Simple and selective high-performance liquid chromatographic method for estimating plasma quinidine levels.

Abstract

A reversed-phase, high-performance liquid chromatographic method employing fluorescence detection is described for the rapid quantification of plasma levels of quinidine, dihydroquinidine and 3-hydroxyquinidine. It involves protein precipitation with acetonitrile followed by direct injection of the supernatant into the chromatograph. For the preparation of plasma standards, pure 3-hydroxyquinidine was isolated from human urine by a simplified thin-layer chromatographic procedure. The mobile phase for the chromatography was a mixture of 1.5 mM aqueous phosphoric acid and acetonitrile (90:10) at a flow-rate of 2 ml/min. The intra-assay coefficient of variation for the assay of quinidine and 3-hydroxyquinidine over the concentration range 2.5-20 mumole/l was less than 1% for both. Interassay coefficients of variation for quinidine (10 mumole/1) and 3-hydroxyquinidine (5 mumole/1) were 3.5% and 4.0% with detection limits of 50 and 25 mumole/l respectively. The method correlated well (r2 = 0.96) with an independently developed gas--liquid chromatographic--nitrogen detection assay for quinidine which also possessed a high degree of precision. (Intra-assay coefficient of variation 3.6% at 20 mumole/l). As expected, comparison of the high-performance liquid chromatographic assay with a published protein precipitation--fluorescence assay showed poor correlation (r2 = 0.78).