We have developed novel precolumn derivatization reagent, p-N,N,N-trimethylammonioanilyl N'-hydroxysuccinimidyl carbamate iodide (TAHS), for sensitive analyses of amino acids using high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS). TAHS, an activated carbamate, was reacted briefly with the amino group to form a ureide bond under mild condition. The derivatives provided selective cleavage at the binding site between the reagent and the amino acid in the collision cell of the mass spectrometer and produced a characteristic fragment derived from the reagent moiety. Using the precursor ion scan mode of the tandem mass spectrometry, amino acids derivatized with the reagents were simultaneously measured on the chromatogram. Selective cleavage also enabled the straightforward isotope ratio analysis of amino acids by the selected reaction monitoring mode, which was applicable in (13)C metabolic flux analysis. TAHS, which contains a cationic quaternary amine, achieved subfemtomole to attomole levels of amino acids detection by measurement in the selected reaction monitoring mode. We also synthesized trideuteriummethyl-substituted TAHS, TAHS-d(3), and demonstrated that the combination of TAHS and TAHS-d(3) is useful in comparing amino acid concentrations between two different samples using a single LC/MS/MS measurement.