Liquid-chromatographic procedure for tricyclic drugs and their metabolites in plasma

We describe a procedure for determining amitriptyline and imipramine and their active metabolites nortriptyline and desipramine, respectively, at therapeutic concentrations in human plasma by use of liquid chromatography. The drugs are extracted at pH 10.5 into hexane/isoamyl alcohol, which is evaporated and the residue chromatographed. Protriptyline is used as the internal standard. As little as 10 microgram of each drug per liter could be detected in plasma, the limit being established by variability in drug-free plasmas. The day-to-day coefficient of variation for each drug at a concentration of about 100 microgram/liter was about 7%. Doxepin and diphenhydramine interfere with the analysis of amitriptyline. Total analysis time for a single sample is 20 min.