Editorial Feature

An Overview of Laser Gas Analysis Technologies

Many gases are colorless and odorless, which means that leakage can readily go undiscovered. It is essential to utilize detectors that can calculate the concentrations of recognized gaseous species in a target atmosphere in many sectors that handle or manufacture gaseous species. This article discusses the properties and importance of laser-based gas analysis technologies.

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Importance of Gas Detection

Examples of typical gas analysis include checking oxygen levels to prevent asphyxiation risks or looking for dangerous gases like carbon monoxide.

Many businesses, including the automotive industry, use gas analysis processes to determine the composition of exhaust gases. This could be done to determine whether regulations are being followed or to assess the capabilities and characteristics of various fuel kinds.

Gases are often difficult to examine because of their relatively low number densities. The detection sensitivity of the analyzer must be high enough to identify concentrations in parts per trillion for various gas analysis applications. The process, known as trace gas analysis, is essential for both the detection of hazardous gases and studies of the composition of the atmosphere, such as determining how specific environmental factors affect the rates at which particular chemical species arise.

Spectroscopic Gas Analysis

Gas analysis can be performed in several ways, including mass spectrometry technologies. One of the most used methods is laser spectroscopy, particularly for the identification of trace gases. To detect and identify various gas species, laser-based spectroscopic techniques use a laser as a light source.

Many absorption-based spectroscopic techniques employ the Beer-Lambert law to determine concentrations and rely on established wavelength-dependent photo-absorption cross-sections. In these tests, the concentration is determined by measuring how much incident laser light the gaseous sample absorbs over a predetermined path length.

However, the fact that the signal levels are low makes it challenging to achieve sufficient signal-to-background levels to precisely quantify gas levels. This is one of the difficulties faced in the absorption techniques for the analysis of gas. As gases typically have relatively low number densities, the number of atoms or molecules per unit volume, even a strongly absorbed wavelength, will frequently only result in a minor reduction in the intensity of incident light.

Increased signal levels for gaseous samples have been a goal of several experimental strategies. One of these is the advancement of cavity ring-down-based techniques, in which the path length of the gaseous sample is significantly extended by building an optical cavity to house the sample.

Lasers in Gas Analysis

Gas analysis may now be conducted on trace gas levels with fast acquisition periods thanks to advancements in light source and detector technology, enabling real-time analysis and monitoring. It has become more possible to apply gas analysis to a wider variety of molecular species with spectroscopic transitions that need the utilization of various wavelengths thanks to tunable diode lasers with lock-in amplifiers.

Designing gas analyzers that can function in challenging environments, such as those found in mines or refineries, where they may be exposed to corrosive or extremely fluctuating circumstances, presents a number of issues.

For measuring gases in more complicated situations, more portable equipment must be developed. This covers blood analysis and other medical applications, as well as dissolved gas analysis to determine the gas composition of liquid samples for environmental studies.

Gas analysis has been profoundly impacted by the introduction of laser sources in a variety of wavelength ranges that are capable of producing high light intensities. There is significantly higher sample absorption in many gaseous species because they have acute resonances and spectroscopic transitions with short line widths.

Due to the availability of tunable wavelength lasers, it is possible to use molecular resonances to increase the sample's absorption in gas analysis tests, hence increasing the signal-to-noise ratio. Particularly for infrared vibrational transitions, the energy positions of these resonances can provide a fingerprint for identifying the molecular species.

Many gas analyzers and sensors make use of molecular resonances to enable selectivity in gas detection when using optical absorption methods and techniques such as photoacoustic spectroscopy, another potent laser-based spectroscopy method for gas analysis.

Laser Absorption Spectroscopy

The laser absorption spectroscopy technique involves measuring how much energy distinct gas molecules absorb from a specific light spectrum to determine the spectrum of the absorption of gas. These absorption spectra provide highly accurate identification of unidentified gases.

The tunable diode laser spectrometer (TDLS), which measures the very low concentration of gases like methane, ammonia, carbon dioxide, and water vapor, uses laser absorption spectroscopy. The signal strength of the emission wavelength is determined by this device using a photodiode.

The target gas' concentration is determined by comparing the observed wavelength to that of the target gas' molecule. It is crucial to choose an appropriate absorption line for the substance being studied to get the desired results. This approach is particularly sensitive, accurate, and specific because of this measure. The diagnostics of combustion use this method.

Literature Studies

In a recent report published by the journal Sensors (Basel), the authors discussed the creation of a laser gas analyzer that could monitor gas concentrations at a 100 Hz data rate. Eddy covariance calculations for gas fluxes in turbulent high wind-speed environments were aided by the quick data rate. The laser gas analyzer was based on derivative laser absorption spectroscopy and was configured to measure carbon dioxide (CO2) and water vapor (H2O, at wavelengths of 1392 and 2004 nm, respectively).

Experimental testing of this device in both aquatic and terrestrial settings was conducted in conjunction with an ultrasonic anemometer. Initially, the laser gas analyzer's accuracy was contrasted with that of a premium commercial device with a maximum data rate of 20 Hz. To confirm the involvement of high-frequency components, the correlation of H2O flux results was subsequently evaluated and compared at data rates of 100 Hz and 20 Hz in both high and low wind speeds.

In a setting with a wind speed of less than 10 m/s, the measurement results demonstrated that the contribution of a 100 Hz data rate to flux estimates was approximately 11% greater than that recorded with a 20 Hz data rate. As a result, it demonstrated that a laser gas analyzer with a high detection frequency is better suited for measurements in windy conditions.

In another study published in the journal Measurement Science and Technology, researchers examined the R(12) line of CO2 in the combination band using direct absorption spectroscopy and a spectrometer made up of a single-pass and a multipass white cell.

The traceable infrared laser spectrometric amount fraction measurement (TILSAM) method was used to measure the concentration of CO2 in gravimetric gas standards that ranged from 300 to 60,000 µmol mol-1 (0.03% to 6%) in N2. The gravimetric reference values and the spectrometric data were compared.

The authors applied infrared laser-spectrometric gas analysis to the expression of uncertainty in measurements. To illustrate the caliber of the findings and software-assisted uncertainty assessment, uncertainty budgets were presented. The relative standard uncertainties of the spectrometrically measured CO2 amount fractions were 1.4% and 0.7%, respectively, at ambient levels of 360 µmol mol-1 and exhaled breath gas levels of 50 µmol mol-1. The limit of detection was 2.2 µmol mol-1.

Individual results had repeatability in the ±1% range. Also, at a distance of 4987.31 cm-1, the collisional broadening coefficients of the CO2 R(12) line were determined. The measured self-, nitrogen-, oxygen-, and air-broadening coefficients had relative standard uncertainties in the range of ±1.7%.

More from AZoOptics: Trace Gas Analysis with Non-Contact Quartz-Enhanced Photoacoustic Spectroscopy

References and Further Reading

Nwaboh, J. A., et al. (2012). Laser-spectrometric gas analysis: CO2–TDLAS at 2 µm. Measurement Science and Technology, 24(1), 015202. 10.1088/0957-0233/24/1/015202

Li, M., et al. (2021). Development of a Laser Gas Analyzer for Fast CO2 and H2O Flux Measurements Utilizing Derivative Absorption Spectroscopy at a 100 Hz Data Rate. Sensors (Basel), 21(10), 3392. https://doi.org/10.3390%2Fs21103392

Nwaboh, J. A., et al. (2011). Molecular Laser Spectroscopy as a Tool for Gas Analysis Applications. Internation Journal of Spectroscopy, 2011, 568913. https://doi.org/10.1155/2011/568913

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Surbhi Jain

Written by

Surbhi Jain

Surbhi Jain is a freelance Technical writer based in Delhi, India. She holds a Ph.D. in Physics from the University of Delhi and has participated in several scientific, cultural, and sports events. Her academic background is in Material Science research with a specialization in the development of optical devices and sensors. She has extensive experience in content writing, editing, experimental data analysis, and project management and has published 7 research papers in Scopus-indexed journals and filed 2 Indian patents based on her research work. She is passionate about reading, writing, research, and technology, and enjoys cooking, acting, gardening, and sports.

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