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Abstract: In this review, we discuss three different approaches to liquid-liquid extraction (LLE) driven by an electrical field (dc): [(1)] electro extraction, by which ionic analytes are extracted from a stagnant organic solution (immiscible with water) into water; [(2)] electrochemically-modulated LLE (known as ITIES extraction, where ITIES means “interface between two immiscible electrolyte solutions”), by which ionic analytes are extracted from a flowing aqueous sample and into a stationary organo-gel; and, [(3)] electro-membrane extraction, by which ionic analytes are extracted from an agitated aqueous sample, through an organic solvent (supported liquid membrane) sustained in the pores in the wall of a porous hollow fiber, and into an aqueous acceptor inside the lumen of the hollow fiber. We review the literature with respect to these techniques, which we compare in terms of performance and working principles, and outline the promising aspects of using electrical fields to enhance and to manipulate LLE. In conclusion, we identify LLE driven by electrical fields as an important field for future development in sample preparation for chromatography and electrophoresis. [Copyright &y& Elsevier]

Abstract: Since 1999, substantial research has been devoted to the development of liquid-phase microextraction (LPME) based on porous hollow fibers. With this technology, target analytes are extracted from aqueous samples, through a thin supported liquid membrane (SLM) sustained in the pores in the wall of a porous hollow fiber, and further into a μL volume of acceptor solution placed inside the lumen of the hollow fiber. After extraction, the acceptor solution is directly subjected to a final chemical analysis by liquid chromatography (HPLC), gas chromatography (GC), capillary electrophoresis (CE), or mass spectrometry (MS). In this review, LPME will be discussed with focus on extraction principles, historical development, fundamental theory, and performance. Also, major applications have been compiled, and recent forefront developments will be discussed. [Copyright &y& Elsevier]

Abstract: The present work has for the first time described and verified a theoretical model of the analytical extraction process electro-membrane extraction (EME), where target analytes are extracted from an aqueous sample, through a thin layer of 2-nitrophenyl octylether immobilized as a supported liquid membrane (SLM) in the pores in the wall of a porous hollow fibre, and into an acceptor solution present inside the lumen of the hollow fibre by the application of an electrical potential difference. The mathematical model was based on the Nernst–Planck equation, and described the flux over the SLM. The model demonstrated that the magnitude of the electrical potential difference, the ion balance of the system, and the absolute temperature influenced the flux of analyte across the SLM. These conclusions were verified by experimental data with five basic drugs. The flux was strongly dependent of the potential difference over the SLM, and increased potential difference resulted in an increase in the flux. The ion balance, defined as the sum of ions in the donor solution divided by the sum of ions in the acceptor solution, was shown to influence the flux, and high ionic concentration in the acceptor solution relative to the sample solution was advantageous for high flux. Different temperatures also led to changes in the flux in the EME system. [Copyright &y& Elsevier]

Abstract: Basic drug substances were transported across a thin artificial organic liquid membrane by the application of 300V d.c. From a 300μl aqueous donor compartment (containing 10mM HCl), the drugs migrated through a 200μm artificial liquid membrane of 2-nitrophenyl octyl ether immobilized in the pores of a polypropylene hollow fiber, and into a 30μl aqueous acceptor solution of 10mM HCl inside the lumen of the hollow fiber. The transport was forced by an electrical potential difference sustained over the liquid membrane, resulting in electrokinetic migration of drug substances from the donor compartment to the acceptor solution. Within 5min of operation at 300V, pethidine, nortriptyline, methadone, haloperidol, and loperamide were extracted with recoveries in the range 70–79%, which corresponded to enrichments in the range 7.0–7.9. The chemical composition of the organic liquid membrane strongly affected the permeability, and may serve as an efficient tool for controlling the transport selectivity. Water samples, human plasma, and human urine were successfully processed, and in light of the present report, electrokinetic migration across thin artificial liquid membranes may be an interesting tool for future isolation within chemical analysis. [Copyright &y& Elsevier]

Abstract: Citalopram, loperamide, methadone, paroxetine, pethidine, and sertraline were extracted exhaustively with electromembrane extraction (EME) by increasing the number of hollow fibers from one to three. Experiments reported recoveries in the range 97–115% from 1000μl spiked water samples. EME was accomplished with 200V as extraction voltage, the extraction time was set to 10min (equilibrium), and the extraction unit was subjected to 1200 revolutions per minute (rpm). The same experiment with different geometry in a stagnant system conducted with 21μl acceptor solution provided recoveries from 50μl undiluted human plasma (pH 7.4) in the range of 56–102% for the six basic model substances. In each experiment the acceptor solution was distributed into three separately hollow fibers in the same sample vial. The importance of an electrical field was verified by comparing EME with liquid-phase microextraction (LPME) under optimal conditions and demonstrated that the time needed to reach equilibrium was reduced by EME. EME–LC/MS provided linearity >0.99 (r 2 values) for the six basic model substances, and the repeatability within the low therapeutic range (10ng/ml) was in the range 5.1–21.4% RSD. LC–MS provided estimated limit of quantification (S/N =10) in the range 0.6–3.2ng/ml. Eventually, patient samples from a reference laboratory were analyzed and provided reliable results with a relative difference <14% compared to stated values from the reference laboratory. [Copyright &y& Elsevier]

Abstract: Amitriptyline, citalopram, fluoxetine, and fluvoxamine were isolated by electro membrane extraction (EME) from 70μl of untreated plasma (pH 7.4), through a supported liquid membrane (SLM) of 1-ethyl-2-nitrobenzene immobilized in the pores of a porous polypropylene hollow fiber, and into 30μl of 10mM HCOOH as acceptor solution inside the lumen of the hollow fiber. The driving force of the extraction was a 9V potential sustained over the SLM with a common battery, with the positive electrode placed in the plasma sample and the negative electrode placed in the acceptor solution. Extractions were performed under totally stagnant conditions with a very simple device for 1min (kinetic regime), and subsequently the acceptor solution was analyzed directly by liquid chromatography–mass spectrometry (LC–MS). Recoveries were 12, 13, 22, and 17% for fluoxetine, amitriptyline, citalopram, and fluvoxamine, respectively. Sample clean-up was comparable to reversed-phase solid-phase extraction (SPE), but EME required substantially less time than SPE. The time advantage of EME was further improved by parallel extraction of three samples (for 1min) with the same 9V battery. EME from plasma combined with LC–MS provided limits of quantification (S/N=10) in the range 0.4–2.3ng/ml, linearity in the range 1–1000ng/ml with r 2-values of 0.998–0.999, and repeatability in the range 3.2–8.9% RSD in the mid-therapeutic window (100ng/ml). [Copyright &y& Elsevier]

Abstract: In hollow fiber membrane liquid-phase microextraction (LPME), target analytes are extracted from aqueous samples and into a supported liquid membrane (SLM) sustained in the pores in the wall of a small porous hollow fiber, and further into an acceptor phase present inside the lumen of the hollow fiber. The acceptor phase can be organic, providing a two-phase extraction system compatible with capillary gas chromatography, or the acceptor phase can be aqueous resulting in a three-phase system compatible with high-performance liquid chromatography or capillary electrophoresis. Due to high enrichment, efficient sample clean-up, and the low consumption of organic solvent, substantial interest has been devoted to LPME in recent years. This paper reviews important applications of LPME with special focus on bioanalytical and environmental chemistry, and also covers a new possible direction for LPME namely electromembrane extraction, where analytes are extracted through the SLM and into the acceptor phase by the application of electrical potentials. [Copyright &y& Elsevier]

Abstract: The present work has for the first time demonstrated electromembrane extraction (EME) at voltages obtainable by common batteries. Five basic drugs were extracted from acidified aqueous sample solutions, across a supported liquid membrane (SLM) consisting of 1-isopropyl-4-nitrobenzene impregnated in the walls of a hollow fiber, and into an acidified aqueous acceptor solution present inside the lumen of the hollow fiber with potential differences of 1–10V applied over the SLM. Extractions from 1ml standard solutions prepared in 10mM HCl for 5min and with a potential of 10V demonstrated analyte recoveries of 50–93% in 25μl of 10mM HCl as acceptor solution. This corresponds to enrichment factors of 20–37. Similar results were obtained with a common 9V battery as power supply. Recoveries from low-voltage EME on human plasma, urine, and breast milk diluted with acetate buffer (pH 4) demonstrated recoveries in the range of 37–55% after 5min of extraction. Excellent selectivity was demonstrated as no interfering peaks were detected. Standard curves in the range of 0.0625–0.625μg/ml demonstrated correlation coefficients of 0.994–0.999. Extraction recoveries from human plasma, urine or breast milk were not found to be sensitive towards individual variations. The results show that low-voltage EME has a future potential as a simple, selective, and time-efficient sample preparation technique of biological fluids. [Copyright &y& Elsevier]

Abstract: The present work has for the first time compared extraction of basic analytes across a supported liquid membrane (SLM) based on (1) passive diffusion in a pH gradient sustained over the SLM and (2) electrokinetic migration in an electrical field sustained over the SLM. For the passive diffusion experiments, performed as liquid-phase microextraction (LPME), five basic drugs were extracted under strong agitation from alkaline samples (10mM NaOH), through 2-nitrophenyl octylether immobilized in the pores of a porous hollow fibre of polypropylene (SLM), and into 25μl of 10mM HCl as the acceptor solution. The experiments based on electrokinetic migration, performed as electro membrane isolation (EMI), were conducted under strong agitation from acidic samples (10mM HCl), through the same SLM as in LPME, and into 25μl of 10mM HCl as the acceptor solution. Whereas LPME relied on diffusion and to some extent also convection as the principal mechanisms of mass transfer, mass transfer in EMI also included a strong contribution from electrokinetic migration. Thus, extraction kinetics was improved by a factor between 6 and 17 utilizing EMI instead of LPME. This major difference in terms of speed was especially pronounced from small sample volumes (150μl), and suggest that EMI may be a very interesting future concept for miniaturized sample preparation. In addition to improved extraction kinetics, extraction rates were strongly compound dependent in EMI, opening the possibility to control the extraction selectivity by the extraction time. [Copyright &y& Elsevier]

Abstract: Electrokinetic cross membrane extraction of acidic drugs was demonstrated for the first time. The acidic drugs were extracted from an alkaline aqueous donor solution (300μl), through a thin supported liquid membrane of 1-heptanol sustained in the pores of the wall of a porous hollow fiber, and into an aqueous alkaline acceptor solution (30μl) present inside the lumen of the hollow fiber by the application of a d.c. electrical potential. The negative electrode was placed in the donor solution, and the positive electrode was placed in the acceptor solution. Optimal extractions were accomplished with 1-heptanol as the supported liquid membrane, with 50V as the driving force, and with pH 12.0 in both the donor and acceptor solutions, respectively (NaOH). Equilibrium extraction conditions were obtained after 5min of operation with the whole assembly agitated at 1200rpm. Eleven different acidic drugs were extracted with recovery values between 8 and 100%, and initial data supported that electrokinetic cross membrane extraction provided repeatable data and linear response between original donor concentration and final acceptor concentration of the acidic model compounds. [Copyright &y& Elsevier]

Abstract: For the first time, electromembrane extraction (EME) of six basic drugs of abuse from undiluted whole blood and post mortem blood in a totally stagnant system is reported. Cathinone, methamphetamine, 3,4-methylenedioxy-amphetamine (MDA), 3,4-methylenedioxy-methamphet-amine (MDMA), ketamine and 2,5-dimethoxy-4-iodoamphetamine (DOI) were extracted from the whole blood sample, through a supported liquid membrane (SLM) consisting of 1-ethyl-2-nitrobenzene (ENB) immobilized in the pores of a hollow fiber, and into an aqueous acceptor solution inside the lumen of the hollow fiber. The SLM acts as a barrier with efficient exclusion of all macromolecules and acidic substances in the sample. Due to the application of the electrical field, only the cationic compounds of interest are extracted efficiently across the membrane, thus providing extremely clean extracts for analysis with liquid chromatography–mass spectrometry, LC–MS. Recoveries in the range 10–30% were obtained from 80μl whole blood within 5min extraction time and an applied voltage of 15V across the SLM. The optimized technique was tested on real forensic whole blood samples taken from three forensic autopsy cases and on five forensic whole blood samples from living persons. The results were in agreement with the analysis using standard sample preparation methods (liquid–liquid extraction) performed on the same samples by Norwegian Institute of Public Health (NIPH), Division of Forensic Toxicology and Drug Abuse Research. Evaluation data were acceptable, with limit of detections (LODs) in the range 40–2610pg/mL, well below concentrations associated with drug abuse; linearites in the range between 10 and 250ng/mL with r 2 values above 0.9939, and with repeatability (RSD) of 7–32%. [Copyright &y& Elsevier]

Abstract: In the current work, droplet-membrane-droplet liquid-phase microextraction (LPME) under totally stagnant conditions was presented for the first time. Subsequently, implementation of this concept on a microchip was demonstrated as a miniaturized, on-line sample preparation method. The performance level of the lab-on-a-chip system with integrated microextraction, capillary electrophoresis (CE) and laser-induced fluorescence (LIF) detection in a single miniaturized device was preliminarily investigated and characterized. Extractions under stagnant conditions were performed from 3.5 to 15μL sample droplets, through a supported liquid membrane (SLM) sustained in the pores of a small piece of a flat polypropylene membrane, and into 3.5–15μL of acceptor droplet. The basic model analytes pethidine, nortriptyline, methadone, haloperidol, and loperamide were extracted from alkaline sample droplets (pH 12), through 1-octanol as SLM, and into acidified acceptor droplets (pH 2) with recoveries ranging between 13 and 66% after 5min of operation. For the acidic model analytes Bodipy FL C5 and Oregon Green 488, the pH conditions were reversed, utilizing an acidic sample droplet and an alkaline acceptor droplet, and 1-octanol as SLM. As a result, recoveries for Bodipy FL C5 and Oregon Green 488 from human urine were 15 and 25%, respectively. [Copyright &y& Elsevier]

Abstract: Electromembrane extraction (EME) of basic drugs from 10μL sample volumes was performed through an organic solvent (2-nitrophenyl octyl ether) immobilized as a supported liquid membrane (SLM) in the pores of a flat polypropylene membrane (25μm thickness), and into 10μL 10mM HCl as the acceptor solution. The driving force for the extractions was 3–20V d.c. potential sustained over the SLM. The influence of the membrane thickness, extraction time, and voltage was investigated, and a theory for the extraction kinetics is proposed. Pethidine, nortriptyline, methadone, haloperidol, and loperamide were extracted from pure water samples with recoveries ranging between 33% and 47% after only 5min of operation under totally stagnant conditions. The extraction system was compatible with human urine and plasma samples and provided very efficient sample pretreatment, as acidic, neutral, and polar substances with no distribution into the organic SLM were not extracted across the membrane. Evaluation was performed for human urine, providing linearity in the range 1–20μg/mL, and repeatability (RSD) in average within 12%. [Copyright &y& Elsevier]

Abstract: Hollow fiber protected liquid-phase microextraction (LPME) was developed for large sample volume extractions in a single step, with special emphasis on extraction of basic drugs from environmental waters. Five antidepressant drugs were extracted from 1100 or 100mL water samples, through approximately 50μL of dihexyl ether immobilized in the pores in the wall of a porous hollow fiber (liquid membrane), and into 20μL of 10mM HCl or HCOOH as the acceptor solution. Extractions were performed for 60 or 120min supported by magnetic stirring at 800rpm, and hereafter the acceptor solution was directly injected in HPLC-UV or HPLC-MS. Compared with earlier work on LPME from small sample volumes, both closing the hollow fiber and the type of liquid membrane was found to be critical for large volume extractions. The hollow fibers were carefully closed with a small piece of metal wire, dihexyl ether was used as the liquid membrane, and pH in the sample was adjusted to 11.8 with NaOH. Recoveries from 1100mL samples were in the range 33–49%, and enrichments were in the range 18,000–27,000 after 120min of extraction. With HPLC-MS, the drugs were detected down to the 5–30pgL−1 level. Within-day precision was within 12.4–20.6% R.S.D. (n =6), whereas between-day precision was within 17.6 and 37.2% R.S.D. Linearity was obtained in the range 1–500ngL−1 with r 2-values between 0.982 and 0.994. The proposed LPME system was utilized to detect the five antidepressants in wastewater from the city of Tromsø in Northern Norway. [Copyright &y& Elsevier]