Electrokinetics gradient

The most familiar type of electrokinetic experiment consists of setting up a potential gradient in a solution containing charged particles and determining their rate of motion. If the particles are small molecular ions, the phenomenon is called ionic conductance, if they are larger units, such as protein molecules, or colloidal particles, it is called electrophoresis. 

Electrokinetics. Electrokinetics is a tested technology that has been used for over half a century to dewater and stabilize soils, and has recently been investigated for in situ use at hazardous waste sites (23). Primarily used for metals removal, the technology utilizes an electrical field to generate a flow and concentration gradient in porous and semiporous soils. 

The potential governing these electrokinetic effects is clearly at the boundary (the face of shear) between the stationary phase (the fixed double layer) and the moving phase (the solution). This potential is called the electrokinetic potential or the zeta potential. An electrokinetic phenomenon in soil involves coupling between electrical, chemical, and hydraulic gradients. 

Kutter, J. P., Jacobson, S. C., and Ramsey, J. M. (1997). Integrated microchip device with electrokinetically controlled solvent mixing for isocratic and gradient elution in micellar electrokinetic chromatography. Anal. Chem. 69, 5165-5171. 

Electroosmosis is the bulk fluid flow that occurs when a voltage gradient is imposed across a charged membrane. Transport by convection allows the delivery and extraction of neutral and zwitterionic compounds and plays a major role in the movement of large, poorly mobile cations. Electroosmosis is an electrokinetic phenomenon, which may be described by nonequilibrium thermodynamics [24] ... 

In 1990, Bushey and Jorgenson developed the first automated system that coupled HPLC with CZE (19). This orthogonal separation technique used differences in hydrophobicity in the first dimension and molecular charge in the second dimension for the analysis of peptide mixtures. The LC separation employed a gradient at 20 (xL/min volumetric flow rate, with a column of 1.0 mm ID. The effluent from the chromatographic column filled a 10 pU loop on a computer-controlled, six-port micro valve. At fixed intervals, the loop material was flushed over the anode end of the CZE capillary, allowing electrokinetic injections to be made into the second dimension from the first. 

Chen, C.H., Lin, H., Lele, S.K., Santiago, J.G., Electrokinetic microflow instability with conductivity gradients. Micro Total Analysis Systems 2000, Proceedings pTAS 2000 Symposium, Enschede, Netherlands, May 14-18, 2000, 983-987. 

For isocratic mode of CEC separations, standard CE instrumentation is sufficient. This applies particularly for equipment that has the provision of column pressurization. In practice this is achieved by applying a gas under a pressure of 2-12 bar to both inlet and outlet vials. Column thermostating in CEC is regarded mandatory to avoid excessive radial temperature gradients within the capillary. In such instruments, sample is typically injected electrokinetically and alternatively by applying the external gas pressure to the sample vial. Detection occurs on-column i.e. directly through a non-packed section of the capillary following immediately the end of the bed. 

Fig. 8.3. Isocratic (a) and gradient (b) separation of PTH amino acids. Column, 250 x 0.075 mm i.d. packed with 3.5 p.m/80 A Zorbax ODS eluents, (A) 2 mmol/1 ammonium acetate, pH 7.0, (B) 2 mmol/1 ammonium acetate, pH 7.0, 90% acetonitrile isocratic elution with 30% B in (a) gadient elution with 30-80% B in 5 min, followed by 80% for 5 min in (b) flow rate of mobile phase through inlet reservoir, 100 pl/min applied voltage, 15 kV Detection, ESI-MS, m/z 100-2000, 0.5 s/spectrum integration time sheath liquid, 1 mmol/1 ammonium acetate, pH 7.0, 90% methanol, 3 pl/min injection, electrokinetic, 2 kV, 2 s sample, PTH-asparagine, PTH-glutamine, PTH-threonine, PTH-glycine, PTH-tyrosine, PTH-alanine (in order of elution). (Reproduced from ref. [82] with permission of Elsevier Sciences B. V.).

Fig. 10.17. Capillary electrochromatography of PTH-amino acids with gradient elution. Column, 207 (127) mm x 50 pm i.d. packed with 3.5 pm Zorbax ODS particles, 80 A pores. Starting eluent (A), 5 mM phosphate, pH 7.55, 30% acetonitrile gradient former (B), 5 mM phosphate, pH 7.55, 60% acetonitrile flow-rate (through inlet reservoir), 0.1 ml/min gradient, 0-100% B in 20 min voltage 10 kV current, 1 pA temperature, 25°C UV detection at 210 nm electrokinetic injection, 0.5 s, 1 kV. Peaks in order of elution formamide PTH-asparagine PTH-glutamine PTH-threonine PTH-glycine PTH-alanine PTH-tyrosine PTH-valine PTH-proline PTH-tryptophan PTH-phenyialanine PTH-isoleucine PTH-leucine. The concentration of the PTH-amino acids dissolved in the mobile phase was 30-60 pg/ml. Reprinted with permission from Huber et al. [68]. Copyright 1997 American Chemical Society.

FIGURE 3 (A) Representative analytical anion exchange (AX) trace for crude 20 mer phosphor-othioate and (B) representative QCGE trace for the same oligonucleotide. Conditions for the analytical AX Column, resource Q I mL column (6.4 X 30 mm) buffer A, 20 mM NaOH buffer B, A + 2.5 M NaCI gradient, 0- 100% B in 50 min, temperature 70°C sample, ISIS 2302 crude, a 20 mer phosphorothioate. Conditions for analytical QCGE column eCap gel-filled capillary (Beckman-Coulter, Fullerton, CA), 14.1 kV run voltage, 40°C, electrokinetic injection 13 s at 10 kV system, Beckman P/Ace 5000 sample ISIS 2302 crude sample at concentration of 0.01 mg/mL... 

In biology, we are typically concerned with the study of electrostatic systems in which electromagnetic interactions are ignored and the electric field is the gradient of the electrostatic potential E = — V. The differential equation for electrokinetic drift follows from Equation (3.45). 

Gradients of electric potential and pressure govern the behavior of ionic systems, selective membranes, and ultra-centrifuges. In electrokinetic phenomena, induced dipoles can cause separations, such as dielectrophoresis and magnetophoresis, which may be especially important in specialized separations. Diffusion potential is the interference between diffusion and electric conduction in an anisotropic crystal where heat conduction occurs in one direction caused by a temperature gradient in another direction. 

The influence of ions on electrokinetic effects can be readily explained with the aid of Stern s concept of the double layer. Substances like silicon carbide, cellulose, sulfur and carbon, which do not ionize, are negatively charged in contact with water and the addition of small amounts of uni-univalent electrolytes tends to increase this charge. It is probable that in these cases the negative zeta-potential is due in the first place to the firm attachment to the surface of hydroxyl ions from the water and possibly also of anions from the electrolyte. An equivalent number of positive ions, some closely held in the fixed part of the double layer and the remainder in the diffuse portion, will be left in the solution. The potential gradient between the solid surface and the bulk of the liquid, which is pure water or a dilute solution, is shown diagrammatically in Pig. 128,1. If the electrolyte concentration is increased, there will be... 

One of the more recent technologies in pollution treatment and remediation is based on the electrokinetic decontamination of soils [126-128], in which a dc potential (a few volts per centimeter) is applied across two inert electrodes embedded in a soil mass. This applied potential causes decomposition of the soil water to occur at the two electrodes. The migration of contaminants in the electric field, water transport, and reactions at the electrodes, as well as reactions caused by the induced pH gradient, can effectively clean soils. Acar et al. [127] reviewed electrokinetic remediation for the removal of metals and other inorganic contaminants from soil as well as its use in the extraction of organics from contaminated soils. 

Alshawabkeh and Acar [134] developed a theoretical model describing contaminant transport during electrokinetic remediation, based on the equilibrium chemistry of the solution and the electrochemical properties of the contaminants within the soil. The model demonstrates that the removal of ions from soils is caused by a combined effect of electrical, hydraulic, and concentration potential gradients that exist in the soil. The factors controlling the rate of change of these potential gradients included the volume compressibility and electrical capacitance of the soil, and the conductivities of the ions. 

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