Ronald E. Majors

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-fiber 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 fiber and into a receiver solution inside the lumen of the hollow fiber. 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.

Figure 1

Experimental

The technical setup for the equipment used in EME is based upon earlier experience with HF-LPME and is shown in Figure 1a. The hollow fiber used is made of porous polypropylene, which is compatible with a broad range of organic solvents. The thickness of the wall of the hollow fiber is 200 µm, with a pore size of 0.2 µm and an internal diameter of 1.2 mm. A piece of the hollow fiber 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 fiber 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 fiber connected to the pipette tip is guided through a punched hole in the sample compartment cap as illustrated in Figure 1a. The pipette tip works as a mechanical support for a 0.5-mm-thick platinum wire placed inside the lumen of the hollow fiber. 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 fiber 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) instrument (11–15) or by high performance liquid chromatography (HPLC) (16).