Anion exchangers, surface-modified silica

Anion 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.

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. 

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. 

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. 

As an example. Fig. 3-57 shows a separation of various inorganic anions on a surface-coated silica. As can be seen in this chromatogram, the chromatographic efficiency of this stationary phase is very poor as compared to modern anion exchangers. However, having the option to elute anions with pure de-ionized water is advantageous, because in this case it means that the suppressor system (normally necessary for a sensitive conductivity detection) is dispensable. This scenario also applies for the separation of the sodium salts of various chloro-containing compounds on silica modified with 4-hydroxymethylbenzo-18-crown-6 shown in Fig. 3-58. Pure methanol was used as a mobile phase for this separation. 

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