H. Gallart-Ayala et al. (2010) recommended methanol mobile phase containing no additives in order to avoid ion suppression of BPA.
On the other hand, the signal of BPA was reported to increase after the addition of acetic acid into acetonitrile mobile phase (P_erez-Palacios et al., 2012) or ammonium acetate into methanol mobile phase (Chu, Haffner, & Letcher, 2005) which is consistent with our results (Fig. 5).