Fluorimetric determination of some thioxanthene derivatives in dosage forms and biological fluids

A simple and highly sensitive method is proposed for the fluorimetric determination of four thioxanthene derivatives, namely: chlorprothixene, clopenthixol, flupentixol and thiothixene, in dosage forms and biological fluids. The method involves the use of nitrous acid as an oxidant to produce the corresponding fluorescent thioxanthenone sulphoxides. The experimental parameters were carefully studied and incorporated into the procedures. The results obtained compare favourably with those obtained by the official methods. The concentration-fluorescence plots were rectilinear over the range of 0.04-0.4 microg/ml for thiothixene, and 0.02-0.25 microg/ml for the other compounds, with minimum detectability (S/N = 2) of 2 ng/ml for all the studied compounds except thiothixene which was 4 ng/ml. The proposed method was applied to the determination of the studied compounds in dosage forms. The results obtained were in good agreement with those obtained adopting the USP XIII method. The proposed method was further applied to the determination of flupentixol in spiked human urine and plasma, the percentage recoveries were 94.39 +/- 1.81 and 96.46 +/- 0.28, respectively. A proposal of the reaction pathway was presented.