Analysis of arginine and methylated metabolites in human plasma by field amplified sample injection capillary electrophoresis tandem mass spectrometry. 
A CE ion trap tandem MS method was optimised for the analysis of arginine, monomethyl- and (symmetric and asymmetric) dimethylarginines in human plasma after a very reduced sample pretreatment step involving a simple protein precipitation with ACN.  Several parameters affecting the analytes MS ionization and the capillary electrophoretic separation were carefully studied and optimised.  The complete separation of arginine, monomethylarginine and symmetric and asymmetric dimethylarginine was obtained in formic acid BGE in short analysis time with high specificity due to MS(2) detection of specific analytes fragments.  In order to achieve the detection sensitivity suitable for the analysis of asymmetric and symmetric dimethylarginine in human plasma, the field amplified sample injection was applied.  Due to stacking effects, this methodology allowed to operate a consistent on-line preconcentration of the analytes before running the electrophoresis.  The method was validated for linearity, repeatability, recovery and accuracy and applied to the quantitative analysis of arginine, monomethyl- and dimethylarginines in human plasma of healthy subjects.