GC-MS profiling of urinary organic acids evaluated as a quantitative method

We assessed the quantitative performances of a classical method for profiling urinary organic acids: ethyl acetate extraction/oxime-trimethylsilyl derivatization/GC-MS. Twenty-seven acids were quantified on the basis of specific ions in both scan and selected-ion monitoring modes. We found that the tuning of the mass detector severely affects the calibration factors, being critical to achieve quantitative results, and we propose a practical procedure for reproducible tuning. Of seven compounds tested, tropic acid was retained as the internal standard suitable for most of the acids of clinical interest; a second internal standard, 2-ketocaproic acid, was used in quantifying keto-acids. The within-day and total relative standard deviations (CVs), estimated from scan-mode analyses of urine, ranged from 2.6% to 12.7% and from 4.2% to 11.8%, respectively. Curvilinear relationships between analytical response and concentration were observed for most of the acids investigated.