Continuous photometric method for the screening of human urines for phenothiazines Original Research Article

A fully automated urinary screening system for five phenothiazines has been developed. The method is based on the oxidation of phenothiazines in acid medium to colourless sulphoxides via orange or purple coloured intermediates, which are continuously monitored at 520 nm. Two innovations have been implemented versus the conventional method; first, sulphuric acid (ideal reaction medium) has been changed by nitric acid (less corrosive, and compatible with pumping tubes) and second, problems associated with the instability of phenothiazonium radical cation are eliminated as the measurements are carried out simultaneously to the formation of the coloured products in the flow system. The configuration adopted permits selective retention of phenothiazines on a LiChrolut®-EN sorbent column before their oxidation with hexacyanoferrate(III) ions in acid medium. The proposed method allows phenothiazines determination within the interval 3–50 μM (1–20 μg ml−1), with a throughput of 12 h−1; an average relative S.D. of 4% (n=11) was obtained for phenothiazine concentration of 5 μM. Finally, a comprehensive study of 50 real urine samples (phenothiazines free) obtained from different individuals provided 6% of false positives and 0% false negatives for chlorpromazine concentrations of 1.5 and 3 μM, respectively.