Electromembrane extraction of highly polar bases from biological samples - Deeper insight into bis(2-ethylhexyl) phosphate as ionic carrier.

Similarly to many other sample extraction techniques, efficient extraction of very polar compounds with electromembrane extraction (EME) is difficult. To date, the best known strategy to improve the mass transfer of these compounds is the addition of an ionic carrier, often bis(2-ethylhexyl) phosphate (DEHP) to the supported liquid membrane (SLM). DEHP is known to work by providing ionic interactions with basic compounds, to improve the partitioning into the SLM. In this work, the behavior of DEHP during extractions was studied for the first time. Interestingly, substantial amounts of DEHP was found to leak from the SLM into the aqueous sample at pH > 4. Due to this leakage, the ion-pair formation between analytes and DEHP was moved from the sample/SLM interface (interfacial complexation) to the bulk of the sample solution (bulk-sample complexation), which improved the mass transfer of polar bases considerably. Based on this, an extraction procedure for eight polar bases with log P values from +0.7 to -5.9 was developed and optimized. The optimization demonstrated that extraction of more polar analytes was favored by bulk-sample complexation. With optimized conditions, extraction from biological samples such as urine, protein-precipitated plasma, and raw plasma were performed with recoveries >40%, except for a few analytes. In addition, the extraction system could be operated under robust conditions with relatively low current (<70 µA for plasma), and provided low variability (<16% RSD), as well as good clean-up efficiency. These findings are an important step in further scientific anchoring of EME, and development of the technique towards selective extraction of very polar substances from complex biological matrices.

Electromembrane Extraction Using Sacrificial Electrodes.

In this paper, we report the first example of employing a sacrificial electrode in the acceptor solution during electromembrane extraction (EME). The electrode was based on a silver wire with a layer of silver chloride electroplated onto the surface. During EME, the electrode effectively inhibited electrolysis of water in the acceptor compartment, by accepting the charge transfer across the SLM, which enabled the application of 500 µA current without suffering gas formation or pH changes from electrolysis of water. The electroplating strategy was optimized with a design-of-experiments (DOE) methodology that provided optimal conditions of electroplating. With an optimized electrode, 1 cm of the electrode in contact with the acceptor solution inhibited electrolysis of water for approximately 30 min at 500 µA current (redox capacity). Further, the redox capacity of the electrode was found to increase through multiple uses. The advantage of the electrode was demonstrated by extracting polar analytes at high-current conditions in a standard EME system comprising 2-nitrophenyl octyl ether (NPOE) as SLM and 10 mM HCl as sample/acceptor solutions. Application of high current enabled significantly higher recoveries than could otherwise be obtained at 100 µA. Sacrificial electrodes were also tested in µ-EME and were found beneficial by eliminating detrimental bubble formation. Thus, the sacrificial electrodes improved the stability of µ-EME systems. The findings of this paper are important for development of stable and robust systems for EME operated at high voltage/current and for EME performed in narrow channels/tubing where bubble formation is critical.

Nanoliter-Scale Electromembrane Extraction and Enrichment in a Microfluidic Chip.

This paper reports for the first time nanoliter-scale electromembrane extraction (nanoliter-scale EME) in a microfluidic device. Six basic drug substances (model analytes) were extracted from 70 µL samples of human whole blood, plasma, or urine through a supported liquid membrane (SLM) of 2-nitrophenyl octyl ether (NPOE) and into 6 nL of 10 mM formic acid as an acceptor solution. A DC potential of 15 V was applied across the SLM and served as the driving force for the extraction. The cathode was located in the acceptor solution. Because of the small area of the SLM (0.06 mm2), the system provided soft extraction with recoveries <1% for the 70 µL samples. Because of the large sample-to-acceptor-volume ratio, analytes were enriched in the acceptor solution. The enrichment capacity was 6-7-fold per minute, and after 60 min of operation, most of the model analytes were enriched by a factor of approximately 400. Because of the SLM and the direction of the applied electrical field, substantial sample cleanup was obtained. The chips were based on thiol-ene polymers, and the soft-lithography-fabrication procedure and the materials were selected in such a way that future mass production should be feasible. The chip-to-chip variability was within 23% RSD (and less than 10% in most cases) with respect to extraction recovery. Our findings have verified that nanoliter-scale EME is highly feasible and provides reliable data, and for future studies, the concept should be tested for applicability in connection with in vitro microphysiological systems, organ-on-a-chip systems, and point-of-care diagnostics. These are potential areas where the combination of soft extraction and high enrichment from limited sample volumes is required for reliable analytical measurements.