Spectral ratio derivative spectrophotometric determination of sulphaquinoxaline and pyrimethamine in veterinary formulations.

Veterinary and human medicines containing a sulphonamide are used to prevent infections in a variety of situations. For sulphonamide products that contain many components (e.g. four or more sulphonamides) chromatographic techniques are recommended [l-5]. However, for products formulated to contain a sulphonamide and another drug which increases the activity of the sulphonamide, simple, rapid and economic methods can be used. Spectrophotometric methods based on the BrattonMarshall reaction are often used for determining total sulphonamides [6-91. If the determination of the two drugs in the formulation containing the sulphonamide and another drug, e.g. sulphamethoxazole and trimethoprim, sulphadiazine and trimethoprim, is required, the derivative spectrophotometric technique is suitable, particularly when the spectra of the two drugs overlap. Recently, the first derivative of the ratio spectra [lo] has been used for resolving different sulphonamides mixtures, e.g. sulphamethizole and nitrofurantoin [ 1 l] or sulphamethoxazole and trimethoprim [ 121. In this paper, the simultaneous determination of sulphaquinoxaline and pyrimethamine in antibacterial formulations used in veterinary practice is described. The procedure was applied to three different veterinary formulations and the results compared with those obtained by using the zero-crossing method in derivative spectrophotometry.