Electrophoretic separation electroosmosis

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. 

In concluding this discussion of electrophoretic separations, we note a general model of these processes developed by Saville Palusinski (1986) in which the effect of chemical reactions on the dissolved species is taken into account. The approach is analogous to that described for the effects of chemical reactions on electroosmosis and electromigration in Section 6.6. 

As a result of electroosmosis, order of elution in a typical electrophoretic separation is, first, the fastest cation followed by successively slower cations, then all the neutrals in a single zone, and ftnally the slowest anion followed by successively faster anions (see Figure. 30-4). In srsme instances, the rate of electroosmotic flow may not be great enough to surpass the rate at which some of the anions move toward the anode, in which case these species move in that direction instead of toward the cathode. 

Lab-on-a-chip separations are reliant upon the pumping of solutions by electroosmosis and the separation of charged ions in an electric field by electrophoresis. Each ion has an electrophoretic mobility, which is proportional to its charge and inversely proportional to the frictional forces that act upon it [13]. The velocity at which an ion migrates in the electric field is dictated by its size, charge, and the applied potential, as seen in Eq. (13.1), where v is the velocity of the ion, /xg is the electrophoretic mobility, E is the applied potential, q is the charge of the ion, q is the viscosity of the solution, and r is the radius of the ion ... 

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. 

Besides electrophoretic migration, analytes in CE move by a process called electroosmosis (or electroendoosmosis). This phenomenon, occurring also in slab gel electrophoresis, produces electroosmotic flow, the electrically driven flow of the liquid within the capillary. However, while in slab gel electrophoresis the gel matrix reduces EOF to an annoyance, in CE this liquid flow can have a significant effect on the separation process. 

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-... 

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. 

Electroosmosis is one of the important processes that affect CE separation. It is a consequence of the surface charge at the capillary wall. Fused silica capillary, the most commonly used material for CE, contains surface silanol (Si-OH) groups which are ionized in contact with an electrophoretic buffer to produce the negatively charged silanoate (Si-O ) groups. The equation is expressed as Eq. 6 ... 

The electroosmotic mobility b is the coefficient of linear response. This ubiquitous approximation, which can be justified in the limits of thin double layers and/or weak fields, greatly limits possible flows and particle motions. With constant zeta and thin double layers, for example, electroosmosis in a capillary is irrotatimial (free of vortices), and particles of different shapes and sizes have the same electrophoretic mobility b = U/E and thus cannot be separated. 

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