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.
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.
Figure 1: Diagram showing the principle of 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