Silica-Based Anion Exchangers

As(lll), As(V). DMA and MMA were determined by an automated hydride AAS method after high-perfomance liquid chromatographic separation using an anion-exchange silica-based column (10-m Ionosphere with a conditioning column) in urines from occupationally... 

Separation Procedure. The petroleum asphaltenes were separated into five fractions acids, bases, neutral nitrogen compounds, saturate hydrocarbons, and aromatic hydrocarbons. Acids were isolated using anion-exchange resin, bases with cation-exchange resin, and neutral nitrogen compounds by complexation with ferric chloride adsorbed on Attapulgus clay. The remaining hydrocarbon fraction is separated on silica gel to produce saturate and aromatic hydrocarbon fractions. 

An inorganic anion-exchange resin based on a zirconia-silica support was prepared by Chicz, Shi, and Regnier. The Lewis acid sites were employed as adsorption sites that permanently retained a PEI coating which then acted as an anion-exchange ligand. In this study, a continuous film of cross-linked PEI was achieved. These surfaces were then tested for the analysis of ova-bumin and bovine serum albumin. 

Like anion exchangers, cation exchangers are divided into polymer-based cation exchangers (PS-DVB, EVB-DVB, polymethacrylate, and polyvinyl copolymers), latex-agglomerated cation exchangers, silica-based, and other (e.g., crown ether, aluminia materials).Modern cation exchangers contain sulfonic, carboxylic, car-boxylic-phosphonic, and carboxylic-phosphonic-crown ether functional groups. 

Unloaded silica does not recover HPA from aqueous solution. The surface of silica gel modified with quarternary ammonium salts (QAS) gets anion-exchange properties. The aim of the work is the elaboration of solid-phase reagents on the base of ion associate of HPA with QAS immobilized onto silica surface for the determination of phosphoms and organic reductants. Heterocyclic (safranine and lucigenine) and aliphatic (trinonyloctadecyl ammonium iodide and tetradecyl ammonium nitrate) compounds have been examined as QAS. 

Nogueira, R., Lubda, D., Leitner, A., Bicker, W, Maier, N.M., Lammerhofer, M., and Lindner, W, Silica-based monohthic columns with mixed-mode reversed-phase/weak anion-exchange selectivity principle for high-performance liquid chromatography, J. Sep. ScL, 29, 966, 2006. 

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. 

Silica-based anion-exchangers tend to have short life-times. This may be extended somewhat by proper sample clean-up and by the use of a silica-gel-saturation column, but this rapid degradation remains the biggest disadvantage of these phases. The resin-based phases are very stable, unless operated above 65°. Unfortunately, because of the slow diffusion processes in these resins, they must be operated at higher temperatures in order to achieve good efficiencies. 

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