Difference spectrophotometric assay of 1,2-diphenolic drugs in pharmaceutical formulations -III. The simultaneous assay of levodopa and benserazide.
A simultaneous assay of levodopa and benserazide in combined formulations is described that utilizes the different spectral properties of the drugs in the presence of germanium dioxide. Difference absorbances are measured at 238 and 292.5 nm between pH 6 solutions of the drugs complexed with germanium dioxide relative to equimolar solutions of the uncomplexed drugs. The concentrations are calculated using two simultaneous equations derived from the difference absorptivities of the individual drugs at 238 and 292.5 nm. The choice of wavelengths for maximum accuracy and precision is discussed. Good recoveries of the drugs in standard mixtures were obtained and satisfactory precision was demonstrated by the relative standard deviations of 0.46% and 1.07% for levodopa and benserazide, respectively, obtained in the replicate analyses of a sample of Madopar capsules. The accuracy of the procedure and the absence of interference from excipients were confirmed by the good agreement in the results for the assay of several batches of Madopar capsules and one of Madopar tablets with those obtained by the manufacturer by an HPLC procedure.
Davidson
A G
AG
Department of Pharmacy, University of Strathclyde, George Street, Glasgow G1 1XW, UK.