In order to screen AA-BAs, the potential AA-BA structures were simulated according to the theoretical amidation reaction of 20 common AAs in combination with 9 types of free C24 BAs (Figure and Table ). Further, their product ions were predicted as [M-nH2O+H] + and [M-nH2O-AA+H] + for positive ion mode and [AA-H] -for negative ion mode, respectively according to the characteristic MS fragmentation patterns. By this means, an MRM transition list of potential AA-BAs under positive and negative ion modes was made (Table ). Based on the predicted MRM transition list, a polarity-switching MRM MS strategy was developed for profiling AA-BAs. The workflow of the polarity-switching MRM MS strategy was shown in Figure .