DETECTING COELUTED IMPURITIES BY SPECTRAL COMPARISON

Impurities can go undetected during chromatographic analysis if the re solution is too low for single-wavelength detectors to reveal shoulder s and valleys. Complete spectral data, obtained by using a photodiode- array detector, provides a greater level of information about seemingl y homogeneous peaks. Coeluted compounds in these peaks can be detected by comparing the apex spectrum mathematically with the spectra at eac h time point across a peak. These spectral comparisons take into accou nt changes in spectral shape caused by noise and other nonideal effect s inherent in the measurement process, thereby identifying only real s pectral changes caused by the presence of more than one compound. This article presents specific examples showing the sensitivity and the li mits of coelution detection.