Spectrophotometric Determination of Sulphamethoxazole and Trimethoprim (co-trimoxazole) in Binary Mixtures and in Tablets

Background: The formulation of sulphamethoxazole (S) and trimethoprin (T) (CO-TRIMOXAZOLE) in a combination mixture is very good pharmacologically since it enhances the efficacy of the individual drugs. However in this combination, difficulties in analysis on ordinary UV spectrophotometry are introduced because the two components give overlapping spectral bands on zero-order. The United States Pharmacopoea (USP)-recommended HPLC analytical method is quite expensive.
Objective: The objective of the present work was to assess whether derivative spectrophotometry could be used to circumvent the overlapping spectral bands of the components and hence use it for routine analysis of the drug.
Study design: Experimental
Methods: The aqueous solution of the individual drugs and their binary mixutres were scanned on zero order and on first derivative at the wave length between 200- 300nm and at the pH of 4.5.
Results: The zero-order spectra of the compounds were completely overlapping. However the first-derivative scan offered better separation and hence T was determined from the absorbance at 237.6nm with negligible contribution from S (since at this point it was reading zero). Likewise S was determined at a wavelength of 259nm when T was reading zero. The linear calibration graphs were obtained for 425mgml^-1 of S and for 4-20mgml^-1 of T.
Conclusion: The method is rapid, simple and can be applied successfully to assay a mixture of the two drugs in pharmaceutical preparations.

Keywords
sulphamethoxazole, trimethoprim, derivative spectroscopy, simultaneous determination.