Charged solutions

First, solutes with larger electrophoretic mobilities (in the same direction as the electroosmotic flow) have greater efficiencies thus, smaller, more highly charged solutes are not only the first solutes to elute, but do so with greater efficiency. Second, efficiency in capillary electrophoresis is independent of the capillary s length. Typical theoretical plate counts are approximately 100,000-200,000 for capillary electrophoresis. 

Retention and stereoselectivity on the BSA columns can be changed by the use of additives to the aqueous mobile phase (30). Hydrophobic compounds generally are highly retained on the BSA, and a mobile-phase modifier such as 1-propanol can be added to obtain reasonable retention times. The retention and optical resolution of charged solutes such as carboxyUc acids or amines can be controlled by pH and ionic strength of the mobile phase. 

FIG. 6 Effect of an electric field on the resultant number of solvent molecules permeating the membrane as a function of time for the case of charged solute molecules, and nonpolar homonuclear solvent molecules. R refers to the field being periodically reversed [26]. 

Electropherograms of a urine sample (8 ml) spiked with non-steroidal anti-inflammatory drugs (10 p-g/ml each) after direct CE analysis (b) and at-line SPE-CE (c). Peak identification is as follows I, ibuprofen N, naproxen K, ketoprofen P, flurbiprofen. Reprinted from Journal of Chromatography, 6 719, J. R. Veraait et al., At-line solid-phase exti action for capillary electrophoresis application to negatively charged solutes, pp. 199-208, copyright 1998, with permission from Elsevier Science. 

Figure 10 Paracellular permeability of charged solutes as a consequence of their molecular radius illustrates the mechanism of molecular restricted diffusion across negatively charged pores. Solid line depicts the curve drawn through the permeability coefficients of neutral solutes, and the neutral image of positively and negatively charged permeants.

Log k appears to correlate with log P for standards between log P —0.5 to 5.0. One limitation of this method is that solutes must be electrically neutral at the pH of the buffer solution because electrophoretic mobility of the charged solute leads to migration times outside the range of Tm and TEof- Basic samples are therefore run at pH 10, and acidic samples at pH 3, thus ensuring that most weak acids and bases will be in their neutral form. This method has been used in a preclinical discovery environment with a throughput of 100 samples per week [24]. 

Ion-exchange chromatography utilizes the dynamic interactions between charged solute ions and stationary phases that possess oppositely charged groups. In separations of this type, sample ions and ions of like charge in the mobile phase, compete for sites (X) on the stationary phase ... 

Montmorillonite, In contrast, appeared to behave as an lon-exchanger with results being Interpreted in terms of competition between all positively charged solution species for the adsorption sites. 

When the force from the applied electric field on the charged solute is counterbalanced by the frictional force, the solute will move with a steady-state velocity (v = dx/dt = QE/f). 

Since the metal can be treated as a nearly perfect conductor, C is high compared with C, and cannot influence the value of the measured doublelayer capacitance. The role of the metal in the double layer structure was discussed by Rice, who suggested that the distribution of electrons inside the metal decides the properties of the double-layer. This concept was later used to describe double-layer properties at the semiconductor/electrolyte interface. As shown later, the electron density on the metal side of the interface can be changed under the influence of charged solution species (dipoles, ions). ... 

Li-dodecyl sulfate, borate, beta-cyclodextrin MEKC/Chiral system with direct UV detection Neutral and charged solutes 52... 

Bedalr, M., and El Rassl, Z. (2003). Capillary electrochromatography with monolithic stationary phases III. Evaluation of the electrochromatographic retention of neutral and charged solutes on cationic stearyl-acrylate monoliths and the separation of water-soluble proteins and membrane proteins. /. Chromatogr. A 1013, 47-56. 

The fluxes of charged solutes depend on the diffusion potential arising from differences in the mobihties of individual ions, as well as on an ion s own concentration gradient (Equation 2.21). The effect of diffusion potentials will be important if the carbonate species are a large part of the total ion concentration, as they often will be. Therefore we have for the net flux of ion B... 

To understand why depends on solution conditions, we must recognize that the behavior of solutes in solution depends upon the presence of other similar and/or dissimilar solutes, and electrolytes (/. e., charged solutes) are especially affected by the presence of all ionic species in solution. Unless we can account for these effects on the value of a for each substance, we cannot know the effective concentration of a substance at any particular analytical concentration, and we cannot comprehend the thermodynamic properties of these substances in solution. The Debye-Hilckel treatment offers us a means for estimating a. ... 

The electrophoretic separation principle is based on the velocity differences of charged solute species moving in an applied electric field. The direction and velocity of that movement are determined by the sum of two vector components, the migration and the electroosmotic flow (EOF). The solute velocity v is represented as the product of the electric field strength E and the sum of ionic mobility uUm and EOF coefficient /a OF ... 

The retention factor k of charged solutes can be determined from migration time data using Eq. (3) (10,11) ... 

As electrostatic interactions exist between the HILIC zwitterionic stationary phase and charged solutes, particular attention has to be paid to the rigorous control of experimental conditions such as pH, buffer type and ionic strength. 

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