Rapid-scannng multi-channel high-performance liquid chromatographic detection of zemildine and metabolites with three-dimensional graphics and contour plotting

The linear photodiode array detector in high-performance liquid chromatography generates a three-dimensional data matrix, which is conventionally presented as a psuedo-isometric plot. A new graphical technique in this context is to present the data as a two-dimensional contour diagram, where isoabsorptive contours are plotted as a function of wavelength and time. The relative merits and demerits of these complementary approaches are discussed with respect to a study on the antidepressant drug zimeldine and its principal metabolites. Several digital methods developed to access the three-dimensional data set are examined, particularly with regard to tests for peak homogeneity. Although spectral slices at wavelengths indicated in the contour plot, and the absorbance-ratio method, are limited in their sensitivity to non-homogeneity, spectral suppression and the second-derivative transformation of the elution profile are shown to be generally applicable to this problem. The total absorbance chromatogram is advocated as a new technique for presenting a rapid, general survey of spectral information within a specified range (e.g., 200-400 nm) as a function of elution time, analogous with total ion current chromatograms in gas chromatography-mass spectrometry. The relative sensitivity of the linear photodiode array detector is shown to be critically dependent on the bandwidth employed for detection. Comparison with a regular single-channel detector indicates that the multi-channel detector is at least six times more sensitive for zemeldine and two metabolites, norzimeldine and zemeldine N-oxide, under identical chromatographic conditions.