Determination of sildenafil citrate and its main metabolite by sample stacking with polarity switching using micellar electrokinetic chromatography

Micellar electrokinetic capillary chromatography (MEKC) coupled with sample stacking and polarity switching was investigated for the determination of Viagra (sildenafil citrate, SC) and its metabolite (UK-103,320, UK) in human serum in the concentration range of clinical interest. Human serum samples spiked with SC and UK were eluted with methanol from a C18 cartridge, the extract was evaporated and regenerated in a solution that contained 1 mM phosphate buffer (pH 12.3) and 20% methanol. The MEKC separation was performed using an injection time of 275 s, a polarity switching time of 93 s, a phosphate buffer, (pH 12.3, 15 mM) containing 25 mM sodium dodecyl sulfate as separation electrolyte and a fused-silica capillary. The analysis takes about 6 min and gives satisfactory inter-day precision with respect to migration times and linear responses over the 80–900 ng/ml concentration range investigated for SC and UK. Intra-day RSDs (n=4 graphs) for the slopes of the calibration graphs were 4.86% for SC and 3.50% for UK. Inter-day RSDs for the slopes were 4.37% for SC and 5.39% for UK. Detection limits (S/N=3) were about 17 ng/ml for both compounds in human serum. A 1-ml volume of blood serum was necessary to do this determination.