Impact of ion balance in electromembrane extraction

Electromembrane extraction (EME) involves transfer of analyte ions from aqueous sample, through a supported liquid membrane (SLM), and into an aqueous acceptor solution under the influence of an external electrical field. In addition to target analyte ions, the sample also contains matrix ions, and both the sample and acceptor contains background buffer ions to control pH. The ratio between the total amount of ions in sample and acceptor defines the ion balance (χ). Previous publications have discussed the impact of ion balance, but conclusions are contradictory. Therefore, the current paper investigated the ion balance in more detail. From a theoretical point of view, low χ-values favor EME; buffer anions at high concentration in the acceptor migrate into the SLM, while target cations enters the SLM from the sample to maintain electroneutrality. A large number of experiments was performed in this paper to investigate the practical impact of ion balance. Twelve basic drugs were used as model analytes (0.0 < log P < 5.0), and 2-nitrophenyl octyl ether (NPOE) and NPOE + 5% di(2-ethylhexyl) phosphate (DEHP) were used as SLM. With formate buffer pH 3.75 as sample and acceptor, the impact of χ in the range 0.01-10 was studied without bias from differences in pH. Here model analytes were unaffected by ion balance. Buffers containing propionic, butyric, and valeric acid were also tested. These buffer ions migrated more into the SLM, and affected recoveries in several cases. However, this was due to ion pairing rather than effects of ion balance. Similar behaviors from sodium chloride and urine samples were observed with different χ-values. Thus, in the systems tested, almost no impact of ion balance was found, and this was attributed to very low partition of background buffer and matrix ions into the SLM. On the other hand, extractions were in several cases influenced by ion pairing phenomena.