A spectrophotometric method for simultaneous estimation of phenytoin and carbamazepine

A 0.5 ml sample of serum, containing different antiepileptic drugs, singly or in combination, was made alkaline, overlayered with isooctane, and steamed in the presence of KMnO4. The spectra of oxidized products in the organic layer were recorded in the UV range. Oxidized phenobarbitone and primidone show no absorption peak; diazepam a delta-max at 228 nm; phenytoin at 247 nm; and carbamazepine at 247 nm and 372 nm. Consequently, phenobarbitone and diazepam do not interfere in phenytoin quantitation, but carbamazepine does. The contribution of carbamazepine at 247 nm was calculated from the absorption at 372 nm and the ratio of its molar extinction coefficients at the two wave lengths. This was subtracted from the total A247 values to get the actual values due to phenytoin. Thus, a method for simultaneous analysis of carbamazepine and phenytoin in a single sample has been developed.