Two-way, unfolded three-way and three-mode partial least squares calibration of diode array HPLC chromatograms for the quantitation of low-level pharmaceutical impurities

A series of two-way diode array chromatograms were recorded at 0.1±0.5% and 1±5% of 3-hydroxypyridine impurity coeluting with 2-hydroxypyridine, recorded at pH 4.9 (good resolution) and 5.0 (poor resolution). Four methods for PLS calibration, namely, using summed spectral pro®les, summed elution pro®les, unfolded three-way PLS and true three-way PLS were applied to the datasets, both using autopredictions and cross-validation. It was found that it was possible to accurately quantify low levels of impurities. Three-way methods often performed worse than two-way methods using the summed spectral pro®le probably due to irreproducibility of elution times. # 1999 Published by Elsevier Science B.V. All rights reserved.