Ion exchange, separations with

Sherman, J.D. (1984) Ion exchange separations with molecular sieve zeolites, in Zeolites Science and Technology (eds G. Ohhlmann, F.R. Ribeiro, A.E. Rodrigues, LD. Rollmann, C. Naccache), NATO Scientific Affairs Division/Martinus Nijhoff, The Hague, pp.583-623... 

Capillary electrochromatography (CEC) is a rapidly emerging technique that adds a new dimension to current separation science. The major "news" in this method is that the hydrodynamic flow of the eluting liquid, which is typical of HPLC, is replaced by a flow driven by electro-endoosmosis. This increases considerably the selection of available separation mechanisms. For example, combinations of traditional processes such as reversed-phase- or ion-exchange- separations with electromigration techniques are now possible. Also, CEC is opening new horizons in the separation of non-polar compounds, and thus represents an alternative to the widely used micellar electrokinetic chromatography. 

Although there is an increasing trend toward precolumn derivatization methods, there are several advantages to an ion-exchange separation with postcolumn derivatization. The first is that there is little sample preparation protein hydrolysates can be injected directly into the column for analysis by LC. Unlike precolumn methods, it is not necessary to separate reaction side-products from the reaction mixture instead, they become a source of background. With postcolumn methods there is more sample-to-sample consistency because of the robustness of the ion-exchange separation and the fact that the reaction time is determined by the column size. 

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. 

Theory. The anion exchange resin, originally in the chloride form, is converted into the nitrate form by washing with sodium nitrate solution. A concentrated solution of the chloride and bromide mixture is introduced at the top of the column. The halide ions exchange rapidly with the nitrate ions in the resin, forming a band at the top of the column. Chloride ion is more rapidly eluted from this band than bromide ion by sodium nitrate solution, so that a separation is possible. The progress of elution of the halides is followed by titrating fractions of the effluents with standard silver nitrate solution. 

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

Complete resolution was not achieved due to the carryover of interfering substances which frequently occurs when separating the components of biological samples. The column carried a reverse phase, but as the mobile phase contained low concentrations of lauryl sulfate, some would have adsorbed on the surface of the stationary phase and significantly modified its interacting properties. The retention mechanism is likely to have involved both ionic interactions with the adsorbed ion exchanger together with dispersive interactions with any exposed areas of the reverse phase. 

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. 

New designs for axial flow process chromatography columns have been examined using ovalbumin separation on Whatman Express-Ion Exchanger Q with a 16 L Side-Pack and a 24 L IsoPak column.38 The Side-Pak column is packed in the transverse direction, so radial inhomogeneity is minimized. 

Cassidy, R. M. and Elchuk, S., Dynamic and fixed-site ion-exchange columns with conductimetric detection for the separation of inorganic ions, /. Chrom. Sci., 21, 454, 1983. 

Nuryono, Huber, C. G., and Kleboth, K., Ion-exchange chromatography with an oxalic acid-alpha-hydroxyisobutyric acid eluent for the separation and quantitation of rare-earth elements in monazite and xenotime, Chromatograph-ia, 48, 407, 1998. 

Ooi, K., Miyai, Y., Makita, Y, and Kanoh, H., Fractionation of lithium isotopes in ion exchange chromatography with titanium phosphate exchanger, Separation Sci. Technol., 34, 1133, 1999. 

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).

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