In the field of inborn errors of metabolism (IEM), "organic acids" are low-molecular weight (relative molecular weight less than ~300), water-soluble carboxylic acids that are intermediates or end products of amino acid, carbohydrate, lipid, or biogenic amine metabolism. Amino acids are excluded from this definition, whereas acylglycine conjugates and some decarboxylated derivatives are included because of their common clinical interest. The analytical procedures for the determination of urinary organic acids usually include oximation, solvent extraction, and silylation followed by gas chromatography with mass detection in scan mode data acquisition. Both the retention time and the mass spectrum allow the identification of the urinary metabolites, with quantification being performed on a specific fragment abundance (1-3). Analytical considerations can be found in the reports by Jellum (4), Chalmers and Lawson (1), Tuchman and Ulstrom (5), Niwa (6), Sweetman (2), and Duez et al. (3).
