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 instalment 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 instalment 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, often 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.Electroextraction
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).