For example, two characteristic product ions of Gly-DCA were generated from the neutral loss of 2H2O (36.0 Da) and Gly+2H2O (111.0 Da) at m/z 416.3 and m/z 341.3 under collision energy (CE) of 30 in positive ion mode, respectively (Figure . highlighted in red). Additionally, a robust product ion was also observed at m/z 74.1, as the result of the cleavage of amide bonds of Gly-DCA at CE of 50 in negative ion mode, corresponding to the mass-to-charge ratio of Gly (Figure , highlighted in blue). The fragmentation patterns of Gly-DCA in positive and negative ion mode provided information about the amino acid moiety on the side chain and the number of hydroxy groups on the BA backbone. Similarly, the MS fragmentation of other 8 AA-BA standards followed the fragmentation patterns of Gly-DCA (Figure and Figure ). To sum up, AA-BAs generated three characteristic product ions via collision-induced dissociation (CID), [M-nH2O+H] + and [M-nH2O-AA+H] + at the CE of 30 in positive ion mode, as well as [AA-H] -at the CE of 50 in negative ion mode.