DETERMINATION OF URINARY MEVALONIC ACID USING ISOTOPE-DILUTION TECHNIQUE

A simplified and highly accurate, stable isotope dilution method using gas chromatography/mass spectrometry (GC/MS) with deuterated (H-2(7)) or (H-2(3))mevalonic acid (MVA) as internal standard was developed fo r the measurement of MVA in urine samples. MVA was converted to its la ctone (MVL) and transferred with the total water of the sample (100 mu l) to a mixture of acetone/methyl t-butyl ether (MTBE). This solution was concentrated and dried by azeotropic removal of MTBE/water in spe cial vessels (used for volume reduction under partial reflux for solut ions containing volatile compounds). After producing MVA by adding NaO H followed by azeotropic drying, MVA was converted to its tris-t-butyl dimethylsilyl (tri-TBDMS) derivative. GC/MS selected ion monitoring me asurements at m/z 317, m/z 320 and m/z 324 were performed in the elect ron impact ionization (EI) mode. Overall recoveries of about 70% were obtained as shown by following the procedure with (C-14)MVL, (H-2(7))- and (H-2(3))MVL as external and/or internal standards. Six replicate analyses of one urine sample revealed a coefficient of variation of 3. 3%. Based on this experience we developed two other methods. The secon d method was based on a fluid/fluid extraction and the third one on an extraction with total water transfer. Both methods are combined with azeotropic drying and showed improved precision, handling and speed.