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SPECTRAL SUBTRACTION


spectralsubtraction.jpg

The acronym RIAS has two syllables that designate a two-step process: RI for region integration and AS for and subtraction. Before the computer was applied to spectrum processing, the region integration could usually be carried out by hand and eye, but the spectral subtraction was quite impractical. Now, however, the spectral subtraction is easily done by the computer and both steps of RIAS are done automatically. The ability to analyze complex samples has been greatly enlarged. Consider, for example, the analysis of polluted air.


The spectrum of the air has all of the individual spectra piled on top of each other. The main spectrum is that of the water vapor, which exists in air in the range of 10,000 to 20,000 PPM. Next is the spectrum of carbon dioxide, at some 400 PPM. Then there is methane near 2 PPM. Carbon monoxide is usually near 0.5 PPM and nitrous oxide is near 0.33 PPM. The sample will also contain many other trace gases at concentrations in the PPM to PPB range.


In RIAS, the computer will measure a designated compound, save the number, and then subtract the entire spectrum of the component from the sample spectrum. The computer will then repeat the process for the next designated compound and then proceed one compound at a time through a list of molecules. The sample spectrum is “peeled down” one compound at a time. If the overlapping of the spectra causes some error, the result will be over-subtraction or under-subtraction. At the end of the peeling process, the residual spectrum will then show the over-subtracted and under-subtracted spectra, either positive or negative. A second run-through of the sequence will then make corrections and clean up the residuals. The computer will report out the sum of the two sets of measurements.

As an example, we show in the figure the peeling process for four compounds in air: water, methane, propane and hexane.


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