Quantitative GLC determination of cis- and trans-isomers of doxepin and desmethyldoxepin.

A GLC method for the simultaneous quantitative determination of the cis- and trans-isomers of doxepin and desmethyldoxepin in human plasma was developed. The method involves the use of a capillary column for efficient separation of the four compounds and the internal standards, amitriptyline and nortriptyline. A high sensitivity is obtained with a nitrogen detector, enabling quantitation of the compounds in plasma of humans treated chronically with doxepin. Confirmation of the identity of the cis- and trans-isomers of doxepin and desmethyldoxepin in biological samples was carried out by selected ion monitoring.