Quantification of l-arginine, asymmetric dimethylarginine and symmetric dimethylarginine in human plasma: A step improvement in precision by stable isotope dilution mass spectrometry

The amino acid l-arginine and its metabolites ADMA and SDMA are important markers for a range of diseases in humans. Increased levels of ADMA and SDMA in plasma point to endothelial dysfunction, hypertension, renal impairment and other pathological states. We present here a method to quantify l-arginine, ADMA and SDMA in human plasma, which is suitable to support clinical research in this field. Sample preparation consisted only of protein precipitation and the analytes were separated using a silica based HILIC column. The analytes were detected by ESI MS/MS, providing high selectivity and sensitivity. The calibration functions were linear in the ranges of 7.5–150 μmol/l for l-arginine, 0.15–3 μmol/l for ADMA and 0.2–4 μmol/l for SDMA. These ranges cover the concentrations encountered in healthy and pathological human plasma. The method employs 13C6-arginine, D7-ADMA and, for the first time in LC–MS/MS, D6-SDMA as internal standards for l-arginine, ADMA and SDMA. Therefore, matrix independency and a high intra-day precision of 0.82% for l-arginine, 2.12% for ADMA and 2.83% for SDMA, were achieved at basal plasma concentrations. The respective inter-day precision values were 4.01% for l-arginine, 3.77% for ADMA and 3.86% for SDMA.