An electron-capture gas chromatographic assay for naltrexone in biological fluids

The electron-capture gas chromatography assay for naltrexone is an adaptation of the method published originally by Sams and Malspeis (1). The current methodology, described in detail, consists of an extraction procedure, derivatization to form an electron-capturing triester, and gas chromatography using an OV-17 column. Extraction efficiencies indicate that either benzene or 0.25% butanol in cyclohexane used as the organic phase yields maximal extraction of naltrexone and minimal extraction of most naltrexone metabolites. Methylene chloride, on the other hand, yields an optimal combination of clean chromatograms and high extraction efficiencies for both naltrexone and its metabolites. Derivatization with either heptafluorobutyric anhydride or pentafluoropropionic anhydride together with a basic catalyst yields a triester derivative. Use of an OV-17 chromatographic column with electron-capture detection permits assay of naltrexone specifically with respect to known metabolites. One minor metabolite, 2-hydroxy-3-O-methyl-beta-naltrexol could interfere with naltrexone quantitation if present in sufficient quantities. This interference could readily be detected if it were to occur. Data on the reproducibility of this assay procedure indicate that it is sensitive to a concentration of 0.25 ng/ml plasma.