Impurities in bulk substances are estimated using HPLC techniques by comparing the chromatograms from a pair of sample solutions. High-low chromatography is described as an HPLC sampling procedure used to improve the detection limits of trace components in bulk substances. Chemical amplification is achieved by comparing the chromatograms from a stock sample solution (high) and an appropriate dilution (low). Three procedures that implement the high-low approach are evaluated, with the advantages and disadvantages of each discussed. Several examples are presented to illustrate the range of applications to which the high-low approach is suited. Finally, special considerations are explored for the use of high-low chromatography as part of a general liquid chromatographic screening process. When combined with optimized chromatographic conditions, high-low chromatography can be an important aid in the determination of impurities in bulk substances.
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Inman et al. (1988) studied this question.