Determination of guaifenesin in human plasma by liquid chromatography in the presence of pseudoephedrine

A sensitive and specific liquid chromatography (LC) procedure was developed and validated for the determination of guaifenesin in human plasma in the presence of pseudoephedrine. Guaifenesin was extracted from plasma at pH 6.9–7.1 using methanol—methylene chloride (5:95, v/v) containing the internal standard mephenesin and pseudoephedrine. The organic layer was separated and evaporated to dryness and the residue reconstituted with the mobile phase containing methanol—acetonitrile—phosphate buffer (0.05 M) (11:11:78, v/v/v) containing 4 mM heptane sulphonic acid and 1% glacial acetic acid. The separation was performed on a μBondapak C 18 column. The flow rate was 1.0 ml min −1. The retention times for guaifenesin and mephenesin were 7.9 and 15.7 min, respectively. Linearity of response was observed in the concentration ranges of 50–1000 ng ml −1 and 1–4 μg ml −1. Accuracy was within 15.4% of the true value for the inter-day and intra-day analysis. The precision, as measured by the RSD, ranged from 4.8 to 8.7% for intra-day. The reproducibility of inter-day ranged from 5.0 to 8.4%. The percent recovery from plasma ranged from 88.6 to 97.6. Data are presented to illustrate the practicality of the method for the evaluation of guaifenesin plasma levels in the presence of pseudoephedrine after multiple oral administration of two sustained release tablets containing 600 mg of guaifenesin per tablet to six male healthy volunteers. The mean half-life of guaifenesin in human subjects was found to be 2.88 h and ranged from 1.36 to 5.25 h.