Spectral study and simultaneous determination of sulfamethoxazole and trimethoprim by digital derivative spectrophotometry

A direct and simple second derivative digital spectrophometric method has been developed for the simultaneous determination of the sulfamethoxazole and trimethoprim in pharmaceutical formulation. A study of the spectral behavior of these drugs in different solvents is included, in order to select the best conditions for their simultaneous determination. Acetonitrile was used as solvent to extract the drugs from the pharmaceutical formulations and then to evaluate directly by second derivative spectra. This sumultaneous determination was carried out by the zero crossing approach, at 272,8 nm for sulfamethoxazole determination and at 256,0 nm for trimethoprim determination. The determination range for sulfamethoxazole was from 6,31x10(-8) to 1,20x10(-4) mol/L and for trimethoprim from 2,47x10(-7) to 1,20x10(-4) Mol/L, and the detection limits for sulfamethoxazole and trimethoprim were 1,89x10(-8) mol/L and 7,40x10(-8) mol/L, respectively. The method was applied successfully in pharmaceutical formulations (tablets). Besides a study of photostability of both drugs with standard solutions and real pharmaceutical formulations was performed