Stability, of bonded phases

Another approach to improve the stability of bonded phases at acidic pH is the nse of sterically hindered silanes, which use, for example, isopropyl groups as side chains, instead of methyl groups. Due to the somewhat lower surface coverage, this technique leaves a larger amount of silanols on the snrface than a high-qnality bonding with a dimethylsi-lane, but this is balanced against the definite improvement in hydrolytic stability at acidic pH. 

Proton donor ability of surface silanols is believed to be the source of peak tailing for analytes with proton acceptor functionaUty (usually basic analytes). The presence of impurities such as iron, boron, and aluminum [70] in bulk silica decreases the silanol pK and decrease the hydrolytic stability of bonded phases. 

A systematic description of bond characteristics in intermetallic phases involves several different approaches. A bond characterization in intermetallics, as related to thermodynamic properties and considerations concerning the stability of intermetallic phases, has been reported by Ellner and Predel (1995). On this subject we observe the peculiar properties of alloys of extraordinary stability formed by... 

The nature and the position of the substituents introduced into the benzene ring of the carbamate derivatives essentially define the chiral recognition ability of these CSPs [82]. Higher long-term stability of polysaccharide phases was achieved with covalent bonding of polysaccharide to the surface of the support [83]. 

The final part of the chapter provides a refresher on pA from an analytical chemist s perspective, the drivers for choosing normal phase versus reversed phase as a separation mode for a particular analysis, and instru-ment/system consideration, and it concludes with a very interesting section on column testing within the framework of bonded phase stability (effect of pH and type of buffer) and probing column selectivity. 

Two excellent reviews that detail procedures for the preparation of bonded phase supports have recently been published by Leonard - and Buchmeiser. One of the most popular methods of surface chemical modification involves the use of organosilanes. These organosilanes react with the surface metal hydroxyl groups and form a surface, which may be represented as M-O-R, where R represents an alkyl chain and M represents the metal (i.e., silica, zirconia, titania, etc.). One important factor that must be stated, however, is that the order of stability of M-O-R bonds increases in the order of M=Si > Zr > Ti > Improvements in the hydrolytic... 

The synthetic methods used for the preparation of bonded phase materials are illustrated in Figure 6.35. One of the first reported bonded phases, the alkoxy silanes (1) also referred to as silicate esters, was prepared by the direct esterification of silanol groups with alcohols. The major disadvantage of this packing material was its hmited hydrolytic stability, as it is readily hydrolysed by aqueous alcohol eluants. 

For alumina, titania, and zirconia, there exists as yet no covalent bonding chemistry that is equivalent to the silanization technique used for silica. Although attempts have been made to silanize these other oxides, the hydrolytic stability of these phases does not match up to the hydrolytic stability of the support itself. Therefore alternative surface modification tet ques have been developed that do not rely on the attachment of the modifier to the surface. The coating can be simply insoluble in the intended mobile phases, or a crosslinked coating can be formed that stretches like a net around the skeleton of the particle. Both techniques are, in principle, independent of the nature of the substrate and can be applied to all inorganic or polymeric packings. 

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