Ion Exchange Media

Retention of one kind of ion in exchange for another kind occurs because ion-exchange media show a clearly defined order of preference for ions of different species (see section 4.4.5 on Selectivity). 

Concentration gradients can exist within the resin pore structure. Ion diffusion is complex since the resin porosity is low and this leads to steric hindrance effects and tortuous diffusion paths. Also, ion diffusion is coupled to the fixed ionic groups and the mobility of each ion within the resin due to charge balance. This coupled diffusion is present in both boundary layer and pore transport. Also, the forward and reverse rates of ion exchange can be affected by the different mobilities of the ions. 

Resin beads are synthesized as gel or macroporous materials. The macroporous resins are polymerized in the presence of a third component that is insoluble in the polymer. After this insoluble component is removed, large pores remain that allow the ions to have improved access to the interior pore structure of the beads. Macroporous resins can be useful for large ions like proteins, but they are more expensive, have lower capacity, and are harder to regenerate than the gel resins. However, they are said to be more resistant to thermal and osmotic shock as well as to oxidation and organic fouling than the gel-type resins [4]. 

The following factors are important in the choice of an ion-exchange resin  

2 Fraction or percent removal of various ions from the liquid phase (selectivity). 

3 Particle size and size distribution (flow throughput considerations). 

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Ion Exchange Resins - Spectra/Gel Ion Exchange resins are ion exchange media for use in low-pressure liquid chromatography. They are based on a polystyrene/divinylbenzene support and are available for both anion and cation exchange applications. This site will give you a reasonable... 

Zeolites are naturally occurring hydrous aluminum-sodium silicates in porous granule form. They are capable of exchanging their sodium base for calcium or magnesium and of expelling these alkaline earth metals for sodium by treatment with salt. Thus, they are a type of ion-exchange media. (Some zeolites act as molecular sieves by adsorption of water and polar compounds.)... 

The polymeric resin beads fill a need that arises from the instability of silica gel and its products to mobile phases of extreme pH (outside a pH range of about 4.0-7.0) and, consequently, are employed in most ion exchange separations. Organic moieties containing ionic groups can be bonded to silica and produce an effective ion exchange media, but the restrictions of pH on phase stability still apply. It follows that ion exchange bonded phases are less popular than the polymer bead alternatives. 

The use of other important phase systems such as exclusion media, ion exchange media and polar stationary phases such as silica gel have not been discussed as this chapter is primarily concerned with sample preparation. The last chapter will give examples of the use of these other phase systems and explain the separations obtained on a basis of molecular interactions and, at that time, the subject of solvent choice will again be discussed. 

The most common and diverse approach to cleanup (and extraction of water samples) in pesticide residue analysis is SPE. Over the last 20 years, improvements and diversifications in SPE formats, sorbent types, and apparatus have made SPE a widely used approach for a variety of applications, including the analysis of pesticide residues. SPE cartridges or disks can be likened to low-resolution HPLC columns in that similar stationary and mobile phases are used. A typical particle size in SPE is 40 pm, and the plastic cartridges are generally packed with 0.1-1 g of sorbent in plastic tubes. The choice of reversed-phase, normal-phase, and ion-exchange media in SPE is very diverse, and Table 2 lists some of the more popular SPE applications for the cleanup of pesticides. 

Ion exchange. In general very selective. Regeneration of the ion exchange media is necessary, as is treatment of the concentrate... 

Although the phenomenon of ion-exchange has been appreciated for many years, it was the development, by D Alelio in 1942, of synthetic ion-exchange media based on the polystyrene resins that extended the use of ion-exchange as an analytical tool. 

Which of the following ion-exchange media could be used effectively at pH 5 ... 

The thermostable CGTase produced by Jhermoanaerobacter sp. ATCC 53,627 is able to liquefy starch at pH 4.5 under standard industrial conditions. It is, therefore, unnecessary to pH adjust the dextrin solution prior to saccharification as is normally done in the industry today. Since there is no need for pH adjustment, significant process advantages are realiz. There is a substantial cost improvement with regard to chemicals, ion-exchange media, charcoal, etc. Also, unwanted by-product formation e.g., maltulose, colored products, base-catalyzed products are reduced. Consequently, these advantages will translate into real savings to the starch industry. 

The fixed-bed operation is usually a semicontinuous process. When the medium gets spent, the fixed-bed operation is stopped and the material is replaced with a fresh batch. In fixed-bed operation, the determination of the medium being spent is usually based on the breakpoint, which is the point at which the exit concentration of the solutes being removed starts to increase sharply to some predetermined level (typically below 10%). If interruptions in the process to replace the adsorbent or the ion exchange media are not desirable, multiple fixed beds can be connected in parallel. While one set is in operation, the other is filled with a fresh medium or, after refilling, is on standby. 

Place, B.G., Treatment Technology for Transuranic Waste Streams Cementation, Vitrification, and Incineration Testing for the Treatment of Spent Ion Exchange Media, Rep. WHC-EP-0462, Westinghouse Hanford Company, Richland, WA (1992)... 

Gravity, Temperature Gradients and Ion-Exchange Media in Formation of Fossil Brines, Am. Assoc. Petrol. Geol. Bull. (1970) 54 (4), 617-626. 

Table 7.2 Inorganic arsenic removal capacities of various sorbents and ion-exchange media in water. Ambient conditions indicate about one atmosphere pressure and room temperature or about 20-25 °C. 

Johansson, H. J., Jagersten, C., and Shiloach, J. (1996). Large scale recovery and purification of periplasmic recombinant protein from E. coli using expanded bed adsorption chromatography followed by new ion exchange media. J. Biotechnol. 48, 9-14. 

Necina, R., Amatschek, K., and Jungbauer, A. (1998). Capture of human monoclonal antibodies from cell culture supernatant by ion exchange media exhibiting high charge density. Biotechnol. Bioeng. 60, 689-698. 

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