Capillary electrophoresis, dispersion

The uniformity of the flow profile is of great importance for high separation efficiencies in capillary electrophoresis. Dispersion effects due to flow non-uniformity, as present in all forms of pressure-driven chromatography, are (ideally) absent in capillary electrophoresis. Thus the ultimate separation efficiency in capillary electrophoresis is determined only by longitudinal diffusion... 

Gotti et al. [42] reported an analytical study of penicillamine in pharmaceuticals by capillary zone electrophoresis. Dispersions of the drug (0.4 mg/mL for the determination of (/q-penicillaminc in water containing 0.03% of the internal standard, S -met hy I - r-cystei ne, were injected at 5 kPa for 10 seconds into the capillary (48.5 cm x 50 pm i.d., 40 cm to detector). Electrophoresis was carried out at 15 °C and 30 kV, with a pH 2.5 buffer of 50 mM potassium phosphate and detection at 200 rnn. Calibration graphs were linear for 0.2-0.6 pg/mL (detection limit = 90 pM). For a more sensitive determination of penicillamine, or for the separation of its enantiomers, a derivative was prepared. Solutions (0.5 mL, final concentration 20 pg/mL) in 10 mM phosphate buffer (pH 8) were mixed with 1 mL of methanolic 0.015% 1,1 -[ethylidenebis-(sulfonyl)]bis-benzene and, after 2 min, with 0.5 mL of pH 2.5 phosphate buffer. An internal standard (0.03% tryptophan, 0.15 mL) was added and aliquots were injected. With the same pH 2.5 buffer and detection at 220 nm, calibration graphs were linear for 9.3-37.2 pg/mL, with a detection limit of 2.5 pM. For the determination of small amounts of (L)-penicillamine impurity, the final analyte concentration was 75 pg/mL, the pH 2.5 buffer contained 5 mM beta-cyclodextrin and 30 mM (+)-camphor-10-sulfonic acid, with a voltage of 20 kV, and detection at 220 nm. Calibration graphs were linear for 0.5-2% of the toxic (L)-enantiomer, with a detection limit of 0.3%. 

In a method of capillary electrophoresis, 1 pg of dextran was dispersed in alkaline buffer containing fluorescein, and the dextran was detected by negative fluorescence (Richmond and Yeung, 1993). 

C. A. Keely, T. A. A. M. van de Goor, and D. McManig-ill, Modeling flow profiles and dispersion in capillary electrophoresis with nonuniform zeta potential, Arml. Chem. 66 4236 (1994). 

A. Vinther and H. Soeberg, Mathematical model describing dispersion in free solntion capillary electrophoresis nnder stacking conditions, J. Chromatogr. 559 3 (1991). 

FIG. 2 pH dependence of the zeta potential of a commercial kaolin dispersion determined by means of capillary electrophoresis ( ) and acoustophoresis (o). 

Capillary electrophoresis is the most used method for determination of the zeta potential of unmodified and polymer-modified diluted kaolin dispersions. 

Ghosal, S., Electrokinetic flow and dispersion in capillary electrophoresis. Anna. Rev. Fluid Mech, 2006, 38 309-338. 

The most common chiral additives used in chiral capillary electrophoresis with micellular solutions (mostly micelles of sodium dodecylsulphate) are derivatives of the three basic cyclodextrins. This system might be considered more of a chromatographic process than one that is electrophoretic, as the solutes are distributed between the aqueous electrolyte phase and the cyclodextrin/micelle phase. The derivatized cyclodextrin additive will also be distributed between the electrolyte and the micelles, the extent of which will depend on the type of derivatized cyclodextrin and its capacity for dispersive or polar interactions with the micelles. As the cyclodextrin additive itself partitions between the electrolyte and the micelle (albeit the distribution under certain circumstances may be small) some of the chiral additive will be distributed on the micelle surface and will act as a chiral stationary phase. 

The zeta potential (Q is thought to be the same as the Stem potential which is defined at the plane dividing the Stem layer and the diffuse layer of the EDL. Zeta potential is an experimentally measurable electrical potential that characterizes the EDL, and it plays an important role in many apphcations such as stability of colloidal dispersion, characterization of biomedical polymers, electrokinetic transport of particles, and capillary electrophoresis, etc. In addition, zeta potentials of the particles and the channel wall are cmcial to the design and process control of microfluidic devices. A review on measuring the zeta potential of microfluidic substrates was provided by Kirby and Hasselbrink [3]. 

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