Although the removal efficiency of lipids was evaluated usingLC-APCI-MS-scan mode, the spiked soybean sample at each 10 ppblevel of PA was analyzed by LC-ESI-MS scan mode and LC-ESI-MS/MS multiple reaction monitoring (MRM) mode as shown inFig. 4A and B, respectively. As shown in Fig. 4A, FLP in combinationwith SCX-SPE cleanup could effectively eliminate a larger amountof matrix components in foodstuffs, as confirmed by LC-ESI-MSscan mode. After FLP and SCX-SPE cleanup, some of PAs compoundswere clearly detected without significant interference effect evenin ESI-MS scan mode. Thus, FLP combined with SCX-SPE cleanupmethod was found to be effective for the precise determination ofPAs in high lipid foodstuffs.For precise qualitative and quantitative analysis, the MS/MSfragmentation pathways of the [M + H]+ions for the PAs were pref-erentially studied to select transition ions in the MRM mode. TheESI spectra of PAs exhibited abundant [M + H]+ions formed bythe protonation of nitrogen in the necine moiety because of thehigh basicity. The MS/MS spectra of [M + H]+ions for mono- anddiester PAs showed similar mass spectral patterns (Fig. S3). Basedon the interpretation of their MS/MS spectra, the MS/MS frag-mentation pathways of the [M + H]+ions for mono- and diesterPAs are proposed in Scheme 1. The initial fragmentation of theester group at the C-1 position occurred through a- and/or b-cleavages with hydrogen migration to produce a-type and b-typeions, respectively. These fragmentations provide the informationon ester structures. Sequential losses of H2O and C5H6O from a-and/or b-type ions, respectively, led to the formation of a commoncharacteristic ion at m/z 138. Another common characteristic ionat m/z 120 was formed by the loss of H2O from the ion at m/z 138.Subsequently, the necine ring fusion could produce a characteristic