96-Well liquid membrane extraction of weakly acidic compounds

Three-phase liquid membrane extraction (LME) of acids involves mass transfer from an acidified sample, through an organic liquid membrane into an alkaline aqueous acceptor. However, this approach presents challenges for acids with pK_a > 9-10, as their efficient extraction often requires extreme pH conditions in the acceptor, which can compromise chemical stability and compatibility with chromatographic analysis. Alternatively, a polar organic solvent can be used as acceptor, but this may challenge the stability of the liquid membrane and the integrity of the extraction system.

In this work, commercial 96-well plates were used for the extraction of nine weakly acidic model analytes (phenols and bisphenols). With an alkaline acceptor, the presence of a boundary layer between the liquid membrane and the acceptor hindered the extraction of the analytes, requiring the use of pH 13.0. To overcome this, an alternative system was developed with acceptor based on dimethyl sulfoxide diluted with pure water. In both systems, different liquid membranes were evaluated, and extraction kinetics were studied. Both systems were applied to human plasma and provided exhaustive extraction of the analytes. Under the final conditions, the DMSO-water acceptor system was evaluated obtaining satisfactory analytical parameters in terms of linearity (r² > 0.990) and precision (RSD ≤15 %).

This new approach enhances the applicability of the three-phase aqueous-organic-organic system, making it milder towards the liquid membrane. It enables the extraction of the analytes as neutral species, broadening the scope of extractable compounds in the three-phase system. Additionally, it offers an alternative to the well-established ionization mode.