“…5,6 However, the detection sensitivity for proteins/peptides, especially for those with low concentrations and poor ionization efficiency, still can hardly meet the requirements of real sample analysis. Recently, many MS-based tags, such as imidazolium derivatives, [9][10][11] piperazine derivatives, 12-15 guanidine derivatives, 16,17 alkyl tertiary amino or quaternary ammonium derivatives, [18][19][20][21][22][23] phenyl derivatives, [24][25][26] and pyridine derivatives, [27][28][29][30] have been exploited for derivatization of various reactive groups on the peptides with improved ionization efficiency and enhanced detection sensitivity. Recently, many MS-based tags, such as imidazolium derivatives, [9][10][11] piperazine derivatives, 12-15 guanidine derivatives, 16,17 alkyl tertiary amino or quaternary ammonium derivatives, [18][19][20][21][22][23] phenyl derivatives, [24][25][26] and pyridine derivatives, [27][28][29][30] have been exploited for derivatization of various reactive groups on the peptides with improved ionization efficiency and enhanced detection sensitivity.…”