Gas chromatographic method using nitrogen-phosphorus detection for the measurement of tramadol and its O-desmethyl metabolite in plasma and brain tissue of mice and rats

A method that allows the measurement of plasma and brain levels of the cent rally-acting analgesic tramadol and its major metabolite (O-desmethyl trama dol) in mice and rats was developed using gas chromatography equipped with nitrogen-phosphorus detection (GC-NPD). Plasma samples were extracted with methyl tert.-butyl ether (MTBE) and were injected directly into the GC syst em. Brain tissue homogenates were precipitated with methanol, the resulting supernatant was dried then acidified with hydrochloric acid, The aqueous s olution was washed with MTBE twice, alkalinized, and extracted with MTBE. T he MTBE layer was dried, reconstituted and injected into the GC system. The GC assay used a DB-1 capillary column. with an oven temperature ramp (135 to 179 degreesC at 4 degreesC/min). Dextromethorphan was used as the intern al standard. The calibration curves for tramadol and O-desmethyl tramadol i n plasma and brain tissue were linear in the range of 10 to 10 000 ng/ml (p lasma) and ng/g (brain). Assay accuracy and precision of back calculated st andards were within 15%. (C) 2001 Elsevier Science B.V. All rights reserved .