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The measurement of cocaine and its major metabolites has been achieved by an HPLC method that compensates for their different solubilities and detection properties. Although ecgonine methyl ester is a major metabolite it is generally not measured by HPLC because it is poorly detectable by UV, and its water solubility makes recovery from urine difficult. Using modified solid-phase extraction procedures recoveries of 85% for ecgonine methyl ester, 97% for cocaine, 106% for benzoylecgonine and 80% for ethylcocaine have been obtained from urine. Increased chromatographic retention and detection sensitivity has been obtained by formation of the t-butyldimethylsilyl derivative of ecgonine methyl ester which was found to be stable in the HPLC mobile phase for at least 1 week. Alkylation of norcocaine and benzoylecgonine has improved their detection sensitivity and also chromatographic resolution. All calibrations were linear over the range 200-1000 ng ml-1 in urine with correlation coefficients > 0.99.

A phase separator is described that is suitable for post-column HPLC applications. It operates with commonly used HPLC eluents and immiscible organic solvents as long as the two phases remain immiscible. It is compatible with gradient elution systems. Separation efficiency is routinely better than 0.8, which ensures that analyte peak heights are about 95% of the maximum height under these conditions. An application for the detection of pethidine, cocaine, methadone, piritramide and dipipanone at 0.8–1.8 ng on-column loadings is described.