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Most gases are extremely difficult to detect because they are odorless. Certain gases are extremely harmful to humans if inhaled. Owing to the above, researchers have developed many potent techniques to analyze different types of gases present in the environment or the ones that are released from industrial processes.
In the past, while digging mines, the presence of harmful gases such as carbon monoxide, methane, and carbon dioxide was determined to ensure miners' safety. Researchers traced back the earliest detecting methods of harmful gases which involved using birds such as canaries. The birds were introduced inside mines owing to their sensitivity to toxic gases. They would stop singing in the presence of such gases. This would work as a signal for the miners to evacuate. At present, there are various effective optical, laser, and spectroscopic methods used to analyze gases.
Mass Spectrometry (MS), Fourier Transform Infrared Technology (FTIR) and Gas chromatography (GC) are the three most widely used techniques for Evolved Gas Analysis (EGA). Recently, the thermal analysis technique for EGA has been coupled with thermogravimetry (TG). As an example, TG-MS or TG-FTIR are used to analyze the emitted gases. The distribution of the emitted gas depends on its flow profile, the volume ratio, and the diffusion coefficients of its molecules. The following are various techniques used to analyze gases.
Laser Absorption Spectroscopy
This is an operating principle based on which various technologies for the analysis of gases have been developed. In this method, different gas molecules absorb a particular light spectrum, and the amount of energy a gas absorbs indicates a characteristic absorption spectrum.
These absorption spectra help identify unknown gases with great accuracy. Laser absorption spectroscopy is applied in Tunable Diode Laser Spectrometer (TDLS), which is used to measure the very low concentration of gases, for example, methane, ammonia, carbon dioxide, and water vapor. This instrument uses a photodiode to measure the emission wavelength’s signal intensity.
The observed wavelength is compared with the wavelength of the targeted gas molecule and thereby, the concentration of the target gas is quantified. To attain a proper result, it is important to select a suitable absorption line for the compound under study. Such a measure makes this technique extremely sensitive, accurate, and specific. This technique is used in combustion diagnostics.
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.
The raw data is processed using a computer and the result is subsequently converted using a Fourier Transform algorithm.
More than 20 different gases can be simultaneously detected using IR radiation. It helps measure gases such as carbon dioxide and unknown organic compounds. It is also used in the development of an ammonia sensor, which is a highly sensitive device used to analyze the levels of atmospheric ammonia. Furthermore, the accurate measurement of cyclical nucleotide-mediated photosynthetic responses to plant hormones can be carried out.
Mass Spectrometry (MS)
Pyrolysis Mass Spectrometry (PYMS) is a technique that is used in EGA. This technique is analogous to Thermal Gravimetric Analysis, however, PYMS does not use a column, unlike other standard techniques. The samples are gradually heated from a low temperature to a higher temperature (50-800 °C) following which they release gases or vapors through desorption or decomposition. These gases bear unique thermal effects and undergo mass losses. These changes are monitored closely. The technique is widely used to identify different polymers.
Environmental Trace Gas Analysis involves the detection of gaseous compounds present in the environment, mostly organic, at a low concentration (parts per million (mol) or lower).
MS is mostly used to detect trace gas owing to its high sensitivity and selectivity. The efficiency of MS has been enhanced by incorporating special inlet techniques or ionization processes. For example, high-pressure ionization of the sample followed by detection of negative ions could bring about an enhancement in the detection range by parts per trillion (mol) of sulfur hexafluoride (a strongly electronegative compound).
Off-Gas Analysis
In the case of evaluation of gases evolved during biological processes, Tandem Gas Analysis is performed. This technique is used when analyzing steady-state cultures. For example, the amount of oxygen used and carbon dioxide released by a microbial culture is analyzed using this method.
Gas Chromatography
In 1906, the concept of "chromatography" was first used by a Russian botanist, Michael Twsett. GC method is widely used in propulsion research. This type of chromatography is used in analytical chemistry, particularly for separating and characterizing compounds that can be vaporized without decomposition.
This technique determines the purity of a substance and separates components from a mixture. GC is a rapid and accurate method of analysis of gaseous combustion products. The most common detectors found in simple gas chromatography are as follows:
- Thermal conductivity detectors
- Gas-density detectors
- Ionization detectors
- Infrared radiometers
The carrier gases used in this technique are helium (very high thermal conductivity) and argon (very low thermal conductivity). Flame ionization detectors are particularly used to detect organic compounds; however, it does not determine the presence of water vapor.
References and Further Reading
Mowry, D.C. et al. (2019). Gas Analysis by Mass Spectrometry. Materials Characterization. AMS International,10. DOI: https://doi.org/10.31399/asm.hb.v10.9781627082136
Rahman, F.A. et al. (2018). A review of methods for measuring the gas emission for combustion analysis in industrial sector. AIP Conference Proceedings 2030, 020291 https://doi.org/10.1063/1.5066932
Valentinem, A. et al. (2013). Infrared gas analysis technique for the study of the regulation of photosynthetic responses. Methods Molecular Biology, 1016, pp. 261-269. https://doi.org/10.1007/978-1-62703-441-8_19
James, J. A. (1964), techniques of gas analysis in metals, Metallurgical Reviews, 9:1, pp. 93-120, DOI: 10.1179/mtlr.1964.9.1.93
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