First- and second-derivative spectrophotometric determination of imipenem and cilastatin in injections

First- and second-derivative spectrophotometry has been used for the quantitation of mixtures of imipenem and cilastatin sodium, compounds that have closely overlapping spectral bands. Beer's law was obeyed at concentrations up to 100 μg ml −1 of imipenem in both the first- and second-derivative modes and up to 75 μg ml − of cilastatin in the first-derivative mode. Detection limits at the P = 0.05 level of significance were calculated to be 0.40 and 0.52 μg ml −1 of imipenem and cilastatin sodium, respectively, in the first-derivative mode, and in a range from 0.45 to 0.68 μg ml −1 for imipenem in the second-derivative mode. The method, which is rapid, simple and does not require a separation step, has been successfully applied to the assay of commercial injections.