HIGHLY SENSITIVE GAS-CHROMATOGRAPHIC MASS-SPECTROMETRIC METHOD FOR MORPHINE DETERMINATION IN PLASMA THAT IS SUITABLE FOR PHARMACOKINETIC STUDIES

A sensitive method was devised to determine morphine plasma concentrat ions by gas chromatography-mass spectrometry (GC-MS) using selected io n monitoring (SIM) with nalorphine as the internal standard. This meth od was rugged, reliable, selective and sensitive and was used to deter mine the morphine content of over 2000 samples. Sample preparation inv olved extraction of basified sample using n-butyl chloride-chloroform (5:1) and evaporation of the extract to dryness. The residue was deriv atised with pentafluoropropionic anhydride, evaporated to dryness, rec onstituted in 40 mu l toluene and injected onto the GC-MS. For a sampl e size of 1 ml, the limit of quantitation was 0.75 ng/ml (SIN ratio 10 :1) and the estimated limit of detection was calculated to be 0.2 ng/m l (SIN ratio 3:1), expressed as morphine base, Precision (n=5) was 4.9 % at 0.75 ng/ml, 6.8% at 1.5 ng/ml, 3.0% at 37.5 ng/ml and 2.3% at 150 ng/ml. Standard curves for the range of 0-750 ng/ml morphine in plasm a were linear with all r(2) values greater than 0.99. No interfering p eaks were seen for either morphine or internal standard in the blank s amples. The method is suitable for pharmacokinetic studies after subcl inical doses of morphine where it has been used to study morphine plas ma concentrations for 6 h after a dose of only 2 mg.