Electrokinetic preconcentration techniques

Fortunately, applied electric fields can be very effectively exploited not only for separation but also for on-line sample preconcentration in capillaries, as evidenced by a number of well-established electrokinetic preconcentration techniques for CE (see, for instance. Chapters 13 and 19 of this book for more details). 

Another novel electrokinetic preconcentration technique is electrokinetic trapping (Fig. 3c), utilizing unique electrokinetic properties of permselective membranes. As a permselective membrane, one can use traditional membranes (such as Nation ), nanochannels, or charged polymer monoliths. When a current is applied through such a permselective membrane/ nanochannel, concentration polarization of ions can occur (even at moderate buffer concentrations), causing the ion concentration of anodic side to decrease (ion depletion) and that of cathodic side to increase (ion emichment). Both phenomena can be (and have been) utilized for concentrating biomolecules. In the cathodic side, biomolecules and ions can be enriched due to the ion emichment process. In the anodic side, ion depletion region can be used in a similar manner... 

Sample preconcentration techniques are used with two purposes (1) to increase concentration in order to achieve detection and (2) to eliminate disturbances of the electrophoretic system during hydraulic or electrokinetic sample introduction when the conductivity of the sample is significantly different from that of the analysis buffer. It is important to keep sample manipulations and modifications to a minimum, and a rule of thumb is to prepare the sample so that its composition is at the same pH as the analysis buffer. It is also advantageous... 

Capillary electrophoresis (CE) coupled to MS has the advantage of high resolution and soft ionization for biomolecules, which may be used to differentiate post-translational modifications and variants of intact proteins and oligonucleotides. Different modes of CE (capillary zone electrophoresis, capillary isoelectric focusing, capillary electrochromatography, micellar electrokinetic chromatography, nonaqueous capillary electrophoresis) to MS as well as online preconcentration techniques (transient capillary isotachophoresis, solid-phase extraction, membrane preconcentration) are used to compensate for the restricted detection sensitivity of the CE methodology [77, 78]. 

In much the same way that standard gel electrophoretic techniques diversified, so did CE. This has resulted in a family of specialized modes that collectively constitute CE. The main modes of CE that have been developed and are presently being exploited include capillary zone electrophoresis (CZE), MEKC, capillary electrokinetic chromatography capillary (CEC), capillary isoelectric focusing (CIEF), capillary gel electrophoresis (CGE), and capillary isotachophoresis (CITP). Over the last decade or so, this latter mode has been primarily used as an on-capillary preconcentration technique. 

As mentioned before, when pressure-based sample injection is employed, the maximum injection volume must be less than the effective length of the capillary. At least 10% of the effective capillary length must remain available for separation. To inject a larger sample volume, electrokinetic injection must be employed. In most online sample preconcentration techniques, the maximum amount that can be injected without loss of separation efficiency is certainly less than the capillary volume. However, with a large volume sample injection under cathodic EOF conditions in SDS MEKC, a sample devoid of the micelle can be continuously electrokinetically injected for a volume equivalent to seven times the capillary volume without significant loss of separation efficiency under favorable... 

Kim, J.B., and Terahe, S. On-line sample preconcentration techniques in micellar electrokinetic chromatography. J. Pharm. Biomed. Anal., 30, 1625, 2003. 

Electrokinetic Preconcentration Field-amplified stacking (FAS) (Fig. 3a) is a technique with a long history, first introduced by Mikkers et al. in the late 1970s [12],... 

FAS can be implemented on microchips in a very similar manner as capillary electrophoresis. However, the requirement of low ionic strength sample buffer for FAS puts limitation on its use as general preconcentration technique. Variations of the technique, such as transient isotacho-phoresis and micellar electrokinetic sweeping, have been more successfully used. Jung et al. reported (Mi-chip transient isotachophoresis... 

Cation- and anion-selective exhaustive injection sweeping (CSEI-sweep and ASEI-sweep) techniques are also available in MEKC. CSEI-sweep and ASEI-sweep are combinations of two online preconcentration techniques, sample stacking with electrokinetic injection and sweeping, that can provide more than 100,000-fold enhancements in detection sensitivity. An electropherograms of tap water analysis by CSEI-sweep-MEKC is illustrated in Figure 7. The results of this study suggest that CSEI-sweep-MEKC can be used for the analysis of quaternary ammonium herbicides in drinking water. 

Several research groups have reported ways to preconcentrate samples in Lab-on-a-Chip devices. While most on-chip preconcentration approaches evolve from conventional capillary electrophoresis and chromatographic column techniques, these preconcentration techniques play an increasingly important role in chip-based system. The basic preconcentration strategies applied on microfluidic devices can be classified into three large categories electrokinetic preconcentration, chromatographic preconcentration and membrane preconcentration. 

Plastic microdevices for high-throughput screening with MS detection were also prepared for detection of aflatoxins and barbiturates. These devices incorporated concentration techniques interfaced with electrospray ionization MS (ESI-MS) through capillaries [2], The microfluidic device for aflatoxin detection employed an affinity dialysis technique, in which a poly (vinylidene fluoride) (PVDF) membrane was incorporated in the microchip between two channels. Small molecules were dialyzed from the aflatoxin/antibody complexes, which were then analyzed by MS. A similar device was used for concentrating barbiturate/antibody complexes using an affinity ultrafiltration technique. A barbiturate solution was mixed with antibodies and then flowed into the device, where uncomplexed barbiturates were removed by filtration. The antibody complex was then dissociated and electrokinetically mobilized for MS analysis. In each case, the affinity preconcentration improved the sensitivity by at least one to two orders of magnitude over previously reported detection limits. 

Preconcentration is another commonly encountered sample pretreatment method that has been successfully integrated onto a CE chip. Ramsey and coworkers incorporated a porous membrane structure into a microfabricated injection valve, enabling electrokinetic concentration of DNA samples using homogeneous buffer conditions [5]. Sample preconcentration in nonhomogeneous buffer systems — a technique known as sample stacking —has also been achieved on-chip [6]. 

The use of recirculating flows for the preconcentration of samples in microfluidic devices is, like TGF and nanofluidic filtering (see later), a relatively new approach (early 2000s). The technique, which has been termed flow-induced electrokinetic trapping (FIET), involves the concentration and... 

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