Page 278 - Tandem Techniques

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solute. In contrast, the spectra for peak (a) all differ extensively, and thus confirms that the peak is not
pure, and is a mixture of at least two unresolved substances.

Figure 7.6
Diode Array Spectra Demonstrating Peak Purity
Courtesy of the Perkin Elmer Corporation

Monitoring the column eluent, by selecting a specific wavelength from which to reconstruct the
chromatogram, is similar in principle to single ion monitoring in mass spectrometry, but not nearly so
discriminating. Whereas single ion monitoring can be used to detect a specific molecular ion or
molecular fragment only, single wavelength monitoring can only broadly enhance the sensitivity of the
system to certain chemical types. Nevertheless, this feature can be extremely useful, particularly in
environmental analysis where maximum sensitivity is often essential. However, the wavelength selected
for high sensitivity detection may also restrict the choice of solvents that can be used, and thus directly
effects the nature of the chromatographic process. For example, in Figure 7.7 it is seen that light of
relatively short wavelengths (210 and 220 nm) is used for detection, because both the alkaloids and the
cardiovascular drugs absorb well at these wavelengths. In order to achieve the separation so