Survey of human and mouse plasma for potential contaminants. Human and mouse plasma were processed with protein precipitation [no solid‐phase extraction (SPE)] or with solid phase extraction using mixed‐mode cation exchange columns (+SPE). For these analyses, plasma samples were not spiked with MEA (internal standard) and photomultiplier tube (PMT) gain remained constant (11) for the duration of the run. Plasma samples were compared with a combined standard (top panel) to identify potential contaminants that may co‐elute with target analytes if not fully eliminated by SPE. Arrows at 20 and 37 min indicate switch to increased sensitivity (PMT gain 16) and back to normal sensitivity (PMT gain 11), respectively. Sensitivity was not increased in the bottom two panels and traces were artificially shifted up (no SPE) and down (+SPE) to aid visualization. Peak identities: 1, arginine; 2, l ‐NMMA; 3, homoarginine; 4, ADMA; 5, SDMA; 6, MEA; 7, alanine; and 8, taurine.