A considerable amount of information has appeared in the literature in the past few years regarding the interesting idea of running differential spectra as a means of detecting small amounts of impurities in a given substance. This technique has been quite well described in a number of specific analytical determinations for small amounts of impurities. As yet, however, no plan has been advanced whereby one could make an orderly qualitative differential scan for impurities from 2–15 microns.
W. H. Washburn, "A Specific Procedure for Running Differential Surveys by Infrared," Appl. Spectrosc. 11, 46-46 (1957)