Aminated silica phases

For the synthesis of pure silica phases, it is possible to use different SDAs in a fluoride media. The SDAs are selected primarily consistent with criteria defined as important in determining a high structure-directing ability (rigidity, size, shape, C/N+, ratio) [97,116], and the availability of the cation or of the parent amine. Therefore, SDAs with polycyclic moieties (giving rise to SDAs with relatively large and rigid portions) predominate. 

The nitrile and amine bonded phases are most popular for normal phase operation. Nonpolar liquids like the hydrocarbons and chlorocarbons are used as the mobile phases, although as noted above, even alcohols and aqueous mixtures can be used. The nitrile phase produces separations somewhat similar to plain silica in LSC. 

Normal phase sorbents CN, NH2, primary and secondary amine, silica, (Toxi-Lab)... 

The synthesis of both amlnated silica phases consisted of adding either 3-aminopropyltrlethoxysilane (for Phase B) or 3-aminopropyldimethylethoxysilane (for Phase D) to dried (130 C) Partisil 10 and refluxing for eight hours in sodlum-drled toluene with stirring over dry nitrogen gas. After Soxhlet-extractlon with methanol, the aminated silica gels were stored In a vacuum desiccator until use. The physical characteristics of these stationary phases are listed in Table I. 

An unfavorable property of silica is its solubiHty at high pH, which, however, is rarely applied with the mobile phases used in NPLC. Silica gel has weak cation-exchange properties (the silanols on the silica gel surface may be ionized to SiO groups), whereas the bonded amine stationary phases behave as weak anion exchangers. This mixed retention mechanism may cause excessive retention and tailing or even split peaks of ionic or ionizable compounds. 

Note The reagent can be employed for qualitative and quantitative analysis on silica gel and RP layers. Ammonia, amine and acid-containing mobile phases should be completely removed beforehand. Amino phases cannot be employed. The NBD-chloride reagent is not as sensitive as the DOOB reagent (qv.) on RP phases. 

The Jordi polyamine column is a polar column for simple sugar and polysaccharide applications. The amine groups are bonded to the DVB backbone and are stable in aqueous mobile phases. This material does not self-hydrolyze as do many silica-based amino packings (Fig. 13.14). 

The Zincke reaction has also been adapted for the solid phase. Dupas et al. prepared NADH-model precursors 58, immobilized on silica, by reaction of bound amino functions 57 with Zincke salt 8 (Scheme 8.4.19) for subsequent reduction to the 1,4-dihydropyridines with sodium dithionite. Earlier, Ise and co-workers utilized the Zincke reaction to prepare catalytic polyelectrolytes, starting from poly(4-vinylpyridine). Formation of Zincke salts at pyridine positions within the polymer was achieved by reaction with 2,4-dinitrochlorobenzene, and these sites were then functionalized with various amines. The resulting polymers showed catalytic activity in ester hydrolysis. ... 

Figure 7. Separation of eight biogenic amines using ion pair partition liquid chromatography. Conditions 30 cm column with 4 fim silica stationary phase, 0.1M HClO /0.9M NaClO mobile phase, ethylacetate/tributyl phosphate/hexane (72.5/10/17.5) velocity, 0.8 cm/sec (47).

This technique is based on the same separation mechanisms as found in liquid chromatography (LC). In LC, the solubility and the functional group interaction of sample, sorbent, and solvent are optimized to effect separation. In SPE, these interactions are optimized to effect retention or elution. Polar stationary phases, such as silica gel, Florisil and alumina, retain compounds with polar functional group (e.g., phenols, humic acids, and amines). A nonpolar organic solvent (e.g. hexane, dichloromethane) is used to remove nonpolar inferences where the target analyte is a polar compound. Conversely, the same nonpolar solvent may be used to elute a nonpolar analyte, leaving polar inferences adsorbed on the column. 

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