Electrical Potential as a Driving Force in Sample Preparation

Jan 01, 2014
Volume 32, Issue 1, pg 14–22

To achieve lower detection limits and better selectivity, researchers are always exploring new ways to enhance sample preparation and separation technology. The addition of electrical potential to existing technologies or for entirely new approaches is relatively unexplored but can create another dimension of selectivity in sample preparation. In addition, speed and sensitivity are added benefits. This installment looks at the use of electroenhancement to solid-phase extraction, solid-phase microextraction, and membrane and liquid extraction.

We are all familiar with the role of electromigration in enhancing separations in capillary and gel electrophoresis and related techniques and in capillary electrochromatography. In addition, electrical potential has been used in electrochemical and other forms of chromatographic detection. The use of electrical potential to enhance and add another dimension of selectivity to sample preparation is still a relatively new, noncommercialized technique. The purpose of this installment is to discuss how electrically driven enhancement has already proven to offer some major advantages to the preparation of samples whose analytes of interest are ionized or can become ionized by the appropriate adjustment of pH to the separation media. We explore five sample preparation methods where the application of a charge across barriers provides enhanced selectivity, oftentimes leaving undesired matrix and impurities behind, thereby accomplishing the goals of sample preparation.

The main purpose of sample preparation is to transport the analytes of interest into a medium that is compatible with the analysis method, to remove interferences, and to concentrate the analytes so that they can be more easily detected and quantitated. Sample preparation using electrolytic potential in bioanalysis is particularly attractive since many of the water-soluble compounds encountered are charged or could be made charged.


Figure 1: Diagram showing the principle of electroextraction.
Most of us are familiar with liquid–liquid extraction (LLE) because it is still one of the most popular techniques used for sample cleanup. In LLE, two immiscible liquids are used to partition analytes from a sample into one of the two phases, an organic phase and an aqueous phase. After shaking the phases together in a separatory funnel, analytes that prefer the organic medium are partitioned into it while those analytes that prefer the aqueous media, usually polar compounds or ionized compounds, are partitioned into it. Electroextraction (EE) is a sample enrichment technique that focuses charged analytes from a large volume of one phase into a small volume of aqueous phase through the application of an electric current. In two-phase EE, the immiscible phases that are electrically conductive are kept between electrodes, and upon addition of an electric field, charged particles travel from one phase to another, thereby separating anions and cations, as depicted in Figure 1. Three-phase systems are also used where anions and cations are attracted into the two outer phases leaving uncharged particles in the middle phase. Generally, organic solvents need to contain a small amount of water so that they can be made conductive. In two-phase EE, a rapid electromigration takes place because ions in the organic phase are subjected to very high electric field strength resulting from low conductivity. Because the electric field of the aqueous acceptor phase is much lower, ions in the organic phase will migrate at high velocity to be concentrated just beyond the liquid–liquid interface.

The apparatus used to conduct electroextraction consists of a vial with a conical bottom, a bottom grounded electrode in contact with the lower aqueous layer and a capillary (injection port) to inject the sample solution, and an upper electrode in contact with the upper organic phase. The EE experiment can also be performed in an electrophoresis-like capillary and this version is referred to as capillary electroextraction (cEE). Lindenburg and colleagues (1) used a wide-bore capillary connected to a two-way, 10-port switching valve interfaced to a liquid chromatography–mass spectrometry (LC–MS) system. This system allowed up to 100 μL of sample to be extracted, and they showed improved detection of peptides in an unspiked plasma sample. For these real samples, limits of detection values were in the 10–50 nM range for several angiotensin peptides, which indicates the potential for cEE as an on-line sample concentrating technique. In another application, this same laboratory applied cEE–LC–MS to urine metabolites including amino acids and acylcarnitines (2).

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