The individual stock solutions of acid standards were prepared in DCM in a concentration range of 9 -12 g L -1 (35 -115 mM) and diluted with DCM, MeOH, or ACN for use as working solutions. Each acid standard in the mixture/model oil was 20 µM for optimization experiments and optimization of ionization efficiency via direct infusion analysis. The stock solutions of EDC and 4-BNMA were prepared freshly in the corresponding solvent before being used with a concentration level of 150 -210 g L -1 (750 -1350 mM). The intermediate solutions of EDC and 4-BNMA at 30 mM were prepared for low-concentration spiking. Solutions of 0.3% hydrochloric acid in solvents (DCM, MeOH or ACN) were prepared to adjust the extractants pH for EDC -4-BNMA couplings.