Electromembrane Extraction: The use of Electrical Potential for Isolation of Charged Substances from Biological Matrices

Mar 01, 2010
Volume 23, Issue 3, pg 152–164

This mini-review of a new sample preparation technique called "electromembrane extraction" demonstrates how the combination of an electroextraction with hollow-fibre liquid-phase microextraction can lead to a very selective, rapid sample preparation method for the extraction of charged substances from complex matrices such as plasma, breast milk and urine. Several important parameters for successful extraction are presented and application examples of various analyte–matrix combinations are tabulated.

A lot of effort has been devoted recently to the miniaturization of existing liquid–liquid extraction methods to make them more versatile and powerful. Reducing the use of hazardous organic solvents due to environmental and cost concerns, time reduction, ease of automation, possible on-line coupling, high-throughput capability and the small amounts of matrix available are major incentives that have motivated scientists working towards miniaturization. In bioanalysis, which in this context is defined as the analysis of small drug molecules and their metabolites in biological samples, the complexity of the matrices requires selective and specific sample preparation methods to isolate the analytes of interest.

Macromolecules, salts, cellular material, fat or lipids in biological matrices such as urine, plasma, whole blood and breast milk can disturb the separation and data analysis steps. In addition, the analytes of interest can often exist in low concentrations (pg/mL–μg/mL). Therefore, a sample preparation method with a high degree of selectivity and enrichment is crucial for a successful analysis.

With these considerations kept in mind, a totally new approach to sample preparation was proposed in 2006.1 The innovative concept, called electromembrane extraction (EME), combined the technical setup for hollow-fibre liquid-phase microextraction (HF-LPME)2,3 with known principles for electroextraction.4–10 This combination offers a highly selective sample preparation method using simple equipment and gains a high degree of enrichment within a short period of time.

The EME method extracts charged substances from a small sample volume through a thin membrane of organic solvent immobilized in the wall of a hollow fibre and into a receiver solution inside the lumen of the hollow fibre. This extraction process is forced by an applied potential difference across the membrane and this combination of well-known liquid–liquid extraction processes with electrokinetic migration yields a rapid and selective sample preparation method for ionic substances. EME has shown to be compatible with a wide range of biological matrices — for example, plasma, whole blood, urine and breast milk — preparing clean extracts in a short period of time with simple and inexpensive equipment.

This instalment of "Sample Preparation Perspectives" is a mini-review of the EME technique and shows the investigation of several parameters that affect recovery of charged analytes. In addition, a number of application examples will illustrate its potential for the successful extraction of drugs from complex matrices.

Experimental


Figure 1: Diagrams of (a) EME set-up and (b) principle, with pethidine as the model substance.
The technical set-up for the equipment used in EME is based upon earlier experience with HF-LPME and is shown in Figure 1(a). The hollow fibre used is made of porous polypropylene, which is compatible with a broad range of organic solvents. The thickness of the wall of the hollow fibre is 200 μm, with a pore size of 0.2 μm and an internal diameter of 1.2 mm.

A piece of the hollow fibre is cut to a length of 25 mm and mechanically closed at the lower end with a pair of pincers. The upper end is sealed to the end of a 22 mm pipette tip by heating. To make the supported liquid membrane (SLM), the fibre is dipped in an organic solvent for 5 s to fill the pores in the walls and the excess of organic solvent gently removed with a medical wipe.

The fibre connected to the pipette tip is guided through a punched hole in the sample compartment cap as illustrated in Figure 1(a). The pipette tip works as a mechanical support for a 0.5 mm-thick platinum wire placed inside the lumen of the hollow fibre. Another platinum wire is introduced directly into the donor phase through the sample compartment cap. When coupled to a power supply, these inert wires act as electrodes, thus creating an electrical field across the SLM. In this way, the equipment makes a closed electrical circuit, where the SLM functions as a resistor.

The volume of the sample varies between 150 μL and 500 μL, depending upon the sample compartment size. However, the compartment is never filled more than half full because of the need for convection space. The sample is shaken on a platform shaker during the extraction to increase the physical movement of the analytes in the bulk donor phase and to reduce the thickness of the stagnant layer at the interface between the donor phase and the SLM. The acceptor phase volume is set to 25 μL and is introduced into the lumen of the hollow fibre by a microsyringe. When the predetermined extraction period is finished, 20 μL of the acceptor phase is collected by the microsyringe and transferred to a vial for analysis in a capillary electrophoresis (CE) instrument11–15 or by high performance liquid chromatography (HPLC).16


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