Identity Of Peaks

In most toxicology laboratories, final identification of a peak will not be made from observed retention or migration behavior. But the decision whether or not to pursue identification will be made solely on the basis of this behavior in either the GC or CE analytical systems. Thus, it is imperative that retention or migration behavior be as reproducible as possible. The more precise the retention behavior, the narrower the window that can be used to decide whether a given peak is a potential positive result. When using a simple UV detector, interpretation of electropherograms and tentative identification of peaks is limited to migration behavior. Use of the DAD adds UV spectral data to migration data and greatly improves the discriminating power of the screen. These two aspects of peak identity are discussed later.

5.1. Migration Behavior

It has been often noted that raw migration time is not highly reproducible. Yang et al. (34), note that this is especially true of micellar systems because the electroosmotic flow (EOF) is a powerful influence on migration in such systems. EOF, in turn, is affected by variables, such as the condition of the interior surface of the capillary, that are difficult to control. Even at pH 2.38 when EOF is reduced almost to zero, raw migration times are less reproducible than is desirable. Migration times have been found to drift with time,

Fig. 2. Comparison results of unpreserved whole human blood stored at room temperature for 56 d by CZE-GC/NPD.

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