Separation ion-exchange

Figure 47.6). By choosing which isotope to mea.sure, all of the rare earth elements can be analyzed accurately and quickly following their ion-exchange separation into just two fractions. 

Ion-exchange separations can also be made by the use of a polymer with exchangeable anions in this case, the lanthanide or actinide elements must be initially present as complex ions (11,12). The anion-exchange resins Dowex-1 (a copolymer of styrene and divinylben2ene with quaternary ammonium groups) and Amherlite IRA-400 (a quaternary ammonium polystyrene) have been used successfully. The order of elution is often the reverse of that from cationic-exchange resins. 

The results of ion exchange separations may be influenced by varying the pH, the solvent or eluant, the temperature, the nature of the ion exchange resin, the particle size, the rate of flow of eluant, and the length of the column. 

An interesting consequence of selective sorption is that conditions for partition chromatography arise which may enhance the normal ion exchange separation factors. This aspect has been utilised by Korkisch34 for separation of inorganic ions by the so-called combined ion exchange-solvent extraction method (CISE). 

Samuelson, Ion Exchanger Separations in Analytical Chemistry, John Wiley,... 

Electrolysis with a mercury cathode or with controlled cathode potential. (g) Application of physical methods utilising selective absorption, chromatographic separations, and ion exchange separations. 

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 exchange of °Co between the EDTA complexes of Co(II) (CoY ) and Co(IlI) (CoY ), in aqueous perchlorate and nitrate media at pH = 2, has been investigated by Adamson and Vorres . Using a ion-exchange separation method, a rate law... 

Table II. Carbohydrate compositions (weight percentage) of individual oligomer peaks purified (QAE-Sephadex or HPLC ion-exchange separation, respectively) from mixtures of citrus pectin oligomers or B fruit extracts Compositions shown are for peaks whose biological activity is described in Figure 4. Uronic acid values are based on colorimetric assay. Proportions of neutral sugars were determined by GC and adjusted so that totals equal 100%. In fact, some oligomers (G7 peaks 8, 9 and 10. B extract peak 10) produced small (less than 1 % of the total integrated area), unknown peaks in the GC chromatograms.

Continuous ion-exchange separation technology, which adopts counter-current processing in a rotating turntable, is currently making an impact and 70% of the world s lysine manufacture units use this technology. 

Fraser CA, Gardner GJ, Maxwell PS, Kubwabo C, Guevremont R, Siu KWM, and Berman SS (1995) Preparation and certification of a biological reference material (CARP-i) for polychlorinated dibenzo-p-dioxin and dibenzofuran congeners. Fresenius J Anal Chem 352 143-147. Gahr A, Huber N, and Niessner R (1998) Fluorimetric determination of bromate by ion-exchange separation and post-column derivatization. Mikrochrm Acta 129 281-290. 

Membranes offer a format for interaction of an analyte with a stationary phase alternative to the familiar column. For certain kinds of separations, particularly preparative separations involving strong adsorption, the membrane format is extremely useful. A 5 x 4 mm hollow-fiber membrane layered with the protein bovine serum albumin was used for the chiral separation of the amino acid tryptophan, with a separation factor of up to 6.6.62 Diethey-laminoethyl-derivatized membrane disks were used for high-speed ion exchange separations of oligonucleotides.63 Sulfonated membranes were used for peptide separations, and reversed-phase separations of peptides, steroids, and aromatic hydrocarbons were accomplished on C18-derivatized membranes. 

In the analysis of isocyanuric acid, a stabilizer used in swimming pools, ion exchange separation on an Omnipac PCX-500 was used to separate isocyanuric acid from ammelide, ammeline, and melamine (Figure 4).104 Since ammelide has one primary amine, ammeline two, melamine three, and isocyanuric acid none, derivatization and RPLC would have been problematic. 

Table 7.89 lists the main characteristics of MDHPLC (see also Table 7.86). In MDHPLC the mobile-phase polarity can be adjusted in order to obtain adequate resolution, and a wide range of selectivity differences can be employed when using the various available separation modes [906]. Some LC modes have incompatible mobile phases, e.g. normal-phase and ion-exchange separations. Potential problems arise with liquid-phase immiscibility precipitation of buffer salts and incompatibilities between the mobile phase from one column and the stationary phase of another (e.g. swelling of some polymeric stationary-phase supports by changes in solvents or deactivation of silica by small amounts of water).

Retention of solutes in ion-exchange chromatography is determined by the nature of the sample, the type and concentration of other ions present in the mobile phase, the pH, temperature, and the presence of solvents. Because there are so many variables, it is often not easy to predict what will happen in an ion-exchange separation if we change the experimental conditions. There are some useful guidelines, and to see how they work we will look at the ion-exchange separation of two weak acids (see Fig. 3.3c). 

Ion exchange equilibria are usually established or altered much faster at higher temperatures. Increasing the temperature will improve efficiency, decrease retention and may alter the selectivity of the separation. The use of organic solvents in the mobile phase will also cause retention to decrease, but because the use of solvents will change many of the variables in an ion-exchange separation, their other effects are not easy to predict. 

Better resolution is obtained here than in the ion-exchange separation of these two (Fig. 3.3c). The benzoic acid, being the more polar, is eluted first. 

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