SUPERCRITICAL-FLUID EXTRACTION AND NEGATIVE-ION ELECTROSPRAY LIQUID-CHROMATOGRAPHY TANDEM MASS-SPECTROMETRY ANALYSIS OF PHENOBARBITAL, BUTALBITAL, PENTOBARBITAL AND THIOPENTAL IN HUMAN SERUM

Four commonly used barbiturates (phenobarbital, butalbital, pentobarbi tal and thiopental) were analyzed in human serum using supercritical f luid extraction (SFE) and negative ionization LC/ESI-MS/MS, Barbital w as used as the internal standard. Carbon dioxide SFE was performed at 40 degrees C and 500 atm, with a total extraction time of 35 min. The analytes were collected off-line in a liquid trap containing absolute methanol. Samples were then concentrated by vacuum centrifugation. The high performance liquid chromatography separation utilized gradient e lution with a total analysis time of 21 min. The precursor and major p roduct ions for the four barbiturates were monitored on a triple quadr upole mass spectrometer with negative ion electrospray ionization (ESI ) in the multiple reaction monitoring mode as follows: (1) thiopental (m/z 241.20 --> 58.00), (2) phenobarbital (m/z 231.10 --> 188.0), (3) pentobarbital (m/z 225.10 --> 181.90) and (4) butalbital (m/z 222.80 - -> 179.90). In the case of phenobarbital, pentobarbital and butalbital , the most abundant product ion arises from the loss of 43 u (HCNO los s). However, in the case of thiopental, the most abundant product ion was observed at mit 58.0 (the [M - 183](-) ion, or NCS-). Mechanisms f or the formation of the collision induced dissociation reaction produc ts of these barbiturates are proposed. (C) 1998 John Wiley & Sons, Ltd .