Capillary electrophoresis electroosmosis

A buffer containing 1 mM Mg804 and 1 mM CaCl2 strongly reduces electroosmotic flow in capillary electrophoresis. Electroosmosis is restored by adding 3 mM EDTA to the buffer. 8uggest an explanation. 

The electroosmotic flow profile is very different from that for a phase moving under forced pressure. Figure 12.40 compares the flow profile for electroosmosis with that for hydrodynamic pressure. The uniform, flat profile for electroosmosis helps to minimize band broadening in capillary electrophoresis, thus improving separation efficiency. 

This chapter introduces the basic concepts and principles of capillary electrophoresis (CE), presenting some background on electrophoresis and capillary electrophesis and describing the components of the system. The two main types of CE, capillary zone and micellar electrokinetic electrophoresis, are described, and a selection strategy, based on the two types of separation, electrophoretic migration and electroosmosis, is presented. 

This chapter discussed the concepts and principles of high performance capillary electrophoresis. A brief history of CE was followed by an analysis of the components of an CE system. Two types of separation were presented, electrophoretic migration and electroosmosis. Several types of CE were dis-... 

An optimum of flow profile has recently been achieved for capillary electrophoresis [76], when the mobile phase migration is done by electroosmosis. It is the situation that has been utilised for electrochromatography. For planar chromatography, the optimum of the linear flow velocity is approximated when the convex shape of a forced-flow profile chiefly counterbalances the concave profile of the advancing meniscus, it is possible to reach optimal efficiency as a function of linear flow velocity [67]. This is demonstrated in Fig. 10.6. At the optimum of efficiency, the microflow profile is nearly linear as the convex and concave forms of laminar flow and the concave form of the advancing meniscus counterbalance each other (Fig. 10.7). 

The instrumental arrangement commonly employed in capillary electrophoresis is shown in Figure 12.1. With untreated silica capillaries, electroosmosis causes the buffer to flow from the anode to the cathode. Samples are introduced at the anodic end, and an on-column or post-column detector is placed at or near the cathodic end of the capillary. The high-voltage produced by the power supply and present in the anodic buffer reservoir is enclosed in a protective shield. 

Electroosmosis is often desirable in certain types of capillary electrophoresis, but in other types it is not. Electroosmotic flow can be minimized by coating the inside capillary wall with a reagent like trimethylchlorosilane to eliminate the surface silanol groups. 

Today, Caliper Life Sciences, MA, USA [255] and Agilent Technologies, CA, USA [256] offer microfluidic chips for DNA and Protein analysis. Liquid propulsion is provided via electroosmosis and combined with capillary electrophoretic separation. The sample is electroosmotically transported and metered inside the chip, then separated via capillary electrophoresis and analysed by fluorescence detection. (Fig. 14). The whole assay is performed within minutes, instead of hours or days. 

Electroosmosis is one of several electrokinetic effects that deal with phenomena associated with the relative motion of a charged solid and a solution. A related effect is the streaming potential that arises between two electrodes placed as in Figure 9.8.1 when a solution streams down the tube (essentially the inverse of the electroosmotic effect). Another is electrophoresis, where charged particles in a solution move in an electric field. These effects have been studied for a long time (37, 38). Electrophoresis is widely used for separations of proteins and DNA (gel electrophoresis) and many other substances (capillary electrophoresis). 

Mosher, R.A., Zhang, C.-X., Caslavska, J., and Thormann, W., Dynamic simnlator for capillary electrophoresis with in situ calculation of electroosmosis, J. Chromatogr. A, 716,17,1995. 

Finally, if heavy beads such as silica (SG = 2.1) are used, they can be assembled into a regular matrix by sedimentation [20]. One particular advantage of silica beads is that, after assembling them in a glass cell, sucrose can be added to the electrophoresis buffer to closely match the index of refraction of silica. This leads to a transparent material that is ideal for optical detection (at the expense of separation time, since sucrose increases the medium s viscosity, and thus reduces electrophoretic mobility). A second advantage is that some of the well-documented surface treatment strategies against electroosmosis developed for capillary electrophoresis can be directly transposed. 

Capillary electrophoresis involves two simultaneous processes called electrophore sis and electroosmosis. Electrophoresis is the migration of ions in an electric field. Electroosmosis pumps the entire solution through the capillary from the anode toward the cathode. Superimposed on this one-way flow are the flow of cations, which are attracted to the cathode, and the flow of anions, which are attracted to the anode. In Figure 23-14, cations migrate from the injection end at the left toward the detector at the right. Anions migrate toward the left. Both cations and anions are... 

The development of novel miniatme modules for FIA with a fibre optic detector (Manz et al. 1991) had a positive effect on further development. In 1992, a complete system for capillary electrophoresis on a planar glass carrier was established successfully (Harrison et aL 1992). Thus, electroosmosis proved to be of value for solution transport as well as for injection operations. 

The mechanism by which analytes are transported in a non-discriminate manner (i.e. via bulk flow) in an electrophoresis capillary is termed electroosmosis. Eigure 9.1 depicts the inside of a fused silica capillary and illustrates the source that supports electroosmotic flow. Adjacent to the negatively charged capillary wall are specifically adsorbed counterions, which make up the fairly immobile Stern layer. The excess ions just outside the Stern layer form the diffuse layer, which is mobile under the influence of an electric field. The substantial frictional forces between molecules in solution allow for the movement of the diffuse layer to pull the bulk... 

The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

The word electrokinetic implies the joint effects of motion and electrical phenomena. We are interested in the electrokinetic phenomena that originate the motion of a liqnid within a capillary tube and the migration of charged species within the liquid that surrounds them. In the first case, the electrokinetic phenomenon is called electroosmosis whereas the motion of charged species within the solution where they are dissolved is called electrophoresis. This section provides a brief illns-tration of the basic principles of these electrokinetic phenomena, based on text books on physical chemistry [7-9] and specialized articles and books [10-12] to which a reader interested to stndy in deep the mentioned theoretical aspects should refer to. 

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