Anion silica-based

Fig. 3-30. Separation of various inorganic anions on a Nucleosil 10 Anion silica-based anion exchanger. - Eluent 0.025 mol/L sodium salicylate (pH 4.0 with salicylic acid) flow rate 1.5 mL/min detection refractive index injection volume 10 pL solute concentrations 1 to 2 g/L of the various anions.

Figure 3.83 Separation of various inorganic anions on a Nucleosii 10 Anion silica-based anion exchanger. Eluent 25 mmol/L sodium...

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. 

For these three materials, covalent bonding technologies cannot be used. With silanes, mixed anhydrides are formed lacking in hydrolytic stability. Coating with organic polymers [32] is the way to go. A bonded phase based on zirconia has been studied widely [43]. Method development strategies established with silica-based RP cannot be transferred to an RP bonded on zirconia. Selectivity is dependent, e.g., on the type of buffer used. Anions in the mobile phase influence retention. The kinetics of analyte interaction with the different active sites may lead to reduced efficiencies. 

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

The main advantage of the zirconium family of columns is their stability from pH 1 to 10 and at temperatures from ambient to 200°C. Their separating character also differs from silica-based columns due to the lack of ionizable surface molecules. Silica above pH 3.0 loses a proton to form anionic silicate moieties, giving the bonded-phase silica column some anionic as well as nonpolar organic column characteristics (Fig. 6.8a). 

The hydrophilic silica-based diol packings have been modified by derivation through some of the diol groups with carboxymethyl and diethy-laminoethyl functions to make weak anionic and cationic protein size-separation columns. These provide the HPLC equivalent of the CM- and DEAE-cellulose columns used in protein purification on open columns and are used with the same type of buffers to provide ion exchange purifications of proteins. 

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