Quantitative urine amino acid analysis using liquid chromatography tandem mass spectrometry and aTRAQ® reagents

Ion-exchange chromatography (IEC) is the most widely used method for amino acid analysis in physiological fluids because it provides excellent separation and reproducibility, with minimal sample preparation. The disadvantage, however, is the long analysis time needed to chromatographically resolve all the amino acids. To overcome this limitation, we evaluated a novel liquid chromatography tandem mass spectrometry (LC–MS/MS) method, which utilizes aTRAQ® reagents, for amino acid analysis in urine. aTRAQ® reagents tag the primary and secondary amino groups of amino acids. Internal standards for each amino acid are also labeled with a modified aTRAQ® tag and are used for quantification. Separation and identification of the amino acids is achieved by liquid chromatography tandem mass spectrometry using retention times and mass transitions, unique to each amino acid, as identifiers. The run time, injection-to-injection, is 25 min, with all amino acids eluting within the first 12 min. This method has a limit of quantification (LOQ) of 1 μmol/L, and is linear up to 1000 μmol/L for most amino acids. The Coefficient of Variation (CV) was less than 20% for all amino acids throughout the linear range. Method comparison demonstrated concordance between IEC and LC–MS/MS and clinical performance was assessed by analysis of samples from patients with known conditions affecting urinary amino acid excretion. Reference intervals established for this method were also concordant with reference intervals obtained with IEC. Overall, aTRAQ® reagents used in conjunction with LC–MS/MS should be considered a comparable alternative to IEC. The most attractive features of this methodology are the decreased run time and increased specificity.