Second-derivative spectrofluorimetric determination of sulphoxide impurity in phenothiazine drug substances and formulations

A direct second-derivative spectrofluorimetric procedure for determining sulphoxide impurity in phenothiazines and their formulations has been developed. The method, which has been applied to the analysis of chlorpromazine hydrochloride and prochlorperazine mesylate, as examples of typical phenothiazine substances, and to their formulations, is based on the measurement of the amplitude taken from the minimum at ca. 270 nm to the longer wavelength maximum in the second-derivative excitation spectrum of the sulphoxide in pH 8 buffer solution. The method is rapid, accurate and precise, and can be used to measure the concentration of sulphoxide in phenothiazines and their formulations at concentrations down to 0.1% m/m of that of the parent phenothiazine.