Cation-exchange materials

Chang, C. and Lenhoff, A.M., Comparison of protein adsorption isotherms and uptake rates in preparative cation-exchange materials, ]. Chromatogr. A, 827, 281, 1998. 

Several limitations on the synthetic techniques that can be employed are imposed by the need for rapidity and minimization of handling because of the radiation hazard, and the low concentration and small physical quantities of the compounds. Purification steps should be eliminated if possible by optimizing yields. Where purification is unavoidable, simple procedures are employed such as use of anion exchange columns to remove perrhenate (the most common contaminant in the final product). A variety of disposable sample preparation columns are well suited to this purpose and are available containing small quantities of anion or cation exchange materials (0.1 to 0.5 g typically) such as quaternary ammonium-, primary ammonium-, or sulfonate-derivatized silica. Reversed phase columns are also often used (C8 or C18-derivatized silica). The purification is often thus reduced to a simple filtration step which can be performed aseptically. 

In general, the acid-sorbing resins may be classified as high molecular weight polyamines or polyimines. Thus, the original Adams and Holmes material was a polymer of m-phenylenediamine. Cation Exchange materials include synthetic resins, such as sulfonated phenol-formaldehyde or polystyrene types, and sulfonated coal. Some manufacturers have a variety of sub-types which are considered superior for particular applications. 

One fact to keep in mind with such phases is that weak acid cation-exchange materials based on carboxylic acid functional groups are subject to esterification in the presence of alcohol containing eluents. Even thongh typical eluent conditions (i.e., weakly acidic aqneous eluents containing alcohol) do not favor ester formation, such stationary phases typically exhibit slowly declining capacity when operated in the presence of alcohol-containing eluents. Consequently, such columns are normally operated with acetonitrile, tetrahydrofuran or acetone rather than with methanol, in order to avoid this problem. 

Cikalo, M. G., Bartle, K. D., and Myers, R (1999). Behavior of cation-exchange materials in capillary electrochromatography. Anal. Chem. 71, 1820-1825. 

The zeolite Si/Al—O framework is rigid, but the cations are not an integral part of this framework and are often called exchangeable cations they are fairly mobile and readily replaced by other cations (hence their use as cation exchange materials). 

Incorporation of Cation-Exchange Materials into the Metal/Epoxy... 

A simple and effective way of incorporating such a cation-exchange material into the interfacial region would be to apply it to the metal surface as a final pretreatment process. This way the released hydrogen ions would be present in the immediate vicinity of the hydroxide ion generation sites. Therefore, the hydroxide ions could be promptly neutralized and the hydrolysis of the epoxy coating by strong alkali minimized 91>. 

Elimination of coextracted materials and concentration of tetracyclines have also been accomplished using mixed-phase extraction membranes with both re-versed-phase and cation-exchange properties (294,295), or solid-phase extraction columns packed with cation-exchange materials such as CM-Sephadex C-25 (301), aromatic sulfonic acid (310), and carboxylic acid (283, 300). For the same purpose, metal chelate affinity chromatography has also been employed. In this technique, the tetracyclines are specifically absorbed on the column sorbent by chelation with copper ions bound to small chelating Sepharose fast flow column (278-281, 294-296). 

Heat stabilizers can be selected from benzothiazoles, benzimidazoles hydrazine compounds, and cation exchange materials (73). Moreover, the addition of heat stabilizers results in an improvement of the impact resistance. 

G.C. Daul, J.D. Reid, and R.M. Reinhardt, Cation exchange materials from cotton and polyvinyl... 

Abd-Alfattah, A. and K. Wada. 1981. Adsorption of lead, copper, zinc, cobalt, and cadmium by soils that differ in cation exchange materials. J. Soil Sci, 32 271. 

The separation of TCAs and related quaternary ammonium compounds on different strong cation exchangers was studied by Enlund et al. [128], Four cation-exchange materials, possessing propanesulfonic acid ligands, were prepared from different 5-pm bare-silica particles ranging from 80 to 800 A in pore size. The best separation was produced on the small-pore materials, but the efficiency and symmetry were similar on all stationary phases compared. 

To optimize the use of the amorphous sodium titanate powders as catalyst substrates, it is important to fully characterize the ion-exchange properties of the material. Further, the solution properties of the active metal to be loaded onto the support will be an important parameter in the control of the adsorption process. For example, exposure of sodium titanate to a nickel salt solution does not guarantee that nickel will be loaded onto the sodium titanate, or that the nickel, if loaded, will be dispersed on an atomic level. Sodium titanate only behaves as a cation exchange material under certain pH conditions. The solution pH also influences the hydrolysis and speciation of dissolved nickel ions (3), which can form large polymeric clusters or colloidal particles which are not adsorbed by the sodium titanate via a simple ion-exchange process. 

Uziele/ al. (U5) used porous divinyl benzene cation-exchange resins to separate the four nucleoside hydrolysates of RNA. Burtis et al. (B34, B35) used a smaller particle cation-exchange material for this separation and also investigated the various parameters that affected the separations. 

---