Micro-analysis for quinidine in serum by thin-layer chromatography followed by fluorescence densitometry

We report a thin-layer-chromatographic micro-analysis for quinidine in serum, with detection by fluorescence densitometry. Quinidine is extracted from 20 microL of serum at pH 13 into 3 mL of hexane/acetone solution (80/20 by vol) containing N-(1-naphthyl)ethylenediamine as internal standard. The extract is concentrated and applied to silica-gel-impregnated plates for conventional thin-layer chromatography. Quinidine is identified from its RF value and quantified from the peak-height ratio between quinidine and the internal standard, relative to that of simultaneously extracted serum standards. The proposed assay is sensitive (to 0.2 mg/L), specific for unmetabolized quinidine, precise (between-run coefficients of variation less than 6%), and readily adaptable to large-scale "batch" analysis. Peak-height ratio is linearly related to concentration to at least 20 mg/L. Quinidine concentrations in the serum of patients, as measured by the proposed method (x) and by a traditional double-extraction spectrofluorometric assay (y), were related as follows: y = 0.994x + 0.276 (r = 0.989, n = 20).