Abstract. Twenty-two sulphonamides and three dihydrofolate reductase inhibitors (DHFR) were included in this study. At pH 7.5, eighteen drugs were resolved in 22 min. Above pH 7.5 the DHFR are not resolved and the pH range of 7-7.5 was found to be optimal. Around pH 7, where the degree of ionization of the SFA is highly variable, pK, and migration time are strongly correlated. Increasing phosphate buffer concentration from 25-100 mM generally improves resolution but increases analysis times. A good correlation between analyte mobility and the ratio of net charge to molecular weight indicates that this ratio may be used as a quick guide to the possibility of separation of related compounds. Our secondary objective was to compare the lead time and effectiveness of CZE with LC. It is clear that method development is much more rapid in CZE but with a sufficiently diverse group of compounds, CZE can be incapable of providing a full analysis without gradient control. Nonionized compounds are not separated at one extreme, and excessively mobile species may not emerge in a reasonable time, or, may migrate to the wrong electrode.

Of the twenty-two sulphonamides and three commonly used dihydrofolate reductase inhibitors investigated for reverse phase separation in previous studies, the recoveries of five representative drugs from the stationary phase have been examined. Sulphanilamide, sulfisomindine, sulphaquinoxaline, diaveridine and pyrimethamine were chosen. Peak areas off the analytical column were compared with those obtained by substitution with stainless steel tubing. After careful correction for flow rate differences where necessary, 100% recoveries were indicated. As a further check, Maloprim tablets were assayed for pyrimethamine and the analysed results were found to be (99.1 f 3155 COpyight C 1996 by Marcel Dekker, Inc. Downloaded by [Michigan State University] at 23:01 08 February 2015 3 156 RICCI AND CROSS 2.9)%. Limits of detection (LOD) for aqueous standards varied from 0.67 (for sulphanilic acid) -0.03 pg mL-' (diaveridine).The linear dynamic range extended from the LOD for each drug to >5 pg mL-' .

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Twenty-two sulphonamides and the three commonly used dihydrofolate reductase inhibitors have been subjected to an investigation of retention behaviour on a silica based reversed phase. Effects of variation in the percentage of methanol and the pH have been determined isocratically and the methanol gradients developed have been modified by variations in the concentration of the phosphate buffer. Significant variations in retention behaviour were observed such that the majority of combinations of drugs could be screened for. No set of conditions studied gave rise to a total separation of all drugs, but around twenty drugs were commonly resolved. #Current address:

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