The chemical structure of AA-BAs contains an amide group and a carboxyl group that can be ionized under both positive and negative ion modes in ESI. To investigate the MS fragmentation of AA-BAs, 9 standards including Gly-LCA, Gly-DHCA, Gly-UDCA, Gly-DCA, Gly-CA, Ala-DCA, Ala-CA, Leu-DCA, and Leu-CA, were analyzed by using ESI-QqQ-MS in both positive ion mode and negative ion mode as follows.