Infrared spectroscopy of gases is a well-established technique for identifying and quantifying gaseous species in an environment. It is especially useful for detecting environmental leakage of gases from process plants and greenhouse emissions from burning fossil fuels because it is not sensitive to diatomic O2 and N2 – the major constituents of air. But it has many other applications too.
For the purpose of measurement, gases are pumped into a sealed cell which is placed into the IR beam. The cells can be single pass cells, limited to pathlengths of up to around 10 cm or multi pass cells in which the IR beam is reflected several times through the gas to give pathlengths of several meters.
The path length of cell required depends on the concentration of gas which must be detected, since absorbance is proportional to both pathlength and concentration. The lower the expected concentration of the gas species under consideration, the longer the pathlength required to detect and quantify it.
Plasma Treatments
Plasma is the fourth state of matter in which clouds of gas are separated by heating or strong magnetic fields into charged ions and free electrons. In a nonthermal plasma, the electrons and ions are not in thermal equilibrium and have different temperatures.
Plasmas have a wide range of uses in science and industry for surface modification and treatment. This includes antimicrobial treatment of food packaging1, modification of metallic and polymer surfaces2, waste composting3, and abatement of waste process gases such as SF6 in the semiconductor industry4 and VOCs in the chemical industry5.
In many of these processes, the mechanism of action is presumed to be that highly energetic free electrons in the plasma promote dissociation of molecules in the ambient air into reactive species that can combine with the treated material. FTIR can be used to monitor the creation of new surface and gaseous species.
For example, Ahmadi et al (2011) used a microwave plasma torch to remove SF6 under a flow of O2 and Ar gas. A Specac 10 cm Storm gas cell attached to the apparatus was used to sample the products formed at different power settings. They were able to monitor the reduction of SF6 and the corresponding formation of products such as SO2 and CO2.4
Waste Composting
When the extinction coefficient of the analyte gas is significantly lower than for gases such as SF6 (resulting in very low absorbance values), or when it is present at very low concentrations, a longer pathlength cell will be required to achieve the same overall absorbance. Multi-pass cells such as the Atmos™ gas cell can be specified with pathlengths in the several metre range.
Volatile organic compounds (VOCs) are produced by a number of processes both anthropogenic and biogenic, the former of which may account for up to 50% of total emissions. Nonthermal plasmas have the potential for abatement of VOCs produced by waste composting facilities whose function is to turn municipal waste into a useful agricultural commodity [3]. In the study by Martini et al (2019) mixtures of synthetic air and VOCs were passed through a nonthermal plasma dielectric barrier discharge (DBD) set up. An FTIR equipped with a Specac Cyclone 2m gas cell was attached to the outlet to measure CO2, ozone, and humidity of the treated gas, since these gases may be produced by the process and are potentially poisonous to the composting process itself. 3
Chemical Vapor Deposition
Vacuum deposition techniques such as chemical vapor deposition (CVD) are commonly used in microfabrication. They are a class of techniques for growing materials such as silicon, diamond, and metals through deposition onto a substrate. The substrate is heated, then volatile reactants are introduced which react at the surface, leaving a deposited layer of material and volatile products to be removed.6
CVD is widely used to deposit coatings onto other materials, giving them improved wear and strength properties, but also to grow synthetic materials such as diamond7 and magnesium oxide [8]. FTIR spectroscopy has been conducted to investigate both the resultant coatings and the vapour phase precursor and reaction products formed. One example of this was Sartori et al (2011) who used a heated 10 cm Storm gas cell to compare the action of two different precursors8.
References
1. Misra, N. N., Tiwari, B. K., Raghavarao, K. S. M. S. & Cullen, P. J. Nonthermal Plasma Inactivation of Food-Borne Pathogens. Food Eng. Rev. 3, 159–170 (2011).
2. Sawtell, D. A. G., Abd-Allah, Z., Bradley, J. W., West, G. T. & Kelly, P. J. Mechanisms of atmospheric pressure plasma treatment of BOPP. Plasma Process. Polym. 15, 1700051 (2018).
3. Martini, L. M. et al. Non-thermal plasma in waste composting facilities: From a laboratory-scale experiment to a scaled-up economic model. J. Clean. Prod. 230, 230–240 (2019).
4. Ahmadi, Z., Khani, M. R., Kooshki, S., Mirzajani, F. & Shokri, B. Investigation of variation power and additive gas effect on the SF 6 destruction using atmospheric microwave plasma torch. IEEE Trans. Plasma Sci. 39, 1834–1841 (2011).
5. Prantsidou, M. & Whitehead, J. C. The Chemistry of Gaseous Dodecane Degradation in a BaTiO3 Packed-Bed Plasma Reactor. Plasma Chem. Plasma Process. 35, 159–172 (2015).
6. The Welding Institute. What is chemical vapour deposition (CVD)? https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-chemical-vapour-deposition-cvd.
7. Gracio, J. J., Fan, Q. H. & Madaleno, J. C. Diamond growth by chemical vapour deposition. J. Phys. D. Appl. Phys. 43, 374017 (2010).
8. Sartori, A. et al. Stability study of a magnesium β-diketonate as precursor for chemical vapor deposition of MgO. Chem. Mater. 23, 1113–1119 (2011).
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