Cation exchangers, surface-modified silica

The separation of the same charged compounds were also accomplished on an ethyl-pyridine bonded silica surface and 30 0% methanol/C02 mobile phases without the need of added sulfonate modifier. Anionic compounds did not elute from the ethyl-pyridinium surface that lead the authors to hypothesize that the surface was positively charged. To further test this hypothesis, the separation of the same compounds on a strong anion exchange column, silica-based propyltri-methylammonium cationic surface, which exhibits are permanent positive charge was attempted. The same retention order was observed on the strong cation exchange surface. 

Cation exchangers based on surface-modified silica gels... 

Scheme V, silica particles are brought into contact with a cation-exchange resin, and if necessary, with an anion-exchange resin, to obtain an acidic silica sols of pH 2-4. This acidic sol is stable because it is negatively charged even at pH 2-4, according to zeta-potential measurements. Starting from such acidic silica sol, surface-modified silica sols are manufactured.

The chromatographic procedure may be carried out using (a) two stainless steel columns in series the first (25 cm x 4.6 mm) packed with particles of silica, the surface of which has been modified with chemically bonded hexysilyl groups (5 /im) (Spherisorb C6 is suitable) and the second (25 cm x 4.6 mm) packed with cation exchange resin (10 /an) (Partisil-10 SCX is suitable) (b) as the mobile phase at a flow rate of 1.0 ml/min, a mixture of 25 volumes of acetonitrile, 25 volumes of a phosphate buffer solution prepared by dissolving 58.5 g of sodium dihydrogen orthophosphate... 

Traditionally, low crosslinked porous polymers modified by sulfonic or carboxylic acid groups (quaternary amines for the separation of cations) were the most widely used stationary phases. In recent years, silica-based chemically bonded or surface-modified (e.g. alumina treated) ion exchangers have found increasing use [159,484-488]. The trend towards increased use of modern porous polymer and silica-based materials is due to their higher performance and greater dimensional stability with different mobile phase compositions. 

With HILIC, various polar stationary phases with differing selectivity are used. Basically, it must be distinguished between three different selective types weak anion exchanger (silica modified with aminopropyl groups) and amide columns, weak cation exchanger (usually unmodified (bare) silica) and neutral supports (diol or zwitterionic stationary phases (ZIC-HILIC)). With ionizable compounds, in addition to the distribution equilibrium between the mobile phase ( pseudo-stationary phase ) near to the polar surface and the less polar mobile phase, ionic interactions can also occur, resulting in differing separation characteristics on the different stationary phases. 

Similar to silica, surface-modified cellulose (84,98,1(X),105,119) e.g. ECTEOLA (a reaction product of epichlorohydrin, triethanolamine and alkali cellulose), carboxymethyl cellulose, amino-benzyl cellulose and diethyl-(2-hydroxypropyl) aminoethyl cellulose have been used as stationary phases for the separation of several inorganic ions. Fixion 50X8 (a polymer based strong acidic cation exchange resin) is used to resolve various inorganic ions by circular TLC (127). 

New nanoporous carbons with extremely high mesopore volumes and a highly dispersed distribution of metals on the carbon surface have been obtained using mesoporous silica. Polystyrene sulfonic acid-based organic salts were used as carbon precursor. The precursor chemistry was modified by cation exchange with catalytically active metals (i.e., copper, nickel, cobalt), prior to carbonization. The carbons have pore sizes predominantly in the range of 10-50 nm. 

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