Simultaneous spectrofluorometric determination of oxatomide and phenylephrine in the presence of a large excess of paracetamol Original Research Article

Oxatomide and phenylephrine have been simultaneously determined in pharmaceutical tablets containing a large excess of paracetamol (ca. 20:1 and 100:1 in weight, respectively) by two spectrofluorometric methods. The strong absorbance of paracetamol at the excitation maxima of the analytes and the severe overlapping between the emission and excitation spectra of the latter gives rise to a highly interfering system. Conventional standard addition was applied to the first-derivative of the emission spectra at two appropriate zero-crossing wavelengths. This method involves adding oxatomide or phenylephrine to an acid aqueous solution (pH=2) of the tablet components, in the ranges 0–6.40 and 0–1.00 mg l−1, respectively. Generalized standard addition was also applied to emission spectra of mixtures obtained by adding both analytes to the unknown samples, and processing the obtained spectral data with a suitable computational program. Both techniques were satisfactorily employed for assaying commercial pharmaceutical tablets. Critical examination of the spectra for the studied analytes as a function of the concentration of paracetamol provides an explanation as to why multivariate calibration methods such as partial least-squares (PLS) cannot be applied to the present system.