Criteria 1-2 address the safety, affordability, and stability of the liquid membrane prior to and during extraction, while criteria 3-5 and 7 address the extraction selectivity, efficiency, and robustness in presence of a complex matrix. In EME, the pH of sample and acceptor solution is typically made basic to induce a negative charge on acidic analytes, which enables migration in an electric field. Criterion 6 was therefore added considering the risk of base-catalyzed hydrolyses of analyte ester and amide groups at elevated pH. During initial experiments, the sample was adjusted to pH 7.4 using 50 mM phosphate buffer. At this pH, all model analytes were negatively charged. During EME of acids, a boundary layer with depressed pH is formed at the liquid membrane-acceptor interface, which may impede the mass transfer into the acceptor solution . To overcome this effect, the acceptor solution was adjusted to pH 11 by 50 mM ammonia. Ammonia was selected as a volatile base compatible with electrospray ionization. All analytes were confirmed to not degrade at pH 7.4 and 11 (Figure ).