Matrix Isolation Spectroscopy in Atmospheric Chemistry

Abstract

Abstract There is a growing concern about our environment and it has been realized that quick and definite measures are required if we are to preserve our planet for posterity. The rising level of greenhouse gases in the atmosphere and its consequent effect on the global climatic conditions, the depletion of stratospheric ozone, acid rain, and photochemical smog are some of the issues that have been addressed by scientists and policy makers the world over. Solutions to most environmental problems can be obtained only through collective efforts and, to ensure that such efforts are effective, it is necessary that policies and legislation are made based on scientific data on our ever‐changing environment. The data must provide information on the nature of trace constituents present in the different layers of the atmosphere, their concentrations, and their chemistry. A variety of experimental techniques have been used for this purpose, such as fluorescence spectroscopy, infrared (IR) and ultraviolet/visible (UV/VIS) absorption, Raman scattering, and photoacoustic spectroscopy. All these techniques exhibit some combination of the following features: multielement detection, low detection limits (DLs), specificity for an unequivocal identification of species, accuracy, and precision. One of the experimental techniques that has found important applications in the study of the atmosphere is matrix isolation (MI) spectroscopy. In this technique, the molecules of interest are diluted in a large excess of an inert gas and sprayed onto a cold substrate held at 10 K. Under these conditions, the molecules are trapped, isolated from each other and only surrounded by inert gas atoms. Such cold, isolated molecules yield spectra that have narrow spectral widths. Furthermore, if the trapped species are reactive molecules or radicals, they are likely to have extended lifetimes in the inert cage, thus allowing their chemistry and spectroscopy to be studied at leisure. These features of MI have made it particularly useful in the study of atmospheric chemistry. The high‐resolution capability enables one to use this technique as an analytical tool, as closely lying spectral features of different molecules can now be resolved – a feature that has been employed to identify and estimate trace constituents in the upper tropospheric and stratospheric samples. In conjunction with gas chromatography (GC), MI and Fourier transform infrared (FTIR), referred to as GC/MI/FTIR, offers a powerful tool with which to resolve isomeric forms of environmentally hazardous chemicals such as polycyclic aromatic hydrocarbons (PAHs), dioxins, and polychlorinated biphenyls (PCBs). Such isomeric resolution is essential, as it is well known that only certain isomeric forms are biologically active, whereas the others are not. In this respect, GC/MI/FTIR even scores over GC mass spectroscopy (MS). Analytical spectroscopy of matrix‐isolated species is also done using fluorescence spectroscopy, where again the advantage of high resolution enables one to resolve isomeric forms of compounds. Where the species of interest is a free radical, electron spin resonance (ESR) is the technique of choice to study the trapped species. This is a particularly powerful tool, as free radicals are known to play an important role in a number of atmospheric processes. Another aspect of MI spectroscopy is its ability to study reaction intermediates, a feature that has been employed to study the reaction mechanisms of atmospherically important chemical and photochemical reactions. All these aspects of the technique are discussed in this article, using examples.