area size of the LT element (see Supplementary Note  5 for the theoretical calculation of the voltage response).

figure 4

a The measured voltage responses of one narrowband detector as a function of the wavelength of the input beam, compared with the absorption spectra of the integrated PMAs. The input beam is from a tunable quantum cascade laser and is modulated by an optical chopper at the frequency of 5 Hz. b The voltage response of the detector as a function of the chopping frequency. The inset is the time-domain voltage response of the narrowband detector with a peak wavelength of 5.52 µm. c The measured fluctuation (total noise) in the output voltage of the detector without input optical power, and the calculated values of two major sources of noise: temperature noise and Johnson noise as a function of the frequency. d The noise equivalent power calculated from the measured total noise and the responsivity of the narrowband detector (5.52 µm). Source data are provided as a Source Data file.

To determine the noise equivalent power (NEP), which is a direct measure of the smallest optical power that can be measured by the detector, we need to evaluate the noise in the detector, or the fluctuation in the output voltage, without any input optical power. The two major sources of noise here are: (1) thermal fluctuation noise \(\tilde u_{{\mathrm{NT}}}\) accounting for the random fluctuations in temperature due to the statistical nature of the heat exchange between the suspended LT detector and the supporting pins on the circuit board; (2) Johnson noise \(\tilde u_{{\mathrm{NR}}}\) originating from the thermal agitation of the electrons inside the electrical conductors at equilibrium. The LT element is a capacitive structure with loss resistance, and its Johnson noise exhibits a frequency dependence due to the product of the loss resistance and the capacitance. Figure  4c plots the calculated thermal fluctuation noise \(\tilde u_{{\mathrm{NT}}}\) and Johnson noise \(\tilde u_{{\mathrm{NR}}}\) (see Supplementary Note  5 for the theoretical calculation of the noises). The total voltage noise of the detector at different modulation frequency is also plotted in Fig.  4c . The measured noise level agrees well with the calculated value when the modulation frequency is below 20 Hz. As presented in Fig.  4d , At the modulation frequency of 5 Hz, the NEP is found to be 1.90 × 10 −8  W Hz −1/2 . From the comparison of the narrowband detectors with commercial LT detectors in Supplementary Note  8 , it is seen that the performance of the narrowband detectors enabled by plasmonic metamaterial absorber developed in this work is comparable to the performance of the commercial LT detectors that use metal black coating as the IR absorber.

The NDIR experiment on single-target gas

We then constructed an NDIR system to examine the performances of the fabricated narrowband detectors in gas sensing. As shown in Fig.  5a , the target gas is mixed with pure nitrogen gas and injected into the gas cell. The gas cell used in this work is a White type multipass cell with an effective optical length of 5 m. See Supplementary Note  9 for the arrangement of the components in the NDIR system. A collimated light beam from a SiC broadband infrared source is modulated by an optical chopper before it entered the gas cell. The light beam passing through the gas cell is then focused onto the sensing area of the packaged pyroelectric detector. The generated pyroelectric current is converted into the output voltage by the integrated circuit (IC) and the output voltage is then measured using the lock-in amplifier. Figure  5b presents the results of the single-target gas sensing experiment for eight gases: H 2 S, CH 4 , CO 2 , CO, NO, CH 2 O, NO 2 , and SO 2 . Since the absolute value of the detector output voltage may vary among each experiment, we use the relative change in the output voltage Δ V / V 0 to represent the voltage response of the detector. V 0 is the initial voltage output of the detector when the chamber is filled with pure nitrogen gas. V is the voltage output of the detector when the target gas is mixed into the chamber and Δ V  ≡  V − V 0 is the change in the output voltage caused by the target gas. To fit the measured detector response Δ V / V 0 as a function of the target gas concentration, we use a modified Beer–Lambert equation 32 , 33 , 34 , 35 , 36 :

The modified version of the Beer–Lambert Law is required by the practical considerations in the NDIR implementation. The coefficient span accounts for the fact that not all the IR radiation that impinges upon the detector is absorbed by the gas, even at high concentrations. The value of span ranges from 0 to 1 because of the optical filter bandwidth and the fine structure of the absorption spectra. The coefficient κ represents the effective absorption coefficient of the gas. See Supplementary Note  10 for the details about the calculation of k from the gas absorption lines obtained from the HITRAN database. l  = 5 m represents the optical path length of the White type multipass cell; x is the gas concentration. The parameter c is added into the power term as a linearization coefficient to account for the variations in the optical path length and light scattering for accurately fitting the equation to the actual absorption data. In practice, the parameters span and c are fitting parameters that are adjusted to match the fitting curves to the measured data as close as possible. See Supplementary Table  7 in Supplementary Note  12 for the detection limit of eight target gases in single-target gas measurement.

figure 5

a The gas sensing system. b The voltage responses of the narrowband detectors as a function of the concentrations of the target gases. The purple squares stand for the measured data while the red dashed lines are the fitting curves based on the Beer–Lambert law. Source data are provided as a Source Data file.

The NDIR experiment on mixed target gases

When there are multiple target gases in the gas cell, one can use multiple narrowband detectors to measure the gas mixture in the cell and back-calculate the concentration of each target gas based on the measured responses of the detectors. For example, assuming there are M target gases in the gas cell, and N narrowband detectors are used for measurement. The voltage response of the i th detector D i  ≡ Δ V i / V 0i is related to the concentration of each target gas by:

Here, i is the number of the detector and j is the number of the target gas. The parameter k ij are calculated using Supplementary Equation ( 8 ) in Supplementary Note  10 . The parameters span ij and c ij are fitting parameters that account for the contribution of the j th target gas to the response D i of the i th detector.

Taking a simple case where two target gases are measured by two narrowband detectors (“two-gas-two-detector”) as an example. The voltage responses of the two detectors are related to the concentrations of the two target gases by:

We chose CO (gas 1) and SO 2 (gas 2) as the two target gases for the mixed-gas experiment. The characteristic absorption wavelengths of the two gases are: λ 1  = 4.67 µm and λ 2  = 7.35 µm, respectively. Correspondingly, the detection wavelengths of the two detectors are tuned to be 4.67 µm (detector I) and 7.35 µm (detector II), respectively. To determine the values of span ij , and c ij , we first performed four single-target gas measurements: (1) CO measured by detector I; (2) SO 2 measured by detector I; (3) CO measured by detector II; (4) SO 2 measured by detector II, as shown in Fig.  6a . By fitting the measured detector responses, we can obtain the values of span ij , and c ij . When the values of span ij , c ij and k ij are determined, the mathematical model of the “two-gas-two-detector” problem is established. We then performed four mixed-gas experiments to verify the mathematical model: (1) CO with varying concentration and SO 2 with fixed concentration measured by detector I; (2) SO 2 with varying concentration and CO with fixed concentration measured by detector I; (3) CO with varying concentration and SO 2 with fixed concentration measured by detector II; (4) SO 2 with varying concentration and CO with fixed concentration measured by detector II, respectively. In each mixed-gas experiment, the fixed concentration is chosen to be 7500 ppm. The measured responses of detector I and detector II are plotted in Fig.  6b using purple squares. We also plot Fig.  6b the calculated detector responses based on Eq. ( 3 ) and Eq. ( 4 ) using red dashed lines (see Supplementary Note  11 for the model parameters of the “two-gas-two-detector” problem). It is seen that the detector responses predicted by Eq. ( 3 ) and Eq. ( 4 ) agree well with the measurements, which confirms the effectiveness of the mathematical model. See Supplementary Note  11 about using the mathematical model to work out the gas concentrations x 1 and x 2 from the detector responses D 1 and D 2 .

figure 6

a The detector responses of four single-target gas measurements: CO measured by detector I; SO 2 measured by detector I; CO measured by detector II; SO 2 measured by detector II, respectively. b The detector responses of four mixed-gas experiments: fixed SO 2 concentration and varying CO concentration measured by detector I; fixed CO concentration and varying SO 2 concentration measured by detector I; fixed SO 2 concentration and varying CO concentration measured by detector II; fixed CO concentration and varying SO 2 concentration measured by detector II, respectively. In each mixed-gas experiment, the fixed concentration is chosen to be 7500 ppm. The purple squares stand for the measured detector responses while the red dashed lines are the calculated detector responses based on the mathematical models provide by Eq. ( 3 ) and Eq. ( 4 ). Source data are provided as a Source Data file.

In summary, we have presented the design, fabrication and measurement of narrowband pyroelectric detectors that are enabled by directly integrating plasmonic metamaterial absorbers onto the sensing area of lithium tantalate elements as on-chip absorption filters. The detection wavelength, or the peak absorption wavelength of the integrated absorber can be tuned to cover the full mid-infrared spectrum by varying the design of the metamaterials. We fabricated eight narrowband detectors whose detection wavelengths are aligned with the characteristic absorption wavelength of eight target gases: H 2 S, CH 4 , CO 2 , CO, NO, CH 2 O, NO 2 , SO 2 , and implement the detectors in a home-built NDIR system to measure the gas concentrations. The mathematical model for a simple case in which two target gases (CO and SO 2 ) in a mixture are measured by two narrowband detectors is also provided. The model can successfully predict the measured responses of the two detectors based on the concentrations of the two target gases. We also build a computer program based on the mathematical model and demonstrate the deduction of the gas concentrations x 1 and x 2 from the detector responses D 1 and D 2 . The presented work thus goes beyond the conventional NDIR gas sensor in multiplexed gas sensing by circumventing the need for multiple pairs of “bandpass filter + optical detector”.

Since pyroelectric materials have long been used to construct low-cost and uncooled detectors throughout the infrared spectrum, the demonstrated narrowband detectors find immediate commercial applications such as the detection of fire flame and human bodies based on their infrared absorption features and non-contact temperature measurement. In particular, pyroelectric detectors based on LT have been massively produced using standard tools for making IC chips 37 . The nanoantenna array was also fabricated using regular fabrication processes in IC industry such as electron beam lithography, electron beam evaporation, and metal lift-off. In the next step, electron beam lithography can be replaced by more standard tools such as project lithography (stepper) for wafer-scale mass production 38 . Although gold is not CMOS compatible, we can use materials such as aluminum 39 , or TiN 40 that are CMOS compatible to make nanoantennas. Thus, our design is compatible with standard fabrication processes in IC industry and meet demands for low-cost and mass production.

As the prospects for improving the limit of detection and make a ~cm long NDIR sensor while maintaining the same integration, the two key aspects we are working on are: (1) Reduce the thickness of the LT element from 75 µm to 700 nm (two orders of magnitude) 20 . This can be done by replacing the self-supported LT plate with thin film LT on silicon. (2) Increase the quality factor of the MIM absorber that determines the overlap between the spectral response of the detector and the gas absorption lines. This can be done by optimizing the design of the nanoantennas in the MIM absorbers 30 .

Since water vapor (humidity) is a strong source of cross-response in the 5–7 μm, we took several measures to minimize the intereference from the water vapor. First, the NDIR system is installed with a pump that can take out the air and water vapor inside the gas pipelines. Second, we also put desiccants at the outlets of the pipelines to remove water vapor from the ambient. Third, the NDIR system is located on an optical table that is covered by a hood. The hood can help isolate the water vapor from the human operator. Finally, the measurement lab is well ventilated to minimize the remaining water vapor in the room. Thus in the future, to make sure that the ~cm long NDIR sensors can accurately measure gases in the 5–7 µm region in standard atmospheric conditions, water vapor need to be removed to a very high degree.

Owing to the high insertion loss caused by the gas cell, the current NDIR system is not implemented with a reference channel. In future designs of ~cm long NDIR sensors, one of the detection elements in the 5–7 μm range can be made as a reference detector. Measuring errors caused by dust or diminishing radiation intensity are removed by the use of the reference channel.

In the future, the demonstrated NDIR sensor architecture can be expanded by increasing the number of narrowband detection elements to analyze more target gases (see Supplementary Note  13 for methods of minimizing the thermal cross-talk between the neighboring elements). It can also be extended to other thermal detector platforms such as thermopile detectors and vanadium oxide microbolometers. When combined with the large size focal plane array technology, the presented device architecture will evolve as on-chip infrared spectrometers, which can serve as tools for spectroscopic analysis of gases, chemicals, explosives, and other types of substances.

Finite element simulations

We used COMSOL, a finite element method-based solver to numerically study the optical properties of the absorbers employing periodic boundary conditions. With plane wave excitation polarized along the y-axis in the electromagnetic waves module. The domain boundaries parallel to the x−z and y−z planes and port 1 is the top face of simulation area, port 2 is the bottom face of simulation area. Using the corresponding parameter in Supplementary Table  1 , we can obtain the optimized absorption curve.

To evaluate the impact of the heat transfer between the sensing area and the environment via convection, we also use the heat transfer module in COMSOL to simulate the temperature change in the detector. The bottom sides of the four silicon posts are all set to be constant temperature T 0  = 293.15 K while all other sides of the structure are set as heat insulation with the dimensions of the LT substrate are: length = 2.4 mm, width = 1.5 mm and thickness = 75 μm. The height and radius of the silicon posts are 250 and 50 μm, respectively. As for the MIM absorber, the top layer of nanodisk antennas is ignored for simplicity while the silicon dioxide spacer and gold backplate are included and set as the heat source with a Gaussian profile: g (x, y) =  g 0 \(*\) exp(− x 2 / r 0 2 ) \(*\) exp(− y 2 / r 0 2 ). Here g 0  =  P 0 /( π r 0 2 t ) is the power density of the heat source; t  = ( t SiO2  +  t backplate ) = 0.2 μm is the total thickness of the spacer and the backplate, the corresponding material parameters are in Supplementary Table  2 .

Device fabrication

Mid-IR detectors were fabricated at the Center of Micro-Fabrication and Characterization (CMFC) of Wuhan National Research Center for Optoelectronics. A schematic of the fabrication and package of the narrowband detector is shown in Supplementary Fig.  3 . The fabrication of the narrowband detectors begins with the deposition of the 80 nm silicon dioxide spacer on top of the gold electrode (100 nm Au/20 nm Cr) pre-deposited on the 75 µm LT substrate (Yamaju Ceramics Co., LTD.) using plasma-enhanced chemical vapor deposition (PECVD). Electron beam resist (AR-P 6200.09) was spin-coated and nanodisk arrays were defined by electron beam lithography (Vistec EBPG 5000plus ES) followed by electron beam evaporation(EB-500S) to deposit 50 nm Au/10 nm Ni(adhesive layer). Another round of electron beam lithography was then performed to define the 600 µm side-length window areas for wire-bonding. The silicon dioxide spacer in the wire-bonding areas was then removed by dry etching (Plasmalab system 100 ICP 180) to expose the gold electrode underneath the spacer. The LT substrate was then cut into separate single-element detectors using a laser cutter, each of which has an antenna array and a wire-bonding area. The detector was then mounted onto a TO-5 packaged impedance matching circuit with a customized special JFET (low drift, low noise) in voltage mode. The bottom electrode of the LT detector is in direct contact with the one input pin of the circuit. The top electrode of the LT detector was then wire-bonded to the other input pin to finish the electrical connection. The optical window is an infrared high transmissive CaF 2 glass.

Measurement of photoresponse

We use a wavelength tunable quantum cascade laser (QCL) to examine the characteristic of the fabricated detectors. The working wavelength of the QCL (Block Engineering, LaserTune) can be continuously tuned from 5.4 to 6.0 μm. See Supplementary Note  6 for the details about the QCL. An optical chopper (Thorlabs, MC2000B, f mod  ≥ 4 Hz) was used to mechanically modulate the output beam from the QCL before it reaches the narrowband detectors. The beam modulated by the chopper was then focused by a reflective objective (Thorlabs, LMM-15X-P01) to the area of antenna array. A power supply (GWINSTEK, GPS-3303C) that provides +5 V bias voltage for the impedance matching circuit in the TO-package and a lock-in amplifier (Stanford Research System, SRS-830) connected to a computer controlled by LabVIEW is used to measure the output electrical signal of the detector. The input reference signal of the lock-in amplifier is same as the chopper input signal.

We also build a gas sensing system. A schematic of the gas sensing system is shown in Fig.  5a . The gas sensing system is composed of three subsystems: the optical subsystem contains a collimated SiC broadband IR source (Thorlabs, SLS203L/M, see Supplementary Note  6 for details), an optical chopper, a gas chamber (GAINWAY, GW-1020IR-5M), a reflective objective (Thorlabs, LMM-15X-P01) and the prepared narrowband pyroelectric detector; the electrical subsystem contains a power supply (GWINSTEK, GPS-3303C) that provides +5 V bias voltage for the detector and a lock-in amplifier (SRS-830); the gas supply subsystem contains mass flow controllers (Sevenstar, CS200C) that control real-time flow of each gas. The target gas gets mixed with pure nitrogen gas and sent into the gas chamber.

Data availability

The data that support the findings of this study are available from the authors on reasonable request; Source data are provided with this paper.

Code availability

The code that support the findings of this study is available from the authors on reasonable request;

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Acknowledgements

F.Y. acknowledges funding support from National Natural Science Foundation of China (NSFC) (11774112, 11604110); National key research and development program of China (2016YFC0201300, 2019YFB2005700); The Fundamental Research Initiative Funds for Huazhong University of Science and Technology (2017KFYXJJ031, 2019kfyRCPY122); Graduates’ Innovation Fund, Huazhong University of Science and Technology (5003182041). We thank Li Pan engineer in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in PECVD fabrication. We thank Zeng Tiantian engineer in the Huazhong University of Science & Technology Analytical & Testing Center for the support in FTIR test. We thank the technical support from Experiment Center for Advanced Manufacturing and Technology in School of Mechanical Science & Engineering of HUST.

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Xiaochao Tan, Heng Zhang, Junyu Li, Haowei Wan, Huan Liu & Fei Yi

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Contributions

X.C.T. and F.Y conceived the idea. X.C.T. and J.Y.L. designed and simulated the prototype structures. X.C.T. fabricated the detector and built up the test system. X.C.T. and H.Z. performed the photoresponse and gas sensing measurement. X.C.T., H.Z., and H.W.W. performed the electrical calculation, data analysis. Q.S.G. helped with data analysis. H.B.Z. provided the Lithium Tantalate materials. F.Y. organized the project, analyzed the results, and provided the support. All authors contributed to the preparation of the manuscript.

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Correspondence to Fei Yi .

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Tan, X., Zhang, H., Li, J. et al. Non-dispersive infrared multi-gas sensing via nanoantenna integrated narrowband detectors. Nat Commun 11 , 5245 (2020). https://doi.org/10.1038/s41467-020-19085-1

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4.2: IR Spectroscopy

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IR Sample Preparation: A Practical Guide

Infrared spectroscopy is based on molecular vibrations caused by the oscillation of molecular dipoles. Bonds have characteristic vibrations depending on the atoms in the bond, the number of bonds and the orientation of those bonds with respect to the rest of the molecule. Thus, different molecules have specific spectra that can be collected for use in distinguishing products or identifying an unknown substance (to an extent.)

Collecting spectra through this method goes about one of three general ways. Nujol mulls and pressed pellets are typically used for collecting spectra of solids, while thin-film cells are used for solution-phase IR spectroscopy. Spectra of gases can also be obtained but will not be discussed in this guide.

Infrared Optical Materials and Handling

While it is all well and wonderful that substances can be characterized in this fashion one still has to be able to hold the substances inside of the instrument and properly prepare the samples. In an infrared spectrometer (Figure \(\PageIndex{1}\) )

the sample to be analyzed is held in front of an infrared laser beam, in order to do this, the sample must be contained in something, consequently this means that the very container the sample is in will absorb some of the infrared beam.

graphics1.jpg

This is made somewhat complicated by the fact that all materials have some sort of vibration associated with them. Thus, if the sample holder has an optical window made of something that absorbs near where your sample does, the sample might not be distinguishable from the optical window of the sample holder. The range that is not blocked by a strong absorbance is known as a window (not to be confused with the optical materials of the cell.)

Windows are an important factor to consider when choosing the method to perform an analysis, as seen in Table \(\PageIndex{1}\) there are a number of different materials each with their own characteristic absorption spectra and chemical properties. Keep these factors in mind when performing analyses and precious sample will be saved. For most organic compounds NaCl works well though it is susceptible to attack from moisture. For metal coordination complexes KBr, or CsI typically work well due to their large windows. If money is not a problem then diamond or sapphire can be used for plates.

Proper handling of these plates will ensure they have a long, useful life. Here follows a few simple pointers on how to handle plates:

  • Avoid contact with solvents that the plates are soluble in.
  • Keep the plates in a dessicator, the less water the better, even if the plates are insoluble to water.
  • Handle with gloves, clean gloves.
  • Avoid wiping the plates to prevent scratching.

That said, these simple guidelines will likely reduce most damage that can occur to a plate by simply holding it other faults such as dropping the plate from a sufficient height can result in more serious damage.

Preparation of Nujol Mulls

A common method of preparing solid samples for IR analysis is mulling. The principle here is by grinding the particles to below the wavelength of incident radiation that will be passing through there should be limited scattering. To suspend those tiny particles, an oil, often referred to as Nujol is used. IR-transparent salt plates are used to hold the sample in front of the beam in order to acquire data. To prepare a sample for IR analysis using a salt plate, first decide what segment of the frequency band should be studied, refer to Table \(\PageIndex{1}\) for the materials best suited for the sample. Figure \(\PageIndex{2}\) shows the materials needed for preparing a mull.

Necessary materials for preparing a KBr plate with a Nujol mull

Preparing the mull is performed by taking a small portion of sample and adding approximately 10% of the sample volume worth of the oil and grinding this in an agate mortar and pestle as demonstrated in Figure \(\PageIndex{3}\). The resulting mull should be transparent with no visible particles.

Mulling ferrocene into mineral oil with a mortar and pestle.

Another method involves dissolving the solid in a solvent and allowing it to dry in the agate pestle. If using this method ensure that all of the solvent has evaporated since the solvent bands will appear in the spectrum. Some gentle heating may assist this process. This method creates very fine particles that are of a relatively consistent size. After addition of the oil further mixing (or grinding) may be necessary.

Plates should be stored in a desiccator to prevent erosion by atmospheric moisture and should appear roughly transparent. Some materials such as silicon will not, however. Gently rinse the plates with hexanes to wash any residual material off of the plates. Removing the plates from the desiccator and cleaning them should follow the preparation of the mull in order to maintain the integrity of the salt plates. Of course, if the plate is not soluble in water then it is still a good idea just to prevent the threat of mechanical trauma or a stray jet of acetone from a wash bottle.

Once the mull has been prepared, add a drop to one IR plate (Figure \(\PageIndex{4}\) ), place the second plate on top of the drop and give it a quarter turn in order to evenly coat the plate surface as seen in Figure \(\PageIndex{5}\). Place it into the spectrometer and acquire the desired data.

Always handle with gloves and preferably away from any sinks, faucets, or other sources of running or spraying water.

The prepared mull from an agate mortar and pestle being applied to a polished KBr plate.

Spectra acquired by this method will have strong C-H absorption bands throughout several ranges 3,000 – 2,800 cm -1 and 1,500 – 1,300 cm -1 and may obscure signal.

Cleaning the plate is performed as previously mentioned with hexanes or chloroform can easily be performed by rinsing and leaving them to dry in the hood. Place the salt plates back into the desiccator as soon as reasonably possible to prevent damage. It is highly advisable to polish the plates after use, no scratches, fogging, or pits should be visible on the face of the plate. Chips, so long as they don’t cross the center of the plate are survivable but not desired. The samples of damaged salt plates in Figure \(\PageIndex{6}\) show common problems associated with use or potentially mishandling. Clouding, and to an extent, scratches can be polished out with an iron rouge. Areas where the crystal lattice is disturbed below the surface are impossible to fix and chips cannot be reattached.

graphics2.jpg

FIgure \(\PageIndex{6}\) A series of plates indicating various forms of physical damage with a comparison to a good plate (Copyright: Colorado University-Boulder).

Preparation of Pellets

In an alternate method, this technique is along the same lines of the nujol mull except instead of the suspending medium being mineral oil, the suspending medium is a salt. The solid is ground into a fine powder with an agate mortar and pestle with an amount of the suspending salt. Preparing pellets with diamond for the suspending agent is somewhat illadvised considering the great hardness of the substance. Generally speaking, an amount of KBr or CsI is used for this method since they are both soft salts. Two approaches can be used to prepare pellets, one is somewhat more expensive but both usually yield decent results.

The first method is the use of a press. The salt is placed into a cylindrical holder and pressed together with a ram such as the one seen in (Figure \(\PageIndex{7}\) ). Afterwards, the pellet, in the holder, is placed into the instrument and spectra acquired.

graphics3.jpg

An alternate, and cheaper method requires the use of a large hex nut with a 0.5 inch inner diameter, two bolts, and two wrenches such as the kit seen in Figure \(\PageIndex{8}\). Step-by-step instructions for loading and using the press follows:

  • Screw one of the bolts into the nut about half way.
  • Place the salt pellet mixture into the other opening of the nut and level by tapping the assembly on a countertop.
  • Screw in the second bolt and place the assembly on its side with the bolts parallel to the countertop. Place one of the wrenches on the bolt on the right side with the handle aiming towards yourself.
  • Take the second wrench and place it on the other bolt so that it attaches with an angle from the table of about 45 degrees.
  • The second bolt is tightened with a body weight and left to rest for several minutes. Afterwards, the bolts are removed, and the sample placed into the instrument.

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Some pellet presses also have a vacuum barb such as the one seen in (Figure \(\PageIndex{8}\). If your pellet press has one of these, consider using it as it will help remove air from the salt pellet as it is pressed. This ensures a more uniform pellet and removes absorbances in the collected spectrum due to air trapped in the pellet.

Preparation of Solution Cells

Solution cells (Figure \(\PageIndex{9}\) ) are a handy way of acquiring infrared spectra of compounds in solution and is particularly handy for monitoring reactions.

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A thin-film cell consists of two salt plates with a very thin space in between them (Figure \(\PageIndex{10}\) ). Two channels allow liquid to be injected and then subsequently removed. The windows on these cells can be made from a variety of IR optical materials. One particularly useful one for water-based solutions is CaF 2 as it is not soluble in water.

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Cleaning these cells can be performed by removing the solution, flushing with fresh solvent and gently removing the solvent by syringe. Do not blow air or nitrogen through the ports as this can cause mechanical deformation in the salt window if the pressure is high enough.

Deuterated Solvent Effects

One of the other aspects to solution-phase IR is that the solvent utilized in the cell has a characteristic absorption spectra. In some cases this can be alleviated by replacing the solvent with its deuterated sibling. The benefit here is that C-H bonds are now C-D bonds and have lower vibrational frequencies. Compiled in Figure \(\PageIndex{11}\) is a set of common solvents.

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This effect has numerous benefits and is often applied to determining what vibrations correspond to what bond in a given molecular sample. This is often accomplished by using isotopically labeled “heavy” reagents such as ones that contain 2 H, 15 N, 18 O, or 13 C.

Basic Troubleshooting

There are numerous problems that can arise from improperly prepared samples, this section will go through some of the common problems and how to correct them. For this demonstration, spectra of ferrocene will be used. The molecular structure and a photograph of the brightly colored organometallic compound are shown in Figure \(\PageIndex{12}\) and Figure \(\PageIndex{13}\).

ferrocene.jpg

Figure \(\PageIndex{14}\) illustrates what a good sample of ferrocene looks like prepared in a KBr pellet. The peaks are well defined and sharp. No peak is flattened at 0% transmittance and Christiansen scattering is not evident in the baseline.

A good spectrum of Ferrocene in a KBr Pellet

Figure \(\PageIndex{15}\) illustrates a sample with some peaks with intensities that are saturated and lose resolution making peak-picking difficult. In order to correct for this problem, scrape some of the sample off of the salt plate with a rubber spatula and reseat the opposite plate. By applying a thinner layer of sample one can improve the resolution of strongly absorbing vibrations.

An overly concentrated sample of ferrocene in a KBr pellet

Figure \(\PageIndex{16}\) illustrates a sample in which too much mineral oil was added to the mull so that the C-H bonds are far more intense than the actual sample. This can be remedied by removing the sample from the plate, grinding more sample and adding a smaller amount of the mull to the plate. Another possible way of doing this is if the sample is insoluble in hexanes, add a little to the mull and wick away the hexane-oil mixture to leave a dry solid sample. Apply a small portion of oil and replate.

An occulted spectrum of Ferrocene in a Nujol mull.

Figure \(\PageIndex{17}\) illustrates the result of particles being too large and scattering light. To remedy this, remove the mull and grind further or else use the solvent deposition technique described earlier.

A sample exhibiting the Christiansen effect on Ferrocene in a Nujol mull.

Characteristic IR Vibrational Modes for Hydrocarbon Compounds

Fourier transform infrared spectroscopy of metal ligand complexes.

The infrared (IR) range of the electromagnetic spectrum is usually divided into three regions:

  • The far-infrared is always used for rotational spectroscopy, with wavenumber range 400 – 10 cm −1 and lower energy.
  • The mid-infrared is suitable for a detection of the fundamental vibrations and associated rotational-vibrational structure with the frequency range approximately 4000 – 400 cm −1 .
  • The near-Infrared with higher energy and wave number range 14000 – 4000 cm −1 , can excite overtone or higher harmonic vibrations.

For classical light material interaction theory, if a molecule can interact with an electromagnetic field and absorb a photon of certain frequency, the transient dipole of molecular functional group must oscillate at that frequency. Correspondingly, this transition dipole moment must be a non-zero value, however, some special vibration can be IR inactive for the stretching motion of a homonuclear diatomic molecule and vibrations do not affect the molecule’s dipole moment (e.g., N 2 ).

Mechanistic Description of the Vibrations of Polyatomic Molecules

A molecule can vibrate in many ways, and each way is called a "vibrational mode". If a molecule has N atoms, linear molecules have 3N-5 degrees of vibrational modes whereas nonlinear molecules have 3N-6 degrees of vibrational modes. Take H 2 O for example; a single molecule of H 2 O has O-H bending mode (Figure \(\PageIndex{18}\) a), antisymmetric stretching mode (Figure \(\PageIndex{18}\) b), and symmetric stretching mode (Figure \(\PageIndex{18}\) c).

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If a diatomic molecule has a harmonic vibration with the energy, \ref{1} , where n+ 1 / 2 with n = 0, 1, 2 ...). The motion of the atoms can be determined by the force equation, \ref{2} , where k is the force constant). The vibration frequency can be described by \ref{3} . In which m is actually the reduced mass (m red or μ), which is determined from the mass m 1 and m 2 of the two atoms, \ref{4} .

\[ E_{n} \ =\ -hv \label{1} \]

\[ F \ =\ -kx \label{2} \]

\[ \omega \ =\ (k/m)^{1/2} \label{3} \]

\[ m_{red} \ =\ \mu \ =\ \frac{m_{1} m_{2}}{m_{1}\ +\ m_{2} } \label{4} \]

Principle of Absorption Bands

In IR spectrum, absorption information is generally presented in the form of both wavenumber and absorption intensity or percent transmittance. The spectrum is generally showing wavenumber (cm -1 ) as the x-axis and absorption intensity or percent transmittance as the y-axis.

Transmittance, "T", is the ratio of radiant power transmitted by the sample (I) to the radiant power incident on the sample (I 0 ). Absorbance (A) is the logarithm to the base 10 of the reciprocal of the transmittance (T). The absorption intensity of molecule vibration can be determined by the Lambert-Beer Law, \label{5} . In this equation, the transmittance spectra ranges from 0 to 100%, and it can provide clear contrast between intensities of strong and weak bands. Absorbance ranges from infinity to zero. The absorption of molecules can be determined by several components. In the absorption equation, ε is called molar extinction coefficient, which is related to the molecule behavior itself, mainly the transition dipole moment, c is the concentration of the sample, and l is the sample length. Line width can be determined by the interaction with surroundings.

\[ A\ =\ log(1/T) \ =\ -log(I/I_{0} )\ =\ \varepsilon c l \label{5} \]

The Infrared Spectrometer

As shown in Figure \(\PageIndex{19}\), there are mainly four parts for fourier transform infrared spectrometer (FTIR):

  • Light source. Infrared energy is emitted from a glowing black-body source as continuous radiations.
  • Interferometer. It contains the interferometer, the beam splitter, the fixed mirror and the moving mirror. The beam splittertakes the incoming infrared beam and divides it into two optical beams. One beam reflects off the fixed mirror. The other beam reflects off of the moving mirror which moves a very short distance. After the divided beams are reflected from the two mirrors, they meet each other again at the beam splitter. Therefore, an interference pattern is generated by the changes in the relative position of the moving mirror to the fixed mirror. The resulting beam then passes through the sample and is eventually focused on the detector.
  • Sample compartment. It is the place where the beam is transmitted through the sample. In the sample compartment, specific frequencies of energy are absorbed.
  • Detector. The beam finally passes to the detector for final measurement. The two most popular detectors for a FTIR spectrometer are deuterated triglycine sulfate (pyroelectric detector) and mercury cadmium telluride (photon or quantum detector). The measured signal is sent to the computer where the Fourier transformation takes place.

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A Typical Application: the detection of metal ligand complexes

Some general absorption peaks for common types of functional groups.

It is well known that all molecules chemicals have distinct absorption regions in the IR spectrum. Table \(\PageIndex{7}\) shows the absorption frequencies of common types of functional groups. For systematic evaluation, the IR spectrum is commonly divided into some sub-regions.

  • In the region of 4000 - 2000 cm –1 , the appearance of absorption bands usually comes from stretching vibrations between hydrogen and other atoms. The O-H and N-H stretching frequencies range from 3700 - 3000 cm –1 . If hydrogen bond forms between O-H and other group, it generally caused peak line shape broadening and shifting to lower frequencies. The C-H stretching bands occur in the region of 3300 - 2800 cm –1 . The acetylenic C-H exhibits strong absorption at around 3300 cm –1 . Alkene and aromatic C-H stretch vibrations absorb at 3200-3000 cm –1 . Generally, asymmetric vibrational stretch frequency of alkene C-H is around 3150 cm -1 , and symmetric vibrational stretch frequency is between 3100 cm -1 and 3000 cm -1 . The saturated aliphatic C-H stretching bands range from 3000 - 2850 cm –1 , with absorption intensities that are proportional to the number of C-H bonds. Aldehydes often show two sharp C-H stretching absorption bands at 2900 - 2700 cm –1 . However, in water solution, C-H vibrational stretch is much lower than in non-polar solution. It means that the strong polarity solution can greatly reduce the transition dipole moment of C-H vibration.
  • Furthermore, the stretching vibrations frequencies between hydrogen and other heteroatoms are between 2600 - 2000cm -1 , for example, S-H at 2600 - 2550 cm –1 , P-H at 2440 - 2275 cm –1 , Si-H at 2250 - 2100 cm –1 .
  • The absorption bands at the 2300 - 1850 cm –1 region usually present only from triple bonds, such as C≡C at 2260 - 2100 cm –1 , C≡N at 2260 - 2000 cm –1 , diazonium salts –N≡N at approximately 2260 cm –1 , allenes C=C=C at 2000 - 1900 cm –1 . The peaks of these groups are all have strong absorption intensities. The 1950 - 1450 cm –1 region stands for double-bonded functional groups vibrational stretching.
  • Most carbonyl C=O stretching bands range from 1870 - 1550 cm –1 , and the peak intensities are from mean to strong. Conjugation, ring size, hydrogen bonding, and steric and electronic effects can lead to significant shifts in absorption frequencies. Furthermore, if carbonyl links with electron-withdrawing group, such as acid chlorides and acid anhydrides, it would give rise to IR bands at 1850 - 1750 cm –1 . Ketones usually display stretching bands at 1715 cm -1 .
  • None conjugated aliphatic C=C and C=N have absorption bands at 1690 - 1620 cm –1 . Besides, around 1430 and 1370cm -1 , there are two identical peaks presenting C-H bending.
  • The region from 1300 - 910 cm –1 always includes the contributions from skeleton C-O and C-C vibrational stretches, giving additional molecular structural information correlated with higher frequency areas. For example, ethyl acetate not only shows its carbonyl stretch at 1750 - 1735 cm –1 , but also exhibits its identical absorption peaks at 1300 - 1000 cm –1 from the skeleton vibration of C-O and C-C stretches.

General Introduction of Metal Ligand Complex

The metal electrons fill into the molecular orbital of ligands (CN, CO, etc.) to form complex compound. As shown in Figure \(\PageIndex{20}\), a simple molecular orbital diagram for CO can be used to explain the binding mechanism.

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The CO and metal can bind with three ways:

  • Donation of a pair of electrons from the C-O σ* orbital into an empty metal orbital (Figure \(\PageIndex{21}\) a).
  • Donation from a metal d orbital into the C-O π* orbital to form a M-to-CO π-back bond (Figure \(\PageIndex{21}\) b).
  • Under some conditions a pair of carbon π electron can donate into an empty metal d-orbital.

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Some Factors to Include the Band Shifts and Strength

Herein, we mainly consider two properties: ligand stretch frequency and their absorption intensity. Take the ligand CO for example again. The frequency shift of the carbonyl peaks in the IR mainly depends on the bonding mode of the CO (terminal or bridging) and electron density on the metal. The intensity and peak numbers of the carbonyl bands depends on some factors: CO ligands numbers, geometry of the metal ligand complex and fermi resonance.

Effect on Electron Density on Metal

As shown in Table \(\PageIndex{8}\), a greater charge on the metal center result in the CO stretches vibration frequency decreasing. For example, [Ag(CO)]+show higher frequency of CO than free CO, which indicates a strengthening o

f the CO bond. σ donation removes electron density from the nonbonding HOMO of CO. From Figure, it is clear that the HOMO has a small amount of anti-bonding property, so removal of an electron actually increases (slightly) the CO bond strength. Therefore, the effect of charge and electronegativity depends on the amount of metal to CO π-back bonding and the CO IR stretching frequency.

If the electron density on a metal center is increasing, more π-back bonding to the CO ligand(s) will also increase, as shown in Table \(\PageIndex{9}\). It means more electron density would enter into the empty carbonyl π* orbital and weaken the C-O bond. Therefore, it makes the M-CO bond strength increasing and more double-bond-like (M=C=O).

Ligation Donation Effect

Some cases, as shown in Table \(\PageIndex{9}\), different ligands would bind with same metal at the same metal-ligand complex. For example, if different electron density groups bind with Mo(CO) 3 as the same form, as shown in Figure \(\PageIndex{22}\), the CO vibrational frequencies would depend on the ligand donation effect. Compared with the PPh 3 group, CO stretching frequency which the complex binds the PF 3 group (2090, 2055 cm -1 ) is higher. It indicates that the absolute amount of electron density on that metal may have certain effect on the ability of the ligands on a metal to donate electron density to the metal center. Hence, it may be explained by the Ligand donation effect . Ligands that are trans to a carbonyl can have a large effect on the ability of the CO ligand to effectively π-backbond to the metal. For example, two trans π-backbonding ligands will partially compete for the same d-orbital electron density, weakening each other’s net M-L π-backbonding. If the trans ligand is a π-donating ligand, the free metal to CO π-backbonding can increase the M-CO bond strength (more M=C=O character). It is well known that pyridine and amines are not those strong π-donors. However, they are even worse π-backbonding ligands. So the CO is actually easy for π-back donation without any competition. Therefore, it naturally reduces the CO IR stretching frequencies in metal carbonyl complexes for the ligand donation effect.

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Geometry Effects

Some cases, metal-ligand complex can form not only terminal but also bridging geometry. As shown in Figure \(\PageIndex{23}\), in the compound Fe 2 (CO) 7 (dipy), CO can act as a bridging ligand. Evidence for a bridging mode of coordination can be easily obtained through IR spectroscopy. All the metal atoms bridged by a carbonyl can donate electron density into the π* orbital of the CO and weaken the CO bond, lowering vibration frequency of CO. In this example, the CO frequency in terminal is around 2080 cm -1 , and in bridge, it shifts to around 1850 cm -1 .

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Pump-probe Detection of Molecular Functional Group Vibrational Lifetime

The dynamics of molecular functional group plays an important role during a chemical process, chemical bond forming and breaking, energy transfer and other dynamics happens within picoseconds domain. It is very difficult to study such fast processes directly, for decades scientists can only learn from theoretical calculations, lacking experimental methods.

However, with the development of ultrashort pulsed laser enable experimental study of molecular functional group dynamics. With ultrafast laser technologies, people develop a series of measuring methods, among which, pump-probe technique is widely used to study the molecular functional group dynamics. Here we concentrate on how to use pump-probe experiment to measure functional group vibrational lifetime. The principle, experimental setup and data analysis will be introduced.

Principles of the Pump-probe Technique

For every function group within a molecule, such as the C≡N triple bond in phenyl selenocyanate (C 6 H 5 SeCN) or the C-D single bond in deuterated chloroform (DCCl 3 ), they have an individual infrared vibrational mode and associated energy levels. For a typical 3-level system (Figure \(\PageIndex{24}\), both the 0 to 1 and the 1 to 2 transition are near the probe pulse frequency (they don't necessarily need to have exactly the same frequency).

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In a pump-probe experiment, we use the geometry as is shown in Figure \(\PageIndex{25}\). Two synchronized laser beams, one of which is called pump beam (E pu ) while the other probe beam (E pr ). There is a delay in time between each pulse. The laser pulses hit the sample, the intensity of ultrafast laser (fs or ps) is strong enough to generated 3 rd order polarization and produce 3 rd order optical response signal which is use to give dynamics information of molecular function groups. For the total response signals we have \label{6} , where µ 10 µ 21 are transition dipole moment and E 0 , E 1 , and E 2 are the energies of the three levels, and t 3 is the time delay between pump and probe beam. The delay t 3 is varied and the response signal intensity is measured. The functional group vibration life time is determined from the data.

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\[ S \ =\ 4 \mu _{10} ^{4} e^{ -i(E_{1} - E_{0} ) t3/h - \Gamma t3} \label{6} \]

Typical Experimental Set-up

The optical layout of a typical pump-probe setup is schematically displayed in Figure \(\PageIndex{26}\). In the setup, the output of the oscillator (500 mW at 77 MHz repetition rate, 40 nm bandwidth centered at 800 nm) is split into two beams (1:4 power ratio). Of this, 20% of the power is to seed a femtosecond (fs) amplifier whose output is 40 fs pulses centered at 800 nm with power of ~3.4 W at 1 KHz repetition rate. The rest (80%) of the seed goes through a bandpass filter centered at 797.5nm with a width of 0.40 nm to seed a picosecond (ps) amplifier. The power of the stretched seed before entering the ps amplifier cavity is only ~3 mW. The output of the ps amplifier is 1ps pulses centered at 800 nm with a bandwidth ~0.6 nm. The power of the ps amplifier output is ~3 W. The fs amplifier is then to pump an optical parametric amplifier (OPA) which produces ~100 fs IR pulses with bandwidth of ~200 cm -1 that is tunable from 900 to 4000 cm -1 . The power of the fs IR pulses is 7~40 mW, depending on the frequencies. The ps amplifier is to pump a ps OPA which produces ~900 fs IR pulses with bandwidth of ~21 cm -1 , tunable from 900 - 4000 cm -1 . The power of the fs IR pulses is 10 ~ 40 mW, depending on frequencies.

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In a typical pump-probe setup, the ps IR beam is collimated and used as the pump beam. Approximately 1% of the fs IR OPA output is used as the probe beam whose intensity is further modified by a polarizer placed before the sample. Another polarizer is placed after the sample and before the spectrograph to select different polarizations of the signal. The signal is then sent into a spectrograph to resolve frequency, and detected with a mercury cadmium telluride (MCT) dual array detector. Use of a pump pulse (femtosecond, wide band) and a probe pulse (picoseconds, narrow band), scanning the delay time and reading the data from the spectrometer, will give the lifetime of the functional group. The wide band pump and spectrometer described here is for collecting multiple group of pump-probe combination.

Data Analysis

For a typical pump-probe curve shown in Figure \(\PageIndex{27}\) life time t is defined as the corresponding time value to the half intensity as time zero.

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Table \(\PageIndex{10}\) shows the pump-probe data of the C≡N triple bond in a series of aromatic cyano compounds: n -propyl cyanide (C 3 H 7 CN), ethyl thiocyanate (C 2 H 5 SCN), and ethyl selenocyanate (C 2 H 5 SeCN) for which the ν C ≡N for each compound (measured in CCl 4 solution) is 2252 cm -1 ), 2156 cm -1 , and ~2155 cm -1 , respectively.

A plot of intensity versus time for the data from TABLE is shown Figure \(\PageIndex{28}\). From these curves the C≡N stretch lifetimes can be determined for C 3 H 7 CN, C 2 H 5 SCN, and C 2 H 5 SeCN as ~5.5 ps, ~84 ps, and ~282 ps, respectively.

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From what is shown above, the pump-probe method is used in detecting C≡N vibrational lifetimes in different chemicals. One measurement only takes several second to get all the data and the lifetime, showing that pump-probe method is a powerful way to measure functional group vibrational lifetime.

Attenuated Total Reflectace- Fourier Transform Infrared Spectroscopy

Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) is a physical method of compositional analysis that builds upon traditional transmission FTIR spectroscopy to minimize sample preparation and optimize reproducibility. Condensed phase samples of relatively low refractive index are placed in close contact with a crystal of high refractive index and the infrared (IR) absorption spectrum of the sample can be collected. Based on total internal reflection, the absorption spectra of ATR resemble those of transmission FTIR. To learn more about transmission IR spectroscopy (FTIR) please refer to the section further up this page titled Fourier Transform Infrared Spectroscopy of Metal Ligand Complexes.

First publicly proposed in 1959 by Jacques Fahrenfort from the Royal Dutch Shell laboratories in Amsterdam, ATR IR spectroscopy was described as a technique to effectively measure weakly absorbing condensed phase materials. In Fahrenfort's first article describing the technique, published in 1961, he used a hemicylindrical ATR crystal (see Experimental Conditions) to produce single-reflection ATR (Figure \(\PageIndex{29}\) ). ATR IR spectroscopy was slow to become accepted as a method of characterization due to concerns about its quantitative effectiveness and reproducibility. The main concern being the sample and ATR crystal contact necessary to achieve decent spectral contrast. In the late 1980’s FTIR spectrometers began improving due to an increased dynamic range, signal to noise ratio, and faster computers. As a result ATR-FTIR also started gaining traction as an efficient spectroscopic technique. These days ATR accessories are often manufactured to work in conjunction with most FTIR spectrometers, as can be seen in Figure \(\PageIndex{30}\).

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Total Internal Reflection

For additional information on light waves and their properties please refer to the module on Vertical Scanning Interferometry (VSI) in chapter 10.1.

When considering light propagating across an interface between two materials with different indices of refraction, the angle of refraction can be given by Snell’s law, \ref{7} , where none of the incident light will be transmitted.

\[ \varphi _{c} \ =\ \varphi _{max} \label{7} \]

The reflectance of the interface is total and whenever light is incident from a higher refractive index medium onto a lower refractive index medium, the reflection is deemed internal (as opposed to external in the opposite scenario). Total internal reflectance experiences no losses, or no transmitted light (Figure \(\PageIndex{31}\)

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Supercritical internal reflection refers to angles of incidence above the critical angle of incidence allowing total internal reflectance. It is in this angular regime where only incident and reflected waves will be present. The transmitted wave is confined to the interface where its amplitude is at a maximum and will damp exponentially into the lower refractive index medium as a function of distance. This wave is referred to as the evanescent wave and it extends only a very short distance beyond the interface.

To apply total internal reflection to the experimental setup in ATR, consider n 2 to be the internal reflectance element or ATR crystal (the blue trapezoid in Figure \(\PageIndex{32}\) )

where n 2 is the material with the higher index of refraction. This should be a material that is fully transparent to the incident infrared radiation to give a real value for the refractive index. The ATR crystal must also have a high index of refraction to allow total internal reflection with many samples that have an index of refraction n 1 , where n 1 < n 2 .

Total internal reflection.jpg

We can consider the sample to be absorbing in the infrared. Electromagnetic energy will pass through the crystal/sample interface and propagate into the sample via the evanescent wave. This energy loss must be compensated with the incident IR light. Thus, total reflectance is no longer occurring and the reflection inside the crystal is attenuated. If a sample does not absorb, the reflectance at the interface shows no attenuation. Therefore if the IR light at a particular frequency does not reach the detector, the sample must have absorbed it.

The penetration depth of the evanescent wave within the sample is on the order of 1µm. The expression of the penetration depth is given in \ref{8} and is dependent upon the wavelength and angle of incident light as well as the refractive indices of the ATR crystal and sample. The effective path length is the product of the depth of penetration of the evanescent wave and the number of points that the IR light reflects at the interface between the crystal and sample. This path length is equivalent to the path length of a sample in a traditional transmission FTIR setup.

\[ d_{p} = \frac{ \lambda }{2 \pi n_{1}} (sin \omega - ( \frac{n_{1}}{n_{2}} )^{2} )^{1/2} \label{8} \]

Experimental Conditions

Refractive indices of atr crystal and sample.

Typically an ATR attachment can be used with a traditional FTIR where the beam of incident IR light enters a horizontally positioned crystal with a high refractive index in the range of 1.5 to 4, as can be seen in Table \(\PageIndex{11}\) will consist of organic compounds, inorganic compounds, and polymers which have refractive indices below 2 and can readily be found on a database.

Single and Multiple Reflection Crystals

Multiple reflection ATR was initially more popular than single reflection ATR because of the weak absorbances associated with single reflection ATR. More reflections increased the evanescent wave interaction with the sample, which was believed to increase the signal to noise ratio of the spectrum. When IR spectrometers developed better spectral contrast, single reflection ATR became more popular. The number of reflections and spectral contrast increases with the length of the crystal and decreases with the angle of incidence as well as thickness. Within multiple reflection crystals some of the light is transmitted and some is reflected as the light exits the crystal, resulting in some of the light going back through the crystal for a round trip. Therefore, light exiting the ATR crystal contains components that experienced different number of reflections at the crystal-sample interface.

Angle of Incidence

It was more common in earlier instruments to allow selection of the incident angle, sometimes offering selection between 30°, 45°, and 60°. In all cases for total internal reflection to hold, the angle of incidence must exceed the critical angle and ideally complement the angle of the crystal edge so that the light enters at a normal angle of incidence. These days 45° is the standard angle on most ATR-FTIR setups.

ATR Crystal Shape

For the most part ATR crystals will have a trapezoidal shape as shown in Figure \(\PageIndex{31}\). This shape facilitates sample preparation and handling on the crystal surface by enabling the optical setup to be placed below the crystal. However, different crystal shapes (Figure \(\PageIndex{33}\) ) may be used for particular purposes, whether it is to achieve multiple reflections or reduce the spot size. For example, a hemispherical crystal may be used in a microsampling experiment in which the beam diameter can be reduced at no expense to the light intensity. This allows appropriate measurement of a small sample without compromising the quality of the resulting spectral features.

crystalshapes.jpg

Crystal-sample contact

Because the path length of the evanescent wave is confined to the interface between the ATR crystal and sample, the sample should make firm contact with the ATR crystal (Figure \(\PageIndex{34}\) ). The sample sits atop the crystal and intimate contact can be ensured by applying pressure above the sample. However, one must be mindful of the ATR crystal hardness. Too much pressure may distort the crystal and affect the reproducibility of the resulting spectrum.

Picture 12.jpg

The wavelength effect expressed in \label{7} shows an increase in penetration depth at increased wavelength. In terms of wavenumbers the relationship becomes inverse. At 4000 cm -1 penetration of the sample is 10x less than penetration at 400 cm -1 meaning the intensity of the peaks may appear higher at lower wavenumbers in the absorbance spectrum compared to the spectral features in a transmission FTIR spectrum (if an automated correction to the ATR setup is not already in place).

Selecting an ATR Crystal

ATR functions effectively on the condition that the refractive index of the crystal is of a higher refractive index than the sample. Several crystals are available for use and it is important to select an appropriate option for any given experiment (Table \(\PageIndex{11}\) ).

When selecting a material, it is important to consider reactivity, temperature, toxicity, solubility, and hardness.

The first ATR crystals in use were KRS-5, a mixture of thallium bromide and iodide, and silver halides. These materials are not listed in the table because they are not in use any longer. While cost-effective, they are not practical due to their light sensitivity, softness, and relatively low refractive indices. In addition KRS-5 is terribly toxic and dissolves on contact with many solvents, including water.

At present diamond is a favorable option for its hardness, inertness and wide spectral range, but may not be a financially viable option for some experiments. ZnSe and germanium are the most common crystal materials. ZnSe is reasonably priced, has significant mechanical strength and a long endurance. However, the surface will become etched with exposure to chemicals on either extreme of the pH scale. With a strong acid ZnSe will react to form toxic hydrogen selenide gas. ZnSe is also prone to oxidation and care must be taken to avoid the formation of an IR absorbing layer of SeO 2 . Germanium has a higher refractive index, which reduces the depth of penetration to 1 µm and may be preferable to ZnSe in applications involving intense sample absorptions or for use with samples that produce strong background absorptions. Sapphire is physically robust with a wide spectral range, but has a relatively low refractive index in terms of ATR crystals, meaning it may not be able to test as many samples as another crystal might.

Sample Versatility

The versatility of ATR is reflected in the various forms and phases that a sample can assume. Solid samples need not be compressed into a pellet, dispersed into a mull or dissolve in a solution. A ground solid sample is simply pressed to the surface of the ATR crystal. For hard samples that may present a challenge to grind into a fine solid, the total area in contact with the crystal may be compromised unless small ATR crystals with exceptional durability are used (e.g., 2 mm diamond). Loss of contact with the crystal would result in decreased signal intensity because the evanescent wave may not penetrate the sample effectively. The inherently short path length of ATR due to the short penetration depth (0.5-5 µm) enables surface-modified solid samples to be readily characterized with ATR.

Powdered samples are often tedious to prepare for analysis with transmission spectroscopy because they typically require being made into a KBr pellet to and ensuring the powdered sample is ground up sufficiently to reduce scattering. However, powdered samples require no sample preparation when taking the ATR spectra. This is advantageous in terms of time and effort, but also means the sample can easily be recovered after analysis.

The advantage of using ATR to analyze liquid samples becomes apparent when short effective path lengths are required. The spectral reproducibility of liquid samples is certain as long as the entire length of the crystal is in contact with the liquid sample, ensuring the evanescent wave is interacting with the sample at the points of reflection, and the thickness of the liquid sample exceeds the penetration depth. A small path length may be necessary for aqueous solutions in order to reduce the absorbance of water.

Sample Preparation

ATR-FTIR has been used in fields spanning forensic analysis to pharmaceutical applications and even art preservation. Due to its ease of use and accessibility ATR can be used to determine the purity of a compound. With only a minimal amount of sample this researcher is able to collect a quick analysis of her sample and determine whether it has been adequately purified or requires further processing. As can be seen in Figure \(\PageIndex{35}\), the sample size is minute and requires no preparation. The sample is placed in close contact with the ATR crystal by turning a knob that will apply pressure to the sample (Figure \(\PageIndex{36}\) ).

Picture 7_jwcg.jpg

ATR has an added advantage in that it inherently encloses the optical path of the IR beam. In a transmission FTIR, atmospheric compounds are constantly exposed to the IR beam and can present significant interference with the sample measurement. Of course the transmission FTIR can be purged in a dry environment, but sample measurement may become cumbersome. In an ATR measurement, however, light from the spectrometer is constantly in contact with the sample and exposure to the environment is reduced to a minimum.

Application to Inorganic Chemistry

One exciting application of ATR is in the study of classical works of art. In the study of fragments of a piece of artwork, where samples are scarce and one-of-a-kind, ATR is a suitable method of characterization because it requires only a small sample size. Determining the compounds present in art enables proper preservation and historical insight into the pieces.

In a study examining several paint samples from a various origins, a micro-ATR was employed for analysis. This study used a silicon crystal with a refractive index of 2.4 and a reduced beam size. Going beyond a simple surface analysis, this study explored the localization of various organic and inorganic compounds in the samples by performing a stratigraphic analysis. The researchers did so by embedding the samples in both KBr and a polyester resins. Two embedding techniques were compared to observe cross-sections of the samples. The mapping of the samples took approximately 1-3 hours which may seem quite laborious to some, but considering the precious nature of the sample, the wait time was acceptable to the researchers.

The optical microscope picture ( Figure \(\PageIndex{37}\) ) shows a sample of a blue painted area from the robe of a 14 th century Italian polychrome statue of a Madonna. The spectra shown in Figure \(\PageIndex{38}\) were acquired from the different layers pictured in the box marked in Figure \(\PageIndex{37}\). All spectra were collected from the cross-sectioned sample and the false-color map on each spectrum indicates the location of each of these compounds within the embedded sample. The spectra correspond to the inorganic compounds listed in Table \(\PageIndex{12}\), which also highlights characteristic vibrational bands.

Picture 9.jpg

The deep blue layer 3 corresponds to azurite and the light blue paint layer 2 to a mixture of silicate based blue pigments and white lead. Although beyond the ATR crystal’s spatial resolution limit of 20 µm, the absorption of bole was detected by the characteristic triple absorption bands of 3697, 3651, and 3619 cm -1 as seen in spectrum d of Figure \(\PageIndex{37}\). The white layer 0 was identified as gypsum.

To identify the binding material, the KBr embedded sample proved to be more effective than the polyester resin. This was due in part to the overwhelming IR absorbance of gypsum in the same spectral range (1700-1600 cm -1 ) as a characteristic stretch of the binding as well as some contaminant absorption due to the polyester embedding resin.

To spatially locate specific pigments and binding media, ATR mapping was performed on the area highlighted with a box in Figure \(\PageIndex{37}\). The false color images alongside each spectrum in Figure \(\PageIndex{38}\) indicate the relative presence of the compound corresponding to each spectrum in the boxed area. ATR mapping was achieved by taking 108 spectra across the 220x160 µm area and selecting for each identified compound by its characteristic vibrational band.

  • Research Guides

BSCI 1510L Literature and Stats Guide: Infrared gas analysis

  • 1 What is a scientific paper?
  • 2 Referencing and accessing papers
  • 2.1 Literature Cited
  • 2.2 Accessing Scientific Papers
  • 2.3 Traversing the web of citations
  • 2.4 Keyword Searches
  • 3 Style of scientific writing
  • 3.1 Specific details regarding scientific writing
  • 3.2 Components of a scientific paper
  • 4 For further information
  • Appendix A: Calculation Final Concentrations
  • 1 Formulas in Excel
  • 2 Basic operations in Excel
  • 3 Measurement and Variation
  • 3.1 Describing Quantities and Their Variation
  • 3.2 Samples Versus Populations
  • 3.3 Calculating Descriptive Statistics using Excel
  • 4 Variation and differences
  • 5 Differences in Experimental Science
  • 5.1 Aside: Commuting to Nashville
  • 5.2 P and Detecting Differences in Variable Quantities
  • 5.3 Statistical significance
  • 5.4 A test for differences of sample means: 95% Confidence Intervals
  • 5.5 Error bars in figures
  • 5.6 Discussing statistics in your scientific writing
  • 6 Scatter plot, trendline, and linear regression
  • 7 The t-test of Means
  • 8 Paired t-test
  • 9 Two-Tailed and One-Tailed Tests
  • 10 Variation on t-tests: ANOVA
  • 11 Reporting the Results of a Statistical Test
  • 12 Summary of statistical tests
  • 1 Objectives
  • 2 Project timeline
  • 3 Background
  • 4 Previous work in the BSCI 111 class
  • 5 General notes about the project
  • 6 About the paper
  • 7 References

Infrared gas analyzer (IRGA)

Rates of photosynthesis in plants can be monitored in several ways, but the most effective is using an infrared gas analyzer (IRGA) .  An IRGA takes advantage of the absorbance of CO 2 molecules at a wavelength of 4260 nm caused by the stretching vibrations of the C=O double bonds. 

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Fig. 1  Leaf chamber of a portable IRGA

Portable IRGAs have been engineered to take relatively rapid readings from air that has passed over a leaf surface (Fig. 1).  Thus the rate of gas exchange can be measured on an intact leaf under field conditions.  A sophisticated IRGA can measure (and even control) the concentration of CO 2 and humidity entering the leaf chamber, the intensity of the incident light and the CO 2 concentration exiting the chamber.  At the same time, it can calculate and record photosynthesis rates using an on-board computer.  With such an IRGA, one can determine photosynthetic light response curves for a plant species under specific temperatures and CO 2 concentrations (Fig. 2).  (It might interest you to know that a portable IRGA can cost more than a new car!)

Fig. 2 Photosynthetic light response curves for Echinacea tennesseensis at 25 ºC.  PAR=photosynthetically active radiation, i.e. light intensity; H and L refer to high and low temperature and moisture pretreatments.  From Baskauf and Eickmeier (1994).

Measuring gas exchange in the BSCI 111 lab

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Fig. 3. IRGA sensor

In this experiment, we will be using an IRGA system (Fig. 3) that can interface with a microcomputer.  Although the apparatus is less sophisticated (and less expensive!), its principles of operation are the same as that discussed above.  The shaft of the sensor is placed in a closed container.  A hot metal filament produces the incident infrared radiation (and the glowing light that you observe winking on and off as the unit operates).  Holes in the shaft allow air to diffuse into the sensor.  At the end of the shaft, an infrared sensor measures the intensity of the radiation after it passes through the air sample.  The higher the concentration of CO 2 , the less radiation that reaches the sensor.  The sensor takes a measurement about once per second.  Because air must diffuse into the sensor, there is a short lag time before changes brought about by leaf activity cause a change to occur in the readings. Note that unlike the IRGA shown in Fig. 1, this is a closed system and that the CO 2 concentration impinging on the leaf cannot be controlled.  Thus the sensor measures changes in CO 2 in the container over time, rather than measuring the difference in CO 2 before and after air passes over the leaf.  Nevertheless, we can measure the rate of gas exchange by observing the rate of change of CO 2 in the container.  The slope of the CO 2 concentration vs. time graph represents the rate of gas exchange. 

Baskauf, C. J. and W. G. Eickmeier (1994) Comparative ecophysiology of a rare and a widespread species of Echinacea (Asteraceae).  American Journal of Botany 81:958-964.

  • Last Updated: Apr 19, 2023 2:37 PM
  • URL: https://researchguides.library.vanderbilt.edu/bsci1510L

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Instrumentation and Control Engineering

Basics of Infrared Gas Analyser

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The region of the IR most used by process analyzers is broken into two parts: the near IR (12,500 to 4000 cm-1) and the mid IR (4000 to 650 cm-1). Except for a small overlap region, sources and detectors that are needed in the near IR will not work in the mid IR and vice versa. Most laboratory IR spectrophotometers work form 4000 cm-1 to between 650 and 200 cm-1,

Infrared radiation interacts with all molecules [except the homonuclear diatomics oxygen (O2), nitrogen (N2), hydrogen (H2), chlorine (Cl2), etc.] by exciting molecular vibrations and rotations. The oscillating electric field of the IR wave interacts with the electric dipole of the molecule, and when the IR frequency matches the natural frequency of the molecule, some of the IR power is absorbed. The pattern of wavelengths, or frequencies, absorbed identifies the molecules in the sample. The strength of absorption at particular frequencies is a measure of their concentration. Analytical laboratory IR is largely concerned with identification, or qualitative analysis, while process IR is concerned with quantitative analysis. Fundamentals of Infrared Analysis : Particular groups of atoms tend to absorb at the same time frequency with very little influence from the rest if the molecule. These group frequencies are a great help in identifying molecules from the IR spectra. On the other hand, similar molecules, such as a series of homologous hydrocarbons, have very similar IR spectra. Infrared analysis is, therefore, most straightforward when the component molecules of the sample have significantly different atomic groupings. A mixture of aliphatic hydrocarbons would be better analyzed by another technique, such as gas chromatography. The part of the spectrum offering the best discrimination between molecules is between 7 and 15 µm, the so-called finger-print region.

The starting point for quantitative analysis is the Beer-Lambert law, which relates the amount of light absorbed to the sample’s concentration and path length.

A= abc=log10 I0 / I

Infrared Gas Analyser Equation

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Gases - Short and Long Path Cells for FTIR Gas Analysis

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International Crystal Laboratories’ Gemini Series Short Path Gas Cells and Long Path Gas Cells are designed for industrial and research applications for FTIR gas analysis. Five product groupings provide users with choices to configure the appropriate solution.   Short path length single pass gas cells constructed of electropolished 316 Stainless Steel, available in standard path lengths of 1cm, 2cm, 3cm, 5cm, 10cm and 15cm.  Long path cells address the most demanding gas sampling requirements of industrial and field deployed FTIR and IR instrumentation.  For applications requiring ultra long path lengths we offer our Saturn™ series multi-pass gas cells that range from 80 to 200 meters.

If you have any questions about our FTIR Gas Analysis Cells, please contact us

 FTIR Gas Analysis Instruments Overview

Mercury™ Single Pass Short Path Length Stainless Steel Specialty Gas Cells

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Short Path Gas cells are useful for FTIR Gas analysis and generally speaking, for concentrations of not less than 100 ppm. Cell utility for various ppm concentrations is dependent not only on path length, but also on the absorbance characteristics of the sample. Feel free to consult with us on these and any other issues concerning selection of an appropriate gas cell.

ICL’s Gemini™ gas cell line begins with our Mercury™ short path length single pass gas cells constructed of electropolished 316 Stainless Steel, available in standard path lengths of 1cm, 2cm, 3cm, 5cm, 10cm and 15cm. Custom path length cells for a variety of applications are also available. These robustly constructed gas cells are ideal for specialty gas analysis, because they are leak tight and capable of high vacuum performance exceeding 1 x 10-8 Torr, and they also can be made pressure-safe with appropriate window material and thickness selection. Each Mercury™ cell is individually helium leak tested or hydrostatically tested and a test certificate can be supplied with any cell for $299.00 (Part No. 0009-9281). Mercury™ cell configurations include base plate mounted, slide mounted and heated versions with temperature controllers. Purge maintaining connectors or couplers extending from the cell body to the sample compartment walls are standard on many base plate mounted models and purge gas connections are an available option. A wide variety of window materials can be used. Wedged windows are available to limit fringing. Instrument specific spectrophotometer interfaces are available. Mercury™ cells feature high-vacuum welded gas fittings without valves comprising the sample inlet and outlet to maintain the leak-free integrity of the cell and assure high vacuum performance. Inlets and outlets comprise a Swagelok® VCR gland, a VCR female nut, a male VCR nut and a 1/4″ Swagelok® adapter. Valve kits are available. Several common Mercury™ cell configurations are shown, but custom configurations are available upon request. Prices do not include windows, which can be selected from our transmission window price list. RFQ for wedged windows. Cell mounting and other hardware, exclusive of the cell body material and valves, is black anodized aluminum.

Mercury Data Sheet

Mars™ Long Path Gas Cells

For applications that demand high performance, durability, reliability, simplicity, dependability and serviceability, Gemini™ Mars™ gas cells are the solution. Mars™ long path cells address the most demanding FTIR gas analysis sampling requirements of industrial and field deployed FTIR Gas Analysis and IR instrumentation with a robust series of metal bodied gas cells with electro-polished stainless steel or electro-less nickel plated aluminum bodies that outperform any gas cell on the market.

A key factor ensuring the durability and serviceability of each Mars™ cell is the use of stainless steel mirrors. These mirrors are extremely durable and they can be reconditioned many times at modest cost, thereby extending the service life of each Mars™ cell for many years. All mirrors are coated with our proprietary Gemgold ™ multilayer gold coatings to insure the highest possible performance in demanding environments. Stainless steel optics are inherently superior to diamond turned aluminum mirrors, from the standpoint of longevity and serviceability. While 316 stainless steel tends to resist the corrosive effects of reactive gases, aluminum tends to react with many gases making aluminum subject to corrosion and therefore an inferior optical substrate.

A more effective low cost option to aluminum mirrors is gold coated glass mirrors, which can be special ordered on Mars™ at a discount to the prices of cells with stainless steel mirrors. Mars™ cells feature modular designs with interchangeable and reconditionable components, enabling us to deliver gas cells that are both versatile and serviceable. Metal bodies and cell hardware can be constructed from many materials. Cells from materials such as stainless steel (passivated or electro polished), pure nickel, nickel coated aluminum, black anodized aluminum, and FEP or PTFE coated aluminum are all available and can be configured to user specifications.

Mars™ gas cells can be configured for high vacuum exceeding 1 x 10 -9 Torr and high pressure applications. Each Mars™ cell is individually helium leak tested or hydrostatically tested and a helium leak test certificate can be supplied with any cell for $299.00 (Part No. 0009-9281). See www.internationalcrystal.net for a sample vacuum test certification.

Mars Cells can be configured with heating jackets and with user programmable temperature and pressure control and monitoring options.

Several common Mars™ cell configurations are shown, but custom configurations are available upon request. Prices include KBr windows, but other materials can be selected from our transmission window price list. RFQ for wedged windows to limit fringing. All Mars™ cells use 25 x 4mm windows. Cell mounting and other hardware, exclusive of the cell body material and valves, is black anodized aluminum.

Mars Data Sheet

Venus™ Glass Bodied Long Path Gas Cells

Venus

Gemini™ Venus™ Glass bodied long path gas cells are classic Hanst cells, first made commercially available by Dr. Phil Hanst , who started producing them after a long career at NASA and the EPA. Hanst cells are in our bloodline at ICL, since we were Phil’s first outlet for his product and Phil’s son Steve, who worked with Phil designing gas cells for 20 years, is now a major shareholder in ICL as well as its Executive Vice President.

Venus™ cells are small to moderate volume long path gas cells designed for FTIR Gas Analysis and for all purpose use in industrial and research applications. The cell design has improved significantly over the years with such advances as multilayer proprietary gold mirror coatings, improved purgeable transfer optics, and instrument specific base plate mountings. All Venus cells are fitted with Swagelok® 1/4″ or 1/8″ tube fittings to which valves, hose barbs or compression fittings can be attached. SS Swagelok® Valves are standard on the models listed.

Venus™ cell configurations include ICL’s popular Ultra Mini Cell which measures 3″ x 3″ x 8″ (3 meters fixed path length, 200cc volume), the Long Path Mini Cell (variable path length to 5 meters, 0.5L volume) and fixed and variable path length cells ranging from 4.8 to 8 meters.  Instrument specific interfaces are available for base plate mounted cell configurations.

Several common Venus™ cell configurations are shown, but custom configurations are available upon request. Prices do include KBr windows, but optional windows can be selected from our transmission window price list. RFQ for wedged windows to limit fringing a/k/a channeling. Cell mounting and other hardware, exclusive of the cell body material and valves, is black anodized aluminum. Fittings and Valves are SS Swagelok®. All cell bodies are borosilicate glass. Mirrors are GemGold ™ multi-layer gold coated, removable and easily serviced.

Venus Data Sheet  

Earth™  Glass Bodied Long Path Gas Cells

ir gas analysis

Gemini™ Earth™ glass bodied long path gas cells are, like Venus™ cells, classic Hanst cells, but with larger volumes and longer path lengths. Pathlengths range from 10 to 20 meters and volumes range from 2 to 8 liters.

For applications requiring ultra long path lengths we offer our Saturn™ series multi-pass gas cells that range from 80 to 200 meters. These cells employ an innovative advancement in White cell configuration that effectively triples the operational range of the cell within a moderate size cell volume, thereby providing path lengths in excess of 100 meters that still produce strong signal throughput to enable part per billion (ppb) trace level detection sensitivity in ambient conditions and when sampling gas mixtures.

The spectra show excellent signal to noise performance at 0.5cm-1 resolution in a 160 meter path length cell in a few minutes of elapsed observation time.

All Earth and Saturn cells feature SS Swagelok® valves, multilayer gold coated mirrors, borosilicate glass bodies, transfer optics and base plate configurations to fit most spectrophotometers, and black anodized aluminum hardware. Earth™ cells use 1/4″ SS flow tubes and Swagelok® connections and Saturn cells use 3/8″ SS flow tubes and Swagelok® connections. All variable path length cells incorporate a laser alignment device with a 6 volt DC power supply for path length verification based on the number of passes made through the cell.

Several typical Earth™ and Saturn™ cell configurations are shown, but custom configurations are available upon request. Prices do include KBr windows, but optional windows can be selected from our transmission window price list. RFQ for wedged windows to limit fringing a/k/a channeling. Cell mounting and other hardware, exclusive of the cell body material and valves, is black anodized aluminum. Fittings and Valves are SS Swagelok®. All cell bodies are borosilicate glass. Mirrors are removable and easily serviced.

Earth Data Sheet  

Saturn™  Long Path Gas Cells

ir gas analysis

Saturn Series Multi-Pass cells are Research-grade long path gas cells from 50 meters to 200 meters or more. Custom transfer optics and mounting hardware for center-focus or side port use on spectrometer systems are provided. An innovative advancement of the White optical configuration for long-long path length effectively triples the operational range of the instrument inside a moderate size cell volume. This provides path lengths in excess of 100 meters that can produce a strong IR signal throughput for trace level detection.

Saturn Series gas cells are ideal accessories for FTIR Gas Analysis and long-long pathlength studies of ambient air pollution or trace gas analysis in the ppb range.  Black anodized aluminum hardware is featured as the primary material for the cell parts. A pyrex glass cell body can be substituted with Quartz, Aluminum, or other materials, which can be protected by a variety of coatings like FEP or nickel plating to prevent reactions on the cell chamber walls or prevent adsorption of trace gases

Heated Vapor Gas Multi-Sampler

This heated cell can be configured as a gas cell (Fig. 2) or for vaporizing samples that are normally solids or liquids at room temperature. Configured for gas sampling, the cell incorporates 2 valves, an inlet and an outlet, that enable flowing gas samples through the cell.

Heat Vapor

In its solid and liquid vaporization configuration, the cell has an outlet needle valve from which a vacuum can be pulled and two (2) sample inlets. One inlet is a needle septa injection port on the top of the cell which is similar to the type used in gas chromatography. There is a second inlet in the form of a side port for inserting solid samples, which also doubles as the thermocouple port. Using the side port eliminates the need to recheck vacuum seals whenever the cell is opened to insert solid samples. The 250w heating jacket extends over the end of the cell so that both the cell body and the optics are heated, and allows operation at temperatures up to 200°C. Condensation on the optics is minimized by heating them to the same temperature as the sample chamber. A type J iron constantan thermocouple is provided with plugs which are compatible with the optional temperature controller. The cell body is type 304 stainless steel and an assortment of seals is provided – silicon rubber, viton and PTFE.

Cells are factory pretested for vacuum leaks using dummy windows and then new windows are shipped with the cell. The cell comes standard with 47 x 6mm KCl windows. The clear aperture is 39mm. Other window materials are available. Requires either a cell mount (0008-5162) (Fig. 1) or base plate mount (0008-9405) (Fig. 2). An optional high stability temperature controller is available. The temperature controller has ramp and soak cycles that can be programmed on the keypad.

Model G-2 Gas Cell – 5cm or 10cm

GasCell

The G-2 Gas Cell is for analysis of pure gases or gas mixtures in which component gases are present at concentrations of 5 percent or more. The G-2 Gas Cell has a fixed pathlength of either 5cm or 10cm. The windows of the cell are easy to remove for cleaning, making the G-2 ideal for working with wet or reactive gases that have a tendency to fog or attack the window material.

The G-2 cell comes completely assembled in a stainless steel universal 2” x 3” mount. It includes a Pyrex® glass body, silicon rubber or Viton® gaskets and a choice of optical materials. The window size is 38mm x 6mm. The clear aperture of the cell is 32mm.

The G-2 is available in a variety of optical window materials, two pathlengths and a choice of ground tapered endings with 12/30 joints (TPE), plain endings joints (PTE) or ground semi-ball joints with 18/9 joints (SBJ). (See notes beneath chart and diagram).

Demountable Beta Gas Cell 5cm or 10cm

Demountable Beta Gas Cell

The demountable Beta Gas Cell comes complete with a 10cm or 5cm Pyrex Cell body, cell mount, 2 Viton® O-rings or gaskets, 2 PTFE gaskets, and 2 septa, but without windows. (Windows and gaskets can be ordered separately). The window size is 25mm x 4mm. When assembled, the cell will be gas tight and operable at both reduced or positive pressures. Filling of the cell is conveniently handled with a gas tight syringe (ICL Part no. 0009-609). The exploded view shows the assembly and disassembly sequence of the cell.

Model G-2SS 10cm Stainless Steel Gas Cell

Stainless Steel Gas Cell

The only functional constraint on the pressure that can be applied to this cell is the window material selected. The clear aperture of the cell is 39mm and it uses 47mm x 6mm windows. Like the G-2, this cell uses a universal 2” x 3” slide mount that fits all spectrophotometers. However, it is also available base plate mounted (See Fig. 1). There are inlet and outlet ports equipped with vacuum valves and hose barbs that are configured to also allow flowing gas through the cell. An 1/4 NPT port in the side of the cell allows the addition of a pressure gauge, a thermocouple and other probes. The cell uses nitrile “O” rings and PTFE gaskets. KBr and NaCl are the basic window material, but other materials are available upon request. Can be heated.

G-3PTFE 10cm PTFE Gas Cell

G-3PTFE 10cm PTFE Gas Cell

All components of the G-3PTFE 10cm gas cell that contact the sample can be made entirely of PTFE, making the cell useful for any gas sample that might attack, or be contaminated by, glass or stainless steel gas cell components.

The G-3PTFE is useful for analysis of pure gases and mixtures in which the constituent gases are present in concentrations of 5% or more. The G-3PTFE has a clear aperture of 39mm and uses 47 x 6mm windows. Swagelok® SS valves are standard, but PFA valves are optional. A heated version is available. Requires a cell mount (Part No. 0008-5162) or optional base plate mount (Fig. 1) (Part No. 0008-9404).

FTIR Gas Analysis

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Gas analysis using IR light sources

Gas analysis using infrared (IR) light sources

Performing gas analysis by infrared absorption requires a light source to provide energy that will be absorbed by gas molecules. Selecting the proper light source can be a complex process, but Hamamatsu is ready to meet the needs of many applications. From detecting extremely low levels of impurities in clean rooms, to medical capnography, to portable LEL/LFL measurements, we can supply a high performing solution that is also cost effective.

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Infrared (IR) light sources

Hamamatsu has a robust line of MIR light sources for your application of choice.

The quantum cascade laser (QCL) represents a technology revolution, achieving 4+ micron light. Distributed feedback (DFB) lasers can provide linewidth resolution that allows parts per trillion accuracy in some cases. These lasers require a great deal of expertise and auxiliary equipment, so in situations where they aren’t desirable, LEDs can be a great alternative. LEDs' reductions in cost and complexity, coupled with long lifetimes, are an excellent combination for instruments that do not require the performance of a QCL.

Quantum cascade lasers (QCLs)

QCLs help instruments detect gases such as CO x , NO x , SO x , CH 4 , NH 3 , and O 3 down to ppb levels or measure isotopes of carbon dioxide and methane. When selecting QCLs, consider these characteristics:

  • Peak emission
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We offer QCLs in continuous wave (CW HHL package) and pulsed emission types. They feature high output, high-speed response, and high reliability. Customization options include wavelength range or peak emission, output power, operating temperature, and operating current.

Mid-infrared LEDs

When selecting mid-infrared (MIR) LEDs, consider these characteristics:

  • Narrow emission spectrum

Our line of RoHS-compliant mid-infrared LEDs can bring 3+ micron light at a fraction of the cost of a QCL, and achieve parts per million sensitivity with proper calibration. Currently, we offer LEDs for measuring CH 4 (3.3 µm), reference (3.9 µm), and CO 2 (4.3 µm). They feature high output, high reliability, and low power consumption.

Near-infrared LEDs

When selecting near-infrared (NIR) LEDs, it is important to consider these characteristics:

We offer many RoHS-compliant near-infrared LEDs with peak emission within 0.83 - 1.55 µm. They feature high output and high reliability, and they consume less power and have a faster response time than NIR-emitting lamps.

Recommended products

Mid-infrared LEDs having high output with peak emission wavelengths of 3.3, 3.9, and 4.3 μm are available in metal or compact ceramic packages. For detection elements, use quantum detectors such as InAsSb photovoltaic detectors.

ir gas analysis

Mid infrared LED L15894-0390C

Mid infrared led l15893-0330c, mid infrared led l15895-0430c.

Quantum cascade lasers are semiconductor lasers that offer peak emission in the mid-IR range (4 μm to 10 μm). They have gained considerable attention as a new light source for mid-IR applications such as molecular gas analysis used in environmental measurement.

Related documents

Application note: CO 2 measurement by InAsSb photovoltaic detectors | Mid-infrared LEDs

Related videos

Infrared product conversations part 2: quantum cascade laser (qcl).

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ir gas analysis

HORIBA's next-generation infrared gas analysis technology IRLAM TM*

Our new measurement technology.

In 2021, HORIBA succeeded in developing a new and proprietary method of Infrared Laser Absorption Modulation (IRLAM™) and putting it into practical use.

IRLAM enables real-time measurement of gases that were difficult to measure with conventional infrared gas analysis technology, due to insufficient sensitivity and interference effects (effects of coexisting gases other than the gas to be measured).

IRLAM is an evolution of the infrared gas analysis technology that HORIBA has refined over the years, and positions this technology, which can achieve High Sensitivity, Low Interference, Compact size, and High Reliability, as a new core analysis technology for HORIBA.

The features of IRLAM are realized by three of HORIBA's proprietary technologies: Quantum Cascade Laser (QCL), Herriott Cell, and the Concentration Calculation Algorithm.

  • Quantum Cascade Laser (QCL): Our original laser source device which has been specifically designed for gas measurement, with improved environmental performance.
  • Herriott Cell: A compact gas cell of our own development that achieves optimal optical path length and small size for various applications.
  • Concentration Calculation Algorithm: HORIBA’s unique calculation algorithm that extracts features from gas absorption signals to achieve low interference and high sensitivity with high speed.

This website provides an easy-to-understand introduction of the principles and features of this new core analytical technology that will become the standard for the next generation gas analysis.

ir gas analysis

Optical Hardware

ir gas analysis

The optical hardware used in IRLAM is developed, designed and manufactured in-house by HORIBA.

Concentration Calculation

ir gas analysis

The unique concentration calculation method uses "feature values" that dramatically reduces the amount of computation.

Application Fields

ir gas analysis

Gas analyzers equipped with IRLAM are expected to play an active role in various fields due to their high performance.

IRLAM Developer’s Column 1 - The thoughts behind the logo

The IRLAM logo was designed to resemble the three lights and multiple reflections of laser light in a Herriott Cell. The three lights represent the three proprietary technologies that support the high performance: QCL, Herriott Cell and Concentration Calculation Algorithm, and the multiple reflections illustrate the multiple features of IRLAM: High Sensitivity, Low Interference, Compact Size and High Reliability.

This logo will be attached as an emblem to various HORIBA gas analyzers equipped with IRLAM technology, proving that they are gas analysis instruments that realize the unparalleled performance.

ir gas analysis

  • K. Shibuya, A. Podzorov, M. Matsuhama, K. Nishimura, and M. Magari, ”High-sensitivity and low-interference gas analyzer with feature extraction from mid-infrared laser absorption-modulated signal,” Meas. Sci. Technol. 32, 035201 (2021). ( https://iopscience.iop.org/article/10.1088/1361-6501/abc5f7 )
  • Y. Onishi, S. Hamauchi, K. Shibuya, K. McWilliams-Ward, M. Akita, and K. Tsurumi, "Development of On-Board NH3 and N2O Analyzer Utilizing Mid-Infrared Laser Absorption Spectroscopy," SAE Technical Paper 2021-01-0610 (2021). ( https://www.sae.org/publications/technical-papers/content/2021-01-0610/ )
  • K. Hara, K. Shibuya, N. Nagura, T. Hanada, and K. Tsurumi, "Formaldehydes Measurement Using Laser Spectroscopic Gas Analyzer," SAE Technical Paper 2021-01-0604 (2021). ( https://www.sae.org/publications/technical-papers/content/2021-01-0604/ )

* IRLAM is a registered trademark or trademark of HORIBA, Ltd. in Japan and other countries

Request for Information

Do you have any questions or requests? Use this form to contact our specialists.

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IMAGES

  1. T-Series Advanced IR Gas Analyzer

    ir gas analysis

  2. Tips of FTIR measurement (Gas analysis)

    ir gas analysis

  3. infrared gas analyser working principle

    ir gas analysis

  4. Analizador de gases OMEGA 5

    ir gas analysis

  5. Analyzing gases by FTIR

    ir gas analysis

  6. How Infra Red(IR) Gas analyzer Work

    ir gas analysis

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COMMENTS

  1. Infrared gas analyzer

    An infrared gas analyzer measures trace gases by determining the absorption of an emitted infrared light source through a certain air sample. Trace gases found in the Earth's atmosphere become excited under specific wavelengths found in the infrared range.

  2. PDF Introduction to Gas Phase FTIR Spectroscopy

    The infrared detector measures the amount of energy at each frequency that has passed through the sample. This results in a spectrum, which is a plot of absorbance intensity versus frequency. Fourier transform infrared spectroscopy is preferred over other gas analyzers for several reasons:

  3. FTIR: A Flexible Tool for Industrial Gas Analysis

    FTIR: A Flexible Tool for Industrial Gas Analysis Instrumentation June 2018 Nenne Nordström, Jim Cornish Fourier transform infrared (FTIR) spectroscopy identifies and quantifies gas and vapor samples. This article outlines how FTIR analyzers work and how they are commonly used in field applications. Figure 1.

  4. Gas Analysis

    Gas Analysis MATRIX-MG and OMEGA 5 Gas Analyzers for the fully automated and high precision real-time monitoring of gas compounds. Contact us Real-Time. Quantitative. Calibration-Free. MATRIX-MG Series: Configurable High Performance

  5. Infrared Spectroscopy

    Infrared (IR) spectroscopy is one of the most common and widely used spectroscopic techniques employed mainly by inorganic and organic chemists due to its usefulness in determining structures of compounds and identifying them. Chemical compounds have different chemical properties due to the presence of different functional groups.

  6. Infrared spectroscopy

    Infrared spectroscopy ( IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms.

  7. How Do FTIR Gas Analyzers Work?

    FTIR gas analyzers efficiently scan the entire infrared spectrum of the entire specimen simultaneously. The interference pattern created by the sensor, known as an "interferogram," encapsulates ...

  8. FTIR

    The Thermo Scientific MAX-iR FTIR Gas Analyzer is designed to meet the challenges of inline process monitoring, batch sampling, gas purity/certification, and more. Combined with Thermo Scientific StarBoost Enhanced Optical Technology, the MAX-iR analyzer enables users to achieve single-digit ppb detection limits for many applications.

  9. Infrared Gas Analyzer IR400

    What is the Infrared Gas Analyzer IR400? Using the non-dispersive infrared method (NDIR), the Infrared Analyzer IR400 measures NO, SO2, CO2, CO, and CH4 concentrations in sample gas. It also measures O2 using an external zirconia sensor or internal paramagnetic sensory. It can simultaneously measure up to 4 or 5 components.

  10. Industrial Gas Analysis with a Disruptive New Technology

    The MAX-iR analyzer's optical enhancement technology, StarBoost, makes it one of the most sensitive FTIR gas analysis systems on the market today. Cavity ring-down spectroscopy (CRDS) is often used for the analysis of impurities in the semiconductor industry, but the main advantage for users of the MAX-iR system is that, unlike CRDS, it can ...

  11. Gas analysis using infrared (IR) detectors

    Infrared (IR) detectors Hamamatsu offers many infrared detector choices for absorption-based gas analyzers, including RoHS-compliant InAsSb photovoltaic detectors. When selecting an appropriate detector, it is important to match the gases' absorption bands to the detector's spectral sensitivity. Other detector characteristics to consider include:

  12. The Different Types of Gas Analysis Techniques

    Infrared (IR) Spectroscopy FTIR, which is based on IR spectroscopy, is used in the analysis of the composition of gases. The detection method includes the introduction of a combination of different light frequencies (IR wavelengths) to gas molecules (sample) and the detector within the instrument measures the amount of light absorbed by the gas.

  13. Non-dispersive infrared multi-gas sensing via nanoantenna ...

    Non-dispersive infrared (NDIR) spectroscopy is one of mid-IR spectroscopic gas sensors that analyzes gases based on their characteristic absorption wavelengths in the mid-IR caused by their ...

  14. 4.2: IR Spectroscopy

    For systematic evaluation, the IR spectrum is commonly divided into some sub-regions. In the region of 4000 - 2000 cm -1, the appearance of absorption bands usually comes from stretching vibrations between hydrogen and other atoms. The O-H and N-H stretching frequencies range from 3700 - 3000 cm -1.

  15. BSCI 1510L Literature and Stats Guide: Infrared gas analysis

    Infrared gas analyzer (IRGA) Rates of photosynthesis in plants can be monitored in several ways, but the most effective is using an infrared gas analyzer (IRGA) . An IRGA takes advantage of the absorbance of CO 2 molecules at a wavelength of 4260 nm caused by the stretching vibrations of the C=O double bonds. Fig. 1 Leaf chamber of a portable IRGA

  16. Basics of Infrared Gas Analyser

    Infrared (IR) absorption (or reflection for solids) is a technique that can be used successfully for continuous chemical analysis of a process. The IR region of the electromagnetic spectrum is generally considered to cover wavelengths from 0.8 to 1000µ m.

  17. PDF Gas Chromatography- GC-IR: Separate & Identify Infrared ...

    Before the advent of FTIR instruments (which are fast), GC-IR measurements were made by depositing the packed column effluent on to a salt window and running in the IR instrument. FTIR instruments allow the real-time analysis of the GC effluent. Not limited to gas effluent - with different interfaces one can perform LC-IR, GPC-IR, TGA-IR, etc.

  18. Gemini FTIR Gas Analysis Cells

    International Crystal Laboratories' Gemini Series short path gas cells and long path gas cells are designed for industrial and research applications for FTIR gas analysis. Five product groupings provide users with choices to configure their appropriate solution including Hanst style fixed path and glass bodied multi-pass cells. Also carrying PTFE Gas Cells, Beta Gas Cells, Stainless Steel Gas ...

  19. Gas analysis using infrared (IR) light sources

    Performing gas analysis by infrared absorption requires a light source to provide energy that will be absorbed by gas molecules. Selecting the proper light source can be a complex process, but Hamamatsu is ready to meet the needs of many applications. From detecting extremely low levels of impurities in clean rooms, to medical capnography, to ...

  20. HORIBA's next-generation infrared gas analysis technology IRLAM

    In 2021, HORIBA succeeded in developing a new and proprietary method of Infrared Laser Absorption Modulation (IRLAM™) and putting it into practical use. IRLAM enables real-time measurement of gases that were difficult to measure with conventional infrared gas analysis technology, due to insufficient sensitivity and interference effects ...

  21. Recent Applications of Evolved Gas Analysis by Infrared Spectroscopy

    The analytical applications of evolved gas analysis (EGA) performed by infrared spectroscopy (IR-EGA) for the period 2010-2012 are reviewed in this article. When the nature of volatile products released by a substance subjected to a controlled temperature program were determined on-line, the results proved a supposed reaction, under either ...

  22. Infrared gas analysis as a method of measuring seagrass ...

    One technique, infrared gas analysis (IRGA), uses a closed gas stream to calculate an accurate carbon budget. Multiple studies have successfully used IRGA with intertidal seagrasses, but it remains unknown how applicable the technology is for underwater plants. Here, we evaluate the potential of IRGA to measure carbon assimilation of subtidal ...

  23. A Guide to Choosing Sensor Technologies for Gas and Flame Detection and

    Broadcom Sensor-Gas-Flame-WP100 August 11, 2022 Executive Summary Mid-IR infrared sensing is widely used today in a broad range of detection and analysis applications, including flame and gas detection, fuel and oil analysis, food safety, and motion and gesture sensing. An innovative approach from both