Resolution and identification of analytes. Arginine (50 µm),NG‐monomethyl‐l‐arginine (l‐NMMA; 0.5 µm), homoarginine (0.5 µm), asymmetric dimethylarginine (ADMA; 0.5 µm), symmetric dimethylarginine (SDMA; 0.5 µm) and monoethyl‐l‐arginine (MEA; 50 µm) were analyzed individually to determine retention times. Peak identities: 1, arginine; 2, l‐NMMA; 3, homoarginine; 4, ADMA; 5, SDMA; and 6, monoethyl‐Larginine (MEA, internal standard). Concentrations in human plasma (middle panel) were 89.7 µm arginine, 0.09 µm l‐NMMA, 0.35 µm ADMA and 0.28 µm SDMA. Concentrations in mouse plasma (bottom panel) were 83.7 µm arginine, 0.14 µm l‐NMMA, 0.51 µm ADMA and 0.13 µm SDMA. Arrows at 20 and 37 min indicate switch to increased sensitivity (PMT gain 16) and back to normal sensitivity (PMT gain 11), respectively.