Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods587rules have been developed to aid in correlating UV/Vis absorption bandsto chemical structure. Similar correlations have been developed for infraredabsorption bands. For example a carbonyl’s C=O stretch is sensitive toadjacent functional groups, occurring at 1650 cm –1 for acids, 1700 cm –1for ketones, and 1800 cm –1 for acid chlorides. The interpretation of UV/Vis and IR spectra receives adequate coverage elsewhere in the chemistrycurriculum, notably in organic chemistry, and is not considered further inthis text.With the availability of computerized data acquisition and storage it ispossible to build digital libraries of standard reference spectra. The identityof an a unknown compound can often be determined by comparing itsspectrum against a library of reference spectra, a process is known as spectralsearching. Comparisons are made using an algorithm that calculatesthe cumulative difference between the sample’s spectrum and a referencespectrum. For example, one simple algorithm uses the following equationnD= ∑ ( A ) −( A )sample i reference ii=1where D is the cumulative difference, A sample is the sample’s absorbance atwavelength or wavenumber i, A reference is the absorbance of the referencecompound at the same wavelength or wavenumber, and n is the number ofdigitized points in the spectra. The cumulative difference is calculated foreach reference spectrum. The reference compound with the smallest valueof D provides the closest match to the unknown compound. The accuracyof spectral searching is limited by the number and type of compoundsincluded in the library, and by the effect of the sample’s matrix on thespectrum.Another advantage of computerized data acquisition is the ability tosubtract one spectrum from another. When coupled with spectral searchingit may be possible, by repeatedly searching and subtracting reference spectra,to determine the identity of several components in a sample withoutthe need of a prior separation step. An example is shown in Figure 10.37in which the composition of a two-component mixture is determined bysuccessive searching and subtraction. Figure 10.37a shows the spectrumof the mixture. A search of the spectral library selects cocaine . HCl (Figure10.37b) as a likely component of the mixture. Subtracting the referencespectrum for cocaine . HCl from the mixture’s spectrum leaves a result(Figure 10.37c) that closely matches mannitol’s reference spectrum (Figure10.37d). Subtracting the reference spectrum for leaves only a small residualsignal (Figure 10.37e).10C.4 Characterization ApplicationsMolecular absorption, particularly in the UV/Vis range, has been used fora variety of different characterization studies, including determining the