Identification and quantification of trihexyphenidyl and its hydroxylated metabolite by gas chromatography with nitrogen-phosphorus detection

A sensitive and specific assay for the simultaneous quantification of trihexyphenidyl and its hydroxylated metabolite in plasma and urine is described. The method is based on the extraction of the drugs with an organic solvent and separation on a 3% OV-17 Chromosorb Q column in a gas chromatograph equipped with a nitrogen-phosphorus detector. The procedure employs SKF 525 A as the internal standard and requires no derivatization. The detection limit was found to be 2 ng/mL for trihexyphenidyl and 1 ng/mL for its metabolite. The precision of the assay procedure for both compounds is about 4 to 7%.