Determination of codeine and morphine in human plasma by high performance liquid chromatography with serial electrochemical detection

A method is described for the determination of codeine and its metabolite, morphine, at low nanogram concentrations in plasma. Analysis is accomplished by high-performance liquid chromatography utilizing a cyanopropyl normal bonded phase (NBP) column in reversed-phase mode and two electrochemical detectors in series configuration. Two internal standards are utilized, ethyl morphine for codeine and nalorphine for morphine. Codeine and morphine concentration data are presented for several patients receiving codeine-containing medications. The lower limit of detectability was 2.00 +/- 0.39 ng per mL for codeine and 1.20 +/- 0.83 ng per mL for morphine. The patient sample mean within-run coefficients of variation for codeine and morphine (at 10 ng per mL) were less than 10 percent, n = 30. The between-run coefficient of variation for codeine was also less than 10 percent (over a range of two to 190 ng per mL, n = 61), and was approximately 15 percent for morphine (over a range of two to 40 ng per mL, n = 31).