High Performance Liquid Chromoatography of some analgesic compounds: an instrumental analysis experiment.

The technique of High Performance Liquid Chromatography (LC) is a useful method for a wide variety of analyses and may be used to complement the older technique of gas chromatography. Much of the work currently puhlished on LC is, however, not directly suitable to illustrate the wide variety of procedures which may he used to enhance the separation characteristics of a mixture, while at the same time, prove interesting and relevant as an undergraduate experiment. Previously published student experiments on LC include analysis of essential oils ( I ) , analysis of urinary compounds (21, and liquid chromatography with electrochemical detection (3). These experiments provide interesting examples of the application of LC but are not designed to illustrate the effects of such fundamental variables as solvent composition, pH, etc. In this paper we present an experiment which demonstrates the techniques of solvent selection, gradient elution, pH control, and ion-pairing in the analysis of an analgesic mixture using reversed phase LC on an octadecylsilane (ODs) column. Although this experiment has been developed using fairly sophisticated apparatus, the majority of the procedures described are directly applicable to inexpensive, isocratic instruments. Selection of a suitahle erouu of solutes for the experiment was based in part on a published evaluation of the applicability of reversed phase LC to drug analysis (4); the selected compounds are shown in Figure 1.