Direct injection micellar liquid chromatographic determination of benzodiazepines in serum

A simple micellar liquid chromatographic (MLC) procedure is reported for the determination of several benzodiazepines in serum: bromazepam, diazepam, flunitrazepam, halazepam, medazepam, nitrazepam, oxazepam and tetrazepam. The optimization studies have been made in C18 and C8 columns, using solutions containing sodium dodecyl sulphate (SDS) modified with butanol or pentanol as mobile phases. The method proposed for the determination of the benzodiazepines uses a hybrid micellar mobile phase of 0.06 M SDS–5% butanol–0.01 M phosphate buffer (pH 7) at 25 °C, and UV detection (230 nm) in a C18 column. The serum samples were injected directly, without any pretreatment, and eluted in less than 22 min, in accordance with their relative polarities, as indicated by their octanol–water partition coefficients. The limits of detection (ng ml−1) were within the ranges of 2–6 and 4–18 for aqueous and serum samples, respectively. Repeatability and intermediate precision were tested for three different concentrations of the drugs, and RSD (%) was below 10 for most of the assays. The MLC results were compared with those obtained from a conventional HPLC method using methanol–water 5:5 (v/v) which requires a previous extraction procedure.