Rapid high-pressure liquid chromatographic determination of quinidine and dihydroquinidine in plasma samples

A high-pressure liquid chromatographic assay for quinidine and dihydroquinidine was developed. A cation-exchange column was utilized with an eluting solvent pH of about 9. The internal standard was cinchonine. The reproducibility and precision of this method were evaluated by analyzing replicate samples and by comparing results with those obtained from a TLC-fluorometric procedure. In addition, several drugs were evaluated to ascertain whether they interfered with the analysis of quinidine and dihydroquinidine.