Laser gas analysis allows for gas detection with high sensitivity and selectivity. The multi-component capability and wide dynamic range of this detection method help analyze a mixture of gases with a broad concentration range. Because this method does not require sample preparation or preconcentration, it is easy to adopt in the laboratory or industry.
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Importance of Gas Analysis in Industry
Gas analysis is crucial for determining the concentrations of known gases in an atmosphere or environment containing a mixture of gases of interest. It is widely used in research, development, and industry. The gases that can be analyzed using gas analytical techniques are broadly classified as dirty and clean.
Environmental pollutants and toxins, including industrial smokestack emissions, diesel engine exhaust, and biomethane from wastewater treatment sites, are categorized as dirty gases. Gas analysis helps detect the concentration of these gases.
Owing to the enhanced research attention towards renewable energy gases, the gas analysis technique is extensively applied to analyze the concentration of hydrogen sulfide or biomethane generated from anaerobic digestion at wastewater and landfill treatment sites.
In contrast, ultra-pure gases emitted from gas supply companies, such as Air Liquide and Linde, can be considered clean. Ultra-pure gases have various applications, a few of which include their use as calibration references for analyzers and feedstocks for industrial processes.
Working Principle of Laser-Based Gas Analysis Systems
Laser-based gas sensors measure the light absorbed by gases when its frequency or wavelength overlaps with the gase’s molecular resonance, governed by Beer-Lambert law. The primary challenge faced by absorption-based gas analysis is that the signal levels are low. Hence, insufficient signal-to-background levels hamper the accurate quantification of gas levels.
To this end, Beer-Lambert law helps deduce a possible method to increase the sensitivity of the optical-based sensing system. There are two possible methods to do this - by increasing the absorption cross-section or by increasing the interaction length. While the first is realized by selecting the spectral region with stronger molecular transitions, the latter (increasing interaction length) is accomplished using multipass cells (MPCs).
Laser-Based Gas Analysis
Although analytical techniques such as mass spectroscopy and conventional optical and gas chromatography served well in detecting environmental and atmospheric trace gases, laser-based gas analysis has been advantageous, especially in industrial applications.
Laser gas analysis is a cost-effective alternative to traditional analytical techniques thanks to recent diode and fiber laser technology. Laser-based gas analysis is employed in air and water quality monitoring, cancer detection, atmospheric chemistry, industrial, traffic and rural emissions, explosives detection, medical applications, national security, vegetation remote sensing, and artwork characterization.
Laser-based gas spectroscopy is a robust tool in trace gas detection, applied in the near- and mid-infrared spectral regions, offers a high selectivity and sensitivity with short acquisition time. Additionally, infrared region-based optical systems integrated with thermo-electrically cooled (TEC) detectors and other semiconductor sources, such as interband cascade lasers, laser diodes, or quantum cascade lasers, enable on-site operations with minimal maintenance.
The commonly used laser- and their-based techniques to detect trace gases include the following:
Semiconductor Lasers: This type of laser is small and highly reliable. However, their industrial applications are hindered by the lack of high-quality, high-power diodes for specific wavelengths. Different wavelengths utilize different semiconductor materials.
For instance, lead salt is used for the 3–30 µm spectral area, antimonides for wavelengths longer beyond 1.8 µm, and gallium arsenide and indium phosphide are used for visible to near-infrared wavelengths. Although lead-salt lasers are effective for trace gas detection in air, they are not suitable for regular industrial applications because they require cryogenic cooling.
Diode Laser Spectroscopy: It uses Beer's law to determine gas concentrations in an absorption spectrometer setup, consisting of a radiation source, detector, and a closed absorption cell. Diode laser spectroscopy is an attractive method to detect trace gases due to the simple instrumentation and allows for electronic implementation of required modulation.
Tunable Diode Laser Absorption Spectroscopy (TDLAS) with long-path absorption cells achieves high-sensitivity local measurements. It’s particularly effective in monitoring most atmospheric trace species that have resolvable infrared line spectra at low pressure.
Diode laser integrated wavelength modulation spectroscopy (WMS) produces signals based on species concentration and reduces laser noise. This sophisticated method entails modulation of the laser wavelength and signal is detected using a computer-controlled signal-averager.
While near-infrared diode lasers operating at ambient temperature target weaker overtone and combination bands, mid-infrared lasers operating at cryogenic temperatures cover fundamental absorption bands for ultrasensitive gas investigations.
Recent Studies
A study published in Optics and Lasers in Engineering proposed a high-sensitivity dynamic analysis technology to achieve real-time online monitoring of dissolved trace gases in transformer oil. The technology used here was based on a low-noise differential photoacoustic cell (DPAC).
The characteristic gas dissolved in the oil was separated using headspace degassing and pumped into the DPAC. The emitted laser light was amplified using an erbium-doped fiber amplifier (EDFA) and reflected in the DPAC (equipped with two expansion mufflers) to form a double-pass excitation enhancement.
The findings showed that both the muffler and the differential detection method reduced the noise during headspace degassing by more than 80%. The detection limit of the system for acetylene dissolved in the transformer oil was determined to be 0.1 μL/L. This study offered a technical solution for dissolved gas analysis and has the advantages of high detection accuracy and quick response time.
Another article published in Sensors and Actuators B: Chemical reported the simultaneous and in situ detection of ammonia and nitric oxide, suitable for flue gas monitoring at selective catalytic reduction (SCR) exhaust, using a mid-infrared fiber-coupled laser absorption sensor.
Two quantum cascade lasers (QCLs) detected the optimal absorption lines of ammonia and nitric oxide at 1103.45 cm−1 and 1929.03 cm−1, respectively. The two QCLs were coupled to a single hollow-core fiber and delivered to an open-path, single-ended optical probe for in situ gas detection.
A series of experiments were conducted to evaluate the sensor performance at different temperatures (296-573 K) and the constant pressure of 1 atm. The minimum detection limits at the average time for nitric oxide and ammonia were 30 ppb at 100 s and 14 ppb at 70 s for nitric oxide and ammonia, respectively. Additionally, the uncertainty in ammonia and nitric oxide detection was less than 5 %.
Conclusion
Overall, laser-based techniques are of great significance for ultra-precise gas measurements. By leveraging semiconductor lasers, these methods enable highly sensitive detection of trace gases through absorption spectroscopy.
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References and Further Reading
Nikodem, M. (2020). Laser-based trace gas detection inside hollow-core fibers: A review. Materials, 13(18), 3983. https://doi.org/10.3390/ma13183983
Laser-Based Spectroscopy and the Analysis of Gas. Accessed on 12 December 2023
Werle, P., Slemr, F., Maurer, K., Kormann, R., Mücke, R., Jänker, B. (2002). Near-and mid-infrared laser-optical sensors for gas analysis. Optics and lasers in engineering, 37(2-3), 101-114. https://doi.org/10.1016/S0143-8166(01)00092-6
Li, C et al. (2023). High-sensitivity dynamic analysis of dissolved gas in oil based on differential photoacoustic cells. Optics and Lasers in Engineering, 161, 107394. https://doi.org/10.1016/j.optlaseng.2022.107394
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