Simultaneous determination of amitriptyline, nortriptyline and four hydroxylated metabolites in serum by capillary gas-liquid chromatography with nitrogen-phosphorus-selective detection.

A method for the simultaneous quantification of the antidepressant drug amitriptyline, its demethylated metabolite nortriptyline and four hydroxy metabolites (E-10-hydroxyamitriptyline, Z-10-hydroxyamitriptyline, E-10-hydroxynortriptyline, Z-10-hydroxynortriptyline) in human serum or plasma has been developed. The method is based on a three-step liquid-liquid extraction followed by gas-liquid chromatography (split-splitless injection, HP-5, 25 m x 0.2 mm I.D., 0.33 microns capillary) with nitrogen-phosphorus-selective detection (GLC-NPD). The limits of detection are 1.5 ng/ml for amitriptyline, nortriptyline, E-10-hydroxyamitriptyline and Z-10-hydroxyamitriptyline and 3 ng/ml for E-10-hydroxynortiptyline and Z-10-hydroxynortriptyline. The within-day and between-day precision is between 6 and 15% at three concentrations (low, moderate and high) for amitriptyline, nortriptyline and E-10-hydroxy metabolites. At low concentrations of 10 ng/ml, the precision of the assay of the Z-10-hydroxy metabolites has been found to be up to 19%. Accuracy is between 91 and 115% for all analytes. The performance of the assay of the hydroxy metabolites is mainly determined by the cleanness and the deactivation of the quartz insert of the injector port. Therefore, every day a freshly cleaned and deactivated insert was used.