High-performance liquid chromatographic determination of (Z)- and (E)-doxepin hydrochloride isomers.

A high-performance liquid chromatographic method for the determination of (Z)- and (E)-doxepin hydrochloride isomers was developed. The analysis employs a column packed with spherical silica microparticles (5-6 mum), and a mobile phase of acetonitrile-chloroform-diethylamine (750:250:0.2) permits baseline resolution and simultaneous determination of the (Z)- and (E)-doxepin isomers. Process-related substances do not interfere. The method is accurate and precise (the relative standard deviation was 0.3% for both isomers). The simple procedure is highly suitable for routine doxepin hydrochloride analysis.