Comprehensive study of buffer systems and local pH effects in electromembrane extraction

Different phosphate-, acetate- and formate buffers in the pH range 2.0-6.8 were tested for electromembrane extraction (EME) in a 96-well system. The five basic drugs haloperidol, loperamide, methadone, nortriptyline, and pethidine were selected as model analytes. The EME performance was tested with respect to extraction recovery, extraction current and pH-stability. The analytes were extracted from 200 μL buffer, through a 100 μm thick supported liquid membrane (SLM) of 2-nitrophenyl octyl ether (NPOE) immobilized in the pores of filters in a 96-well plate, and into 100 μL buffer acceptor phase. The extraction voltage was 50 V and the extraction time was 10 min. The acceptor phase was analyzed by HPLC-UV. The extraction current was ≤6 μA with all buffers, and pH was effectively stabilized during EME using buffers as donor (sample) and acceptor phase. For buffers with pH ≤ 4.8 as acceptor phase, the extraction recoveries were in the range 66-97% and with RSD <15%. With pH in the range 5.8-6.8 in the acceptor phase, the extraction recoveries decreased and were in the range 21-62%. This was attributed to elevated pH conditions in the acceptor/SLM interface. The presence of elevated pH conditions was visualized with phenolphthalein as pH sensitive color indicator. Increasing the buffer strength from 10 to 500 mM in an attempt to offset the elevated pH conditions gave no improvement, and elevated pH conditions remained. Elevated pH conditions in the acceptor/SLM interface were also observed when voltage was increased, and when NPOE was replaced with tributyl phosphate as SLM. The presence of elevated pH conditions close to the SLM in EME was discussed for the first time, and this information is highly important for future development of EME.