HYDROCARBONS, C1 THROUGH C4
From methane through the butanes and butenes there are some 20 significant molecules that need to be measured. Members of this group of molecules are encountered in measurements of the environment, in the monitoring of chemical processes, in combustion studies, as indoor air pollutants and in many other situations. The identification and quantification of these compounds is facilitated by the great detail in their infrared bands. Most prominent are the C-H stretch bands near 3000 cm-1, but in addition, most of these compounds have strong highly structured bands in the lower frequency regions of the spectrum.
Acetylene. Acetylene is a product of incomplete combustion and it is only slowly oxidized in the atmosphere. It is therefore seen in urban smog and it is found at sub-PPB levels in the clean troposphere. The acetylene is most readily measured by its extremely strong spectral feature near 730 cm-1. It helps in seeing this band if the CO2 interference is first subtracted.
Ethane. Ethane is present in natural gas and in the atmosphere. In doing a hydrocarbon analysis on an air sample, it is necessary to take the ethane into account. This is easy to do in the infrared spectrum because of the extensive detail in the ethane C-H band.
Methane. Methane is a constituent of clean air at about 1.7 parts-per-million. The compound has a basic role in the atmospheric chemistry. Methane is the most prominent hydrocarbon seen in atmospheric infrared spectra. In a detailed analysis of hydrocarbons in air, methane is the first to be measured and subtracted.
Ethylene. Ethylene is encountered as a basic compound in industrial processes, as an effluent from petroleum facilities, and as a constituent of engine exhausts. Like all the olefins, ethylene shows a strong characteristic absorption band in the readily observed region near 1000 cm-1. Its strong spectral feature near 950 cm-1 makes identification and quantification easy.
Propylene and Isobutylene. These two alkenes occur in engine exhausts and are highly reactive in the atmosphere, contributing to photochemical smog. Each can readily be measured by its strong spectral features in the low frequency region.
Propane, Butane and Isobutane. These three molecules can be distinguished from each other by the details in their C-H bands.
Because of the narrowness of spectral features for these light hydrocarbons, a spectrometer of limited resolution will cause the spectra to exhibit absorbance non-linearities (also called Beer's law failure). Quantitative measurements should be made using only weak spectral features with maximum absorbance no higher than 0.1 A.U. See SPECTRA, chapter A.