Assessment of an electron-impact GC-MS method for organic acids and glycine conjugates in amniotic fluid

We assessed the reliability of a method designed for common electron-impact GC-MS systems to determine in a single run most organic acids and glycine conjugates of clinical interest in amniotic fluid. Suitable sensitivity was achieved by dividing the selected-ion chromatogram into 12 time segments during which the monitoring dwelt on specific ions. Twelve metabolites were simultaneously quantified in amniotic fluid, with performances ranging from very good to clinically acceptable. The total coefficient of variation was 2.5-14.1% and the detection limit was well below the lower value of the physiological range. For five other metabolites, the precision was lower and/or the detection limit was near the physiological range. The method was clinically assessed by the prenatal detection of three cases of tyrosinaemia type I and one case of propionic acidaemia. Analytical and clinical evaluation of the method showed that GC-MS with electron-impact fragmentation can be an informative analytical approach for low-level organic acids in physiological fluids. Apart from the case of glycine conjugates, the method shows a fair reliability for amniotic fluid analysis, which might warrant its use for prenatal diagnosis of organic acidurias. However, this method cannot replace procedures using isotopic internal standards, nor GC-MS based on chemical ionization fragmentation, which remain confirmatory analytical methods of choice.