Electromembrane extraction – looking closer into the liquid membrane

Electromembrane extraction (EME) is a microextraction technique, where basic and acidic analytes are extracted as ionic species across a supported liquid membrane (liquid membrane) and into an acceptor solution. Extraction is based on an electrical field sustained across the liquid membrane, and selectivity is based on the direction and magnitude of the electrical field, the chemical composition of the liquid membrane, and pH in the sample and acceptor. EME shows potential for extraction of basic and acidic analytes from complex sample matrices, including biological fluids, environmental samples, food, and beverages. EME is green sample preparation, and provides efficient cleanup, high selectivity, and pre-concentration. Acceptor solutions are aqueous, and can be injected directly in liquid chromatography. We expect more activity with EME in the future. For this activity to be successful, understanding the fundamentals of EME is crucial. In this article, we are looking closer into the liquid membrane. We know extraction efficiency is highly affected by the chemical composition of the liquid membrane, but the fundamental understanding is limited. Therefore, selection of liquid membranes has been based on trial and error type of experiments up to date. In the last couple of years, however, the molecular understanding has improved. In light of this, we have reviewed liquid membranes reported in the literature; for nonpolar bases (log P ≥ 2), medium polar bases (0 ≤ log P < 2), polar bases (log P < 0), and correspondingly for acids. We have selected and presented a small number of liquid membranes, based on reported extraction performance and operational stability.