Reverse electroosmotic flow

The optimum pH for separating cations is pK + 0.30 K. K.-C. Yeung and C. A. Lucy, Isotopic Separation of [14N]- and fI5N] Aniline by Capillary Electrophoresis Using Surfactant-Controlled Reversed Electroosmotic Flow, Anal. Chem 1998, 70. 3286. 

Figure 13.34 Use of cationic surfactant to reverse electroosmotic flow (a) normal EOF toward cathode (no surfactant) and (b) reversed EOF toward anode (cationic surfactant bilayer). (Katz et al., used with permission.)...

Hardenborg, E., Zuberovic, A., Ullsten, S., Soderberg, L., Heldin, E., and Markides, K. E., Novel polyamine coating providing non-covalent deactivation and reversed electroosmotic flow of fused-sihca capillaries for capillary electrophoresis. Journal of Chromatography A 2003, 1003, 217-221. 

Schematic diagram showing the reversal of electroosmotic flow.

The direction of electroosmotic flow and, therefore, the order of elution in CZE can be reversed. This is accomplished by adding an alkylammonium salt to the buffer solution. As shown in Figure 12.45, the positively charged end of the alkylammonium ion binds to the negatively charged silanate ions on the capillary s walls. The alkylammonium ion s tail is hydrophobic and associates with the tail of another alkylammonium ion. The result is a layer of positive charges to which anions in the buffer solution are attracted. The migration of these solvated anions toward... 

Separation is performed using free-zone electrophoresis, where the capillary is filled with a separating buffer at a defined pH and molarity. This buffer is also called a BGE. During separation, the polarity is set to cathodic or anodic mode, also called normal and reverse mode, depending on the charge of the molecule cation or anion. For anions, the capillary is usually dynamically coated with an electroosmotic flow (EOF) modifier to reverse the EOF and separate the analytes in the co-electroosmotic mode. 

Diress, A. G., and Lucy, C. A. (2004). Electroosmotic flow reversal for the determination of inorganic anions by capillary electrophoresis with methanol-water buffer. /. Chromatogr. A 1027, 185-191. 

While the electroosmotic flow is a positive attribute of electrophoresis in most cases, there are instances where EOF needs to be carefully controlled. For example, too high an electroosmotic flow may decrease resolution, especially of cations with similar mobility. In a different case, when analyzing anions of very different mobilities (anorganic and organic) in one run, the electroosmotic flow needs to be reversed (5). Furthermore, alternative elec-... 

For an analyte cation moving in the same direction as the electroosmotic flow, xep and xeo have the same sign, so xapp is greater than xcp. Electrophoresis transports anions in the opposite direction from electroosmosis (Figure 26-20b), so for anions the two terms in Equation 26-11 have opposite signs. At neutral or high pH, brisk electroosmosis transports anions to the cathode because electroosmosis is usually faster than electrophoresis. At low pH, electroosmosis is weak and anions may never reach the detector. If you want to separate anions at low pH, you can reverse the polarity of the instrument to make the sample side negative and the detector side positive. 

Figure 26-24 Charge reversal created by a cationic surfactant bilayer coated on the capillary wall. The diffuse part of the double layer contains excess anions, and electroosmotic flow is in the direction opposite that shown in Figure 26-20. The surfactant is the didodecyldimethylammonium ion, (n-C,2H25)N(CH3)2, represented as in the illustration.

The type of electrophoresis we have been discussing so far is called capillary zone electrophoresis. Separation is based on differences in electrophoretic mobility. If the capillary wall is negative, electroosmotic flow is toward the cathode (Figure 26-20) and the order of elution is cations before neutrals before anions. If the capillary wall charge is reversed by coating it with a cationic surfactant (Figure 26-24) and the instrument polarity is reversed, then the order of elution is anions before neutrals before cations. Neither scheme separates neutral molecules from one another. 

In summary, there is evidence that the skin presents a weak cation permselectivity [25,76,77,80,93,125], which can be reversed by acidifying the pH of the solutions bathing the skin [10,23,76,77]. At pH>p/, the skin is negatively charged and electroosmotic flow proceeds in the anode-to-cathode direction. At pH < pi, the skin becomes positively charged and electroosmotic flow reverses to the cathode-to-anode direction. Under the application of an electric field, counterions (cations at physiological pH) are preferentially admitted into the skin. As a consequence, the sodium and chloride transport numbers are 0.6 and 0.4, respectively, during transdermal iontophoresis (in contrast to their values in a neutral membrane tNa = 0.45 rCi = 0.55) [126]. 

Santi, P., and R.H. Guy. 1996. Reverse iontophoresis—Parameters determining electroosmotic flow I. pH and ionic strength. J Control Release 38 159. 

The capillary gel electrophoresis separation of a three-component, 3 P-labeled, nucleotide mixture is illustrated in Figure 10. It is interesting to note that the migration order of ATP and CTP is reversed with respect to the free solution separations presented in Figures 8 and 9. This is caused by the absence of electroosmotic flow in the gel-filled capillary. 

A. E. Nassar, S. Lucas, W. Jones, and L. Holland, Separation of chemical warfare agent degradation products by the reversal of electroosmotic flow in capillary electrophoresis, Anal. Chem. 70, 1085-1091 (1998). 

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