Determination of sulfamethoxazole and trimethoprim by ratio spectra derivative spectrophotometry

Summary A new method is described for analysing the binary mixture of sulfamethoxazole and trimethoprim, using the ratio spectra derivative spectrophotometry. The procedure does not require any separation step. Calibration graphs were linear up to 25 mg/l of sulfamethoxazole and up to 50 mg/l of trimethoprim; their 1:5 and 5:1 (m/m) mixtures can be resolved with recoveries between 97 and 104%. The method was satisfactorily applied for determining sulfamethoxazole and trimethoprim in different pharmaceutical products.