Determination of low plasma concentrations of 3-methoxy-4-hydroxyphenylethylene glycol using gas chromatography-negative-ion chemical ionization mass spectrometry.

Gas chromatography-negative-ion chemical ionization mass spectrometry (GC-NICI-MS) allowed the detection of extremely low plasma concentrations of 3-methoxy-4-hydroxyphenylethylene glycol (MHPG). Glucuronide and sulphate conjugates of MHPG were determined after enzymatic hydrolysis of plasma with beta-glucuronidase-arylsulphatase. A 1-ml plasma sample was extracted at the pH of the hydrolysis (pH 4.8) with ethyl acetate, and the dry extract was derivatized with pentafluoropropionic anhydride in ethyl acetate. After evaporation of the solvent, the residue was dissolved in benzene and an aliquot was analysed by GC-NICI-MS. A trideuterated analogue of MHPG was used as an internal standard. Negative-ion chemical ionization of the pentafluoropropionyl derivatives was carried out using ammonia. The ion-molecule adducts at m/e 766 and 785 (MHPG) and m/e 769 and 788 (internal standard) were formed from the pentafluoropropionyl derivatives with the ions of m/e 163 (CF3CF2COO-) and m/e 144 (loss of fluorine from m/e 163). The concentrations of the ions of m/e 163 and 144 play a major role in the sensitivity and precision of this technique, which allows the detection of free MHPG plasma concentrations as low as 100 pg/ml in routine analysis.