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Our body comes with several, in-built gas analyzers. Our noses are adept at picking up the distinctive rotten-egg aroma of trace amounts of hydrogen sulfide. Our eyes rapidly begin to water in the presence of gaseous chlorine. However, given many gases are hazardous to human health, outsourcing gas detection and analysis work to external devices is a well-advised move for ensuring workplace health and safety. Dedicated gas sensors can also be used not just for chemical identification of gases, but for quantitative analysis and continual online monitoring with much greater reliability.
At the heart of many modern gas monitors are optical laser technologies. In such devices, a laser beam is passed through the gas sample of interest, onto a detector, or sensor, that converts the incoming laser light into electrical signals. Laser-based sensing technologies have become widely adopted for gas detection and analysis due to their quick response times, high sensitivity and reliability.
Laser sensors work by monitoring the changes between the incident laser beam and the light that is ultimately detected by the sensor. One approach to doing this is to compare the laser beam that passes through the sample to a reference beam that is not passed through any gas. These changes are caused by the absorption of the light by the gas sample. Each gas has a unique absorption profile, which means it will absorb different wavelengths of light by different amounts, which provides the chemical fingerprint that means that laser sensors can be used for chemical identification.
While theoretically laser sensors can be designed for any region of the electromagnetic spectrum, many gas analysis devices operate in the infra-red. This is because many small gaseous species, like methane, other hydrocarbons, and carbon dioxide, absorb infra-red light very strongly, so it is easy to design devices with a sensitivity that extends to parts per billion. The other advantage is that the absorption profile of these gases in the infra-red is characterized by many different spectral lines. This means there are many features in the spectra that can be used to identify chemical species with greater accuracy.
Extensive, unique chemical fingerprints mean that laser sensors can be used to identify not just single gases, but which components are present in even very complex mixtures. As the amount of absorption at a given wavelength is proportional to the concentration of the gaseous species, laser sensors can retrieve quantitative information, identifying not just which of several chemical species are present, but also their relative ratios.
The wealth of information that can be provided with laser sensors makes gas analysis a powerful tool in industrial processing and even wine-making. Gaseous carbon dioxide is produced as part of the fermentation process and the amount of carbon dioxide produced is an indicator of how rapidly fermentation is proceeding. Given the influence fermentation has on the final flavor profile and alcohol content of the wine, the information from online gas analysis can be used to control other factors, such as temperature, to ensure the fermentation rate occurs to the winemaker’s preference.
As not all gas samples absorb infra-red light, laser sensors are typically designed for the detection of specific gases. This also allows for optimization of the device for the best sensitivity and performance. However, diode lasers, that have proved a popular choice for gas analysis devices, are available in a range of wavelengths across the electromagnetic spectrum, so there are options for detecting molecules that only absorb either in the ultraviolet or visible. Another attractive feature of diode light sources is they have sufficiently low power draw and are small enough they can be built into portable devices. This is ideal for monitoring changes in gas composition of the atmosphere as part of climate studies in remote places only accessible to unmanned aerial vehicles.
Laser sensors are a powerful tool for gas analysis, making them widely used in the workplace and home safety, as well as the for the optimization and control of industrial processes.