Electroosmotic flow modifier

Burgi, D.S., Large-volume stacking of anions in capillary electrophoresis using an electroosmotic flow modifier as a pump. Area/. Chem., 1993, 65 3726-3729. 

Phosphate running buffer (pH 8) of 20 mM containing electroosmotic flow modifier, 1 mM cetyltrimethylammonium bromide, provides good separation with —20 kV voltage. Detection wavelength of 214 nm can be applied. 

Track-etched polycarbonate filters and hydrophilic cellulose membrane filters (both 0.2 micron pore diameter, Whatman Filter Co.) were used to recover supernatant solutions. Total chelating agent and metal ion-chelating agent complex concentrations were determined using a Quanta 4000E capillary electrophoresis instrument (Waters Corp.) and bare fused-silica capillaries (75 microns wide, 60 cm long. Polymicro Technol.). The capillary electrolyte consisted of 25 mM phosphate buffer (pH 7.0) and 0.5 mM tetradecyl trimethylammonium bromide (TTAB) electroosmotic flow modifier (see (29)). 

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. 

The low electroosmotic flow (EOF) of the PMMA chip material facilitated the rapid switching between analyses of explosive-related cations and anions using the same microchannel and run buffer (and without an EOF modifier) [29], This led to a rapid (<1 min) measurement of seven explosive-related cations and anions down to the low micromolar level. The presence of an 18-crown-6 ether modifier in the run buffer allowed separation of the peaks of the co-migrating ammonium and potassium ions. 

While the external electrical field approach is a method directly modifying the zeta-potential of the capillary wall, it is not applicable with commercial apparatuses. The back-pressure technique, on the other hand, has the disadvantage that the flat electroosmotic flow profile is disrupted by superposition of a pressure-driven laminar flow profile hence, the efficiency of separation deteriorates. 

Wang, B., Chen, L., AbdulaM-Kanji, Z., Horton, J.H., Oleschuk, R.D., Aging effects on oxidized and amine-modified poly(dimethylsiloxane) surfaces studied with chemical force titrations Effects on electroosmotic flow rate in microfluidic channels. Langmuir, 2003, 19, 9792-9798. 

Henry, A.C., Waddell, E.A., Shreiner, R., Locascio, L.E., Control of electroosmotic flow in laser-ablated and chemically modified hot imprinted poly(ethylene tereph-thalate glycol) microchannels. Electrophoresis 2002, 23, 791-798. 

The key operational parameter in free-solution capillary electrophoresis is the pH of the running buffer, as the electroosmotic flow and ionization of the analyte can be regulated by this variable. The role of buffers in capillary electrophoresis has been discussed in detail, with emphasis on buffer concentration, buffer type, and pH effects [10]. The effect of organic solvents on separation and migration behavior has been studied for dipeptides [11] and somatostatin analog peptides [12]. The order of migration as well as the selectivity may be manipulated by organic modifiers in... 

Capillary zone electrophoresis (CZE) is the most simple and widely used mode in CE. Separations take place in an open-tube, fused silica capillary under the influence of an electric field. The velocity of the analytes is modified by controlling the pH, viscosity, or concentration of the buffer, or by changing the separation voltage. The electroosmotic flow is often used in this mode to improve resolution or to shorten analysis times. 

Magnuson, M.L., Creed, J.T., Brockhoff, C.A. Speciation of selenium and arsenic compounds by capillary electrophoresis with hydrodynamically modified electroosmotic flow and on-line reduction of selenium(VI) to selenium(IV) with hydride generation inductively coupled plasma mass spectrometric detection. Analyst 122, 1057-1062 (1997)... 

The concentration of 25 mM was shown to increase resolution as well as migration times. Acetonitrile was added as an organic modifier due to its capability to change the zeta potential as well as the buffer viscosity, which resulted in changes in the electroosmotic flow (EOF). Therefore, the distribution of solutes between the buffer and the micelle was strongly affected by the involvement of acetonitrile. This was then shown to improve the poor resolution between ezetimibe and simvastatin [57]. 

An alternative method, recently proposed, involves the use of modifiers in the running buffers, where the modifiers interact with the capillary walls to reduce, but not eliminate, electroosmotic flow. A continuous slow migration toward the detector thus occurs throughout the run. Using 0.1% methylcellulose as a modifier, species having pi values of 0.01 pH unit can be separated. 

Many separations of anions require that the direction of EOF be reversed. This is accomplished by adding a flow modifier, such as a quaternary ammonium salt with a long hydrocarbon chain, to the BGE. A thin layer of the flow modifier is adsorbed on the capillary surface. This gives the surface a positive charge and causes electrolyte anions to give an electroosmotic flow towards the anode. 

Figure 10.6 shows that the greatest differences between electrophoretic mobilities and electroosmotic mobility occur around 200 mM salt in the BGE. Figure 10.7 shows an excellent separation of inorganic anions at pH 8.5 in 220 niM sodium chloride. The high salt concentration suppresses the EOF sufficiently that no flow modifier is... 

Organic solvents Modify electroosmotic flow, increase solubility of organic ions, modify ion solvation, reduce wall interactions... 

Metal ions Modify mobility of anions and electroosmotic flow... 

---