Automated on-line solid-phase extraction gas chromatography with nitrogen-phosphorus detection: Determination of benzodiazepines in human plasma

An automated sample preparation module, the ASPEC (automated sample preparation with extraction columns) was interfaced with a capillary gas chromatograph (GC) by means of a loop-type interface. The system was optimized for the determination of four benzodiazepines in plasma. Extraction from the untreated plasma was carried out on disposable C-18 cartridges and involved several washing steps. The analytes were desorbed with 2 ml of ethyl acetate and a 110-mu l aliquot of the eluate was injected into the gas chromatograph via the loop-type interface using fully concurrent solvent evaporation conditions. Detection of the benzodiazepines was carried out with a nitrogen-phosphorus detector (NPD). The ASPEC-GC-NPD was fully automated and could run unattended overnight. With a sample volume of 1 ml the procedure showed good linearity and repeatability in the range 5-50 ng/ml using a sample volume of 1 ml. The limits of detection in plasma were 0.5-2 ng/ml.