Preparation of chemically bonded phases

Various forms of amorphous silica exist but all may be considered to have the general formula Si(OH)4. Preparation of bonded phase 

A number of methods are available for the chemical modification of silica (Brust et al., 1973 Hastings et al., 1971) but the most popular is shown schematically in Fig. 10.3. 

In practice, dry silica is refluxed for several hours with a solution of the organosilane (RS1X3) in toluene or carbon tetrachloride under a dry inert gas atmosphere (Little et al., 1979a). The silica is recovered by filtration, washed (when X = Cl it is hydrolysed with water) and then capped to mask all unreacted silanol groups with trimethylchlo- 

Alternative bonded phases may be generated either by chemically modifying the attached group or by modifying the silane reagent prior to attachment to the bonded phase (Wheals, 1975 Chang et al., 1976). 

Accurate quantitative analyses of bonded phases are difficult but the most successful has been provided by C, H and N analysis. As the carbon loading can have a major influence on chromatographic retention it is important that some indication of this parameter is provided and, indeed, most commercial suppliers of columns quote the carbon loading. Similarly, as the number of free hydroxyl groups can influence retention, especially in reversed phase chromatography, it is important to know whether or not a column has been end-capped. 

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It is generally accepted that the only important polar adsoiption sites on the silica surface are the silanol functions, i.e., hydroxyl groups, that are attached to silicon atoms (2). They can interact with the sample molecules by hydrogen bonding and various physical observations ctAi be used to prove this statement. Complete dehydration of silica by beating, i.e., removal of all surface hydroxyl groups, yields a hydrophobia silica which no longer shows adsorption for unsaturated and polar molecules and is no more wetted by water (15). Chemical modification of the surface hydroxyls such as used in the preparation of chemically bonded phases also eliminates the selective adsorption properties of the silica. ... 

Sandoval, I.E. and Pesek, I.I., Synthesis and characterization of a hydride-modified porous silica material as an intermediate in the preparation of chemically bonded chromatographic stationary phases, AnaZ. Chem., 61, 2067, 1989. 

The chemical stfuctuie of silanixing agents employed in the preparation of hydroearbonaceous bonded phases for RPC can be represented by the gen formula j... 

There are several basic rules for the preparation of chemically bonded stationary phases. 

Porous silica (or silica gel) is by far the most important adsorbent for liquid-solid chromatography and is also the substrate used to prepare most chemically bonded phases, giving it a preeminent position in modem column technology [3-15]. Silica particles are mechanically strong and easily prepared in a wide range of particle size ranges and pore diameters suitable for chromatography. As a substrate for the... 

Akapo, S.O. (1989). Gas solid fluidization an improved method for the preparation of chemically bonded stationary phases. PhD Thesis, University of London. 

Berendsen, GE. and de Galan, L., Preparation and chromatographic properties of some chemically bonded phases for reversed-phase liquid chromatography, J. Liq. Chromatogr, 1, 561, 1978. 

On the other hand, the direct chromatographic approach involves the use of the chiral selector either in the mobile phase, a so-called chiral mobile phase additive (CMPA), or in the stationary phase [i.e., the chiral stationary phase (CSP)]. In the latter case, the chiral selector is chemically bonded or coated or allowed to absorb onto a suitable solid support. Of course chiral selectors still can be used as CMPAs, but the approach is a very expensive one owing to the high amount of chiral selector required for the preparation of the mobile phase, and the large amount of costly chiral selector that is wasted (since there is very little chance of recovering this compound). Moreover, this approach is not successftd in the preparative separation of the enantiomers. 

Miller, N.T. and Shieh, C.H., Preparative hydrophobic interaction chromatography of proteins using ether based chemically bonded phases, J. Liq. Chromatogr., 9, 3269, 1986. 

G.E.Berendsen, Preparation and Characterization of well-defined Chemically Bonded Phases for HPLC, Thesis, Delft, 1980. 

Chemical stability of carbon over the entire pH range has led to considerable interest in the development of carbon-based stationary phases for RPC. Porous graphitised carbon with sufficient hardness, well-defined and stable pore structure without micropores, which ensures sufficient retention and fast mass transfer can be prepared by a complex approach consisting of impregnation of the silica gel with a mixture of phenol and formaldehyde followed by formation of phenol-formaldehyde resin in the pores of the silica gel, then thermal carbonisation and dissolution of the silica gel by hydrofluoric acid or a hot potassium hydroxide. solution [48. The retention and selectivity behaviour of carbon phases significantly differs from that of chemically bonded pha.ses for RPC. Carbon adsorbents have greater affinity for aromatic and polar substances so that compounds can be separated that are too hydrophilic for adequate retention on a Cix column. Fixed adsorption sites make these materials more selective for the separation of geometric isomers [49]. 

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