Spectral Searching and Spectral Subtraction

With the availability of computerized data acquisition and storage it is possible to build digital libraries of standard reference spectra. The identity of an a unknown compound can often be determined by comparing its spectrum against a library of reference spectra, a process is known as spectral searching. Comparisons are made using an algorithm that calculates the cumulative difference between the sample’s spectrum and a reference spectrum.

Another advantage of computerized data acquisition is the ability to subtract one spectrum from another. When coupled with spectral searching it may be possible, by repeatedly searching and subtracting reference spectra, to determine the identity of several components in a sample without the need of a prior separation step. An example is shown below in which the composition of a two-component mixture is determined by successive searching and subtraction: (a) the mixture’s spectrum ; (b) a search of the spectral library selects cocaine•HCl as a likely component of the mixture; (c) the resulting spectrum after subtracting the reference spectrum for cocaine•HCl; (d) a search yields mannitol as the mixture’s likely second component; (e) subtracting the reference spectrum for mannitol leaves only a small residual signal.


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