Electrolyte compositions, separation

In a typical CEC arrangement, a positive separation voltage is applied to the inlet of a CEC column exhibiting a cathodic EOF, which means that the EOF flows toward the detector. Any unretained cation (exhibiting no interaction with the stationary phase) will migrate before the EOF, and any unretained anion will migrate after the EOF. Because of the experimental similarity of CEC to CE, the major experimental parameters that can be optimized are the electrolyte composition, separation voltage, and temperature. Further parameters, which are specific to CEC are the stationary phase and dimensions of the column. 

Capillary Electrophoresis. Capillary electrophoresis (ce) is an analytical technique that can achieve rapid high resolution separation of water-soluble components present in small sample volumes. The separations are generally based on the principle of electrically driven ions in solution. Selectivity can be varied by the alteration of pH, ionic strength, electrolyte composition, or by incorporation of additives. Typical examples of additives include organic solvents, surfactants (qv), and complexation agents (see Chelating agents). 

CE has been used for the analysis of anionic surfactants [946,947] and can be considered as complementary to HPLC for the analysis of cationic surfactants with advantages of minimal solvent consumption, higher efficiency, easy cleaning and inexpensive replacement of columns and the ability of fast method development by changing the electrolyte composition. Also the separation of polystyrene sulfonates with polymeric additives by CE has been reported [948]. Moreover, CE has also been used for the analysis of polymeric water treatment additives, such as acrylic acid copolymer flocculants, phosphonates, low-MW acids and inorganic anions. The technique provides for analyst time-savings and has lower detection limits and improved quantification for determination of anionic polymers, compared to HPLC. 

I. Pinnau and L.G. Toy, Solid Polymer Electrolyte Composite Membranes for Olefin/ Paraffin Separation, 7. Membr. Sci. 184, 39 (2001). 

Caron (C9) developed an electrolytic method for separating cobalt and nickel from alloys of the two metals. The ammoniacal electrolyte composition is chosen to favor the plating of cobalt from nickel. The electrolyte is passed through a number of cells in series, with continuous deposition of cobalt at the cathode and the discharge of a cobalt-free high-nickel electrolyte. Distillation of this solution after the addition of Ca(OH)2 or NaOH converts the nickel to the hydroxide and the regenerated ammonia is returned to the process. 

In particular, a molecularly imprint-based CEC system was compared with an immobilised protein-based system and a cyclodextrin-based system for the enantiomer separation of )8-adrenergic antagonists. It was shown that the MIP-based system could separate all the analytes tested without any change in the experimental conditions (i.e electrolyte composition). This was not possible using the other systems. The chromatographic efficiency, however, was superior for the cyclodextrin-based system. The poor efficiency is a rather discouraging, yet common, feature of molecular imprint-based separation systems. Also, the protein-based systems showed poor efficiency in this study, but optimised conditions can improve this [72,73]. Nevertheless, the MIP-based and protein-based systems showed good selectivity. 

Chcrkaoui and Veuthey have used nonaqueous CE for the simultaneous separation of nine nonsteroidal anti-inflammatory drugs. Separation was achieved using methanol/acetonitrile (40/60 v/v) with 20 mM ammonium acetate. Perhaps more important, the elution order of the individual components was shown to change with both solvent and electrolyte composition, indicating that selectivity can be manipulated. 

Process or device development is intimately linked to the availability of materials suitable as active or passive cell components. Design, even in its conceptual stage, is inseparable from what materials are available for electrodes or for containment, what electrolyte compositions may come into consideration, and what separators (if any) are needed. Electrochemical engineering involves not only the cell or cell process but also the often considerable chemical and physical operations (separations, chemical reactors, heat exchangers, control, etc.) that precede and follow the electrochemical step. 

In a detailed laboratory investigation of the effect of cell variables on the deuterium separation factor in electrolysis of water, Brun and co-workers [B13] have found that a depends on the cathode material, electrolyte composition, and cell temperature, generally as follows. The separation factor is higher for an alkaline electrolyte than for an add. With KOH, at 15°C, a pure iron cathode gave the highest value reported, 13.2. The separation factor for mild steel, the material used in most commercial electrolyzers, was 12.2. Values as low as S were reported for tin, zinc, and platinized steel. At 2S°C the separation factor with a steel cathode was 10.6, and at 75°C it had dropped to 7.1. 

Following this reasoning, by variation of the electrolyte compositions on the two aqueous phases independently, the B2 and the hypothetical B2 components may be separated. An experimental test of this idea is shown in Figure 11 (41). Bacteriorhodopsin was first reconstituted into a lipid bilayer with both aqueous phases maintained initially at pH 7. Acidification of the cytoplasmic aqueous phase caused reversible inhibition of the negative phase as expected for the known behavior of the B2 component. Subsequent acidification of the extracellular aqueous phase leads to the appearance of a new negative phase that has a different time course. Furthermore, this negative component has a pH dependence that is opposite to the pH dependence of the known B2 component and is enhanced by acidification. This new component is thus identified with the hypothetical B2 component. 

The membrane has two purposes. Firstly it separates the internal components of the sensor from the external working environment. This is useful in that the electrolyte composition may be maintained and that fouling of the electrode by components of the analyte mixture may be prevented. Secondly, the membrane forms a well defined diffusion barrier for the analyte to pass through. The steady state current observed under potentiostatic control is a function of the kinetics of electron transfer at the electrode and of mass transport to the electrode surface. At high potentials when electron transfer is fast the current is solely a function of mass transfer. This may be controlled by changing the thickness of the membrane and changing the membrane material. The sensitivity and selectivity of the sensor may therefore be controlled to some extent by judicious choice of the membrane material. 

Fillet M., Servais A.C., Crommen J., Effects of background electrolyte composition and addition of selectors on separation selectivity in nonaqueous capillary electrophoresis. Electrophoresis, 24, 1499-1507 (2003). 

CZE separations are based solely on the differences in the electrophoretic mobilities of charged species, either in aqueous or nonaqueous media (this latter often referred to as nonaqueous capillary electrophoresis, NACE). In CZE, the migration of a species within the capillary column is the net result of mass transport phenomena and chemical equilibria. Two modes of migration are possible, that is, under suppressed electroosmotic flow (EOF), achieved at low pH buffers or by the use of surface modified capillaries, and in the presence of EOF in the latter, two possibilities arise separations under co- and counter-EOF, depending on the relative mobility of the analyte and EOF itself. With the proper control of electrolyte composition (buffer type regarding both co- and counterions, buffer pH and concentration, as well as additives), the analyte mobility can be altered. Flow characteristics are also dependable on the electrolyte composition as well as on the capillary surface condition. 

Once sufficiently porous monoliths are created, it is still necessary to optimize the MIP system to achieve good CEC separations. Choice of functional monomer, cross-linking monomer, and molar ratio of imprint molecule to monomers will all affect the separation ability of the column. Furthermore, the electrophoretic conditions must be optimized including electrolyte composition and pH (see Chapter 20.11). 

Fig. 39.1a shows a schematic representation of the HUP/C supercapacitor a pure solid electrolyte membrane separates two composite electrodes composed of a mixture of highly polarizable electrode particles and electrolyte crystallites. Current collectors and plastic encapsulation complete the component. The manufacturing process involves (i) synthesis of electrolyte powder for the pure electrolyte membrane (ii) synthesis of the intimate mixture of composite electrode parts (iii) pressure performing of the membrane (iv) assembly of the two composite electrodes and the... 

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