EFFECT OF SPECTRAL RESOLUTION, DETECTOR LINEARITY AND CHROMATOGRAPHICRESOLUTION ON PEAK PURITY CALCULATIONS

The detection of inhomogeneity in chromatographic peaks is one of the principal benefits of photodiode array detection for HPLC. The peak ho mogeneity can be estimated by comparing the angles between the vector representations of the instantaneous spectrum and the peak apex spectr um, and the instantaneous spectrum and the spectral noise. When the sp ectral or purity angle exceeds the noise angle, the peak is not homoge neous because the differences between the instantaneous spectra and th e apex spectrum cannot be explained by statistical variation of the ap ex spectrum. In this paper, we consider the impact of detector lineari ty and slit width on the purity (homogeneity) measurements of a separa tion of a series of compounds related to vanillin. The peak purity ang le, i.e., the spectral contrast of spectra within a chromatographic pe ak, varies with the maximum absorbance. At small maximum absorbances ( less than 0.1 AU), the variation in purity angle is dominated by syste m noise. At large maximum absorbances (greater than 0.5 AU), the varia tion in purity angle is dominated by the photometric uncertainty. The smallest measurable purity angle for a homogeneous peak is inversely p roportional to the slit width and spectral bandpass of the polychromet er of the photodiode array detector.