High-performance liquid chromatography method for assay of diltiazem hydrochloride and its related compounds in bulk drug and finished tablets.

The method provides for the resolution of trans-diltiazem and seven known and several unknown related compounds from diltiazem HCl. Minimum detectable amounts were less than 0.1%, except for an intermediate which originates early in the synthetic process, for which the sensitivity is approximately 2%. The relative standard deviation of the assay procedure is 0.15%. Total related compounds in four bulk drug and four tablet samples were less than 0.25%. The specific rotation of four samples of diltiazem HCl analyzed in duplicate was between +112 and +114 degrees. The UV absorption spectra of all compounds exhibited two maxima, one between 203 and 213 nm and the other between 230 and 244 nm.