Determination of codeine in plasma by high-performance liquid chromatography

An automated, sensitive, and selective reverse-phase high-performance liquid chromatographic assay has been developed to measure codeine in plasma. The analysis requires only 1 ml plasma and is accomplished by detection of the fluorescence of codeine following extraction and concentration. The method is simple and rapid, involving a one-step extraction of codeine from alkalinized (pH 10.0) plasma into an organic layer of hexane/dichloromethane, 2/1. The organic layer was evaporated under nitrogen and the residue reconstituted with the mobile phase. The samples were chromatographed on a reverse-phase C-18 column using a mobile phase of acetonitrile-phosphate buffer, 80/20 (pH 5.80). The codeine and internal standard, N-allylnorcodeine, peaks were detected using a fluorescence detector. The retention times were 8.6 min for the internal standard and 11.3 min for codeine. Standard curves were linear from 10 to 250 ng/ml. The assay was validated by direct comparison with a gas chromatographic procedure that employed nitrogen-phosphorus detection. The assay has been employed for the analysis of several codeine studies using human, dog, and rat plasma.