Bonded phase hydrolytic stability

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. 

The nomenclature of the RP is not consequent. The RP most often used contains octyl (RP C8) or octadecyl (RP C18) groups. There is no differentiation even when two methyl groups are introduced additionally with the silane (as with monofunctional silanes) or only one (difunctional) or none (trifunctional silane). Some manufacturer use silanes with bulky side groups (e.g., isopropyl groups) to improve the hydrolytic stability of the bonded phases, but here also, only the longest alkyl group is used in nomenclature. RP C8 and RP C18 are the work horses in HPLC. Shorter chains (RP4) are used in protein separations, and special selectivity can be obtained with bonded phenyl, cyano, amino or fluoro groups. 

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. 

The term bonding density is constantly used throughout this book. This is probably the most important characteristic of the bonded phase. The higher the bonding density, the more hydrophobic the adsorbent surface, the better the shielding of residual silanols, and the higher the hydrolytic stability. 

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. 

The type of pH modifier to make a desired mobile phase pH also has an effect on the column stability, and this is indirectly related to the peak efficiency and the retention of the analyte. As an increasing number of column volumes of the mobile phase are traversed through the column, the stability of the packing material could be comprised. Rearrangement of the packing bead leads to the loss of efficiency, dissolution of silica leads to loss in efficiency and retention, and hydrolytic decomposition of the bonded phase could impact the peak shape and retention. Different compounds, such as neutral compounds, acidic compounds, and basic compounds, could show different behaviors. 

Properties which are relevant for bond stabilities are closely related to orbital energies, and the latter can be measured directly by electrochemical procedures. Hence the observed relationship between the electrochemical ligand parameter E (L) and bond stability -even including hydrolytic stability rather than bond energies in the gas(eous) phase - might have been anticipated. This correlation is based upon the log-linear regression equation... 

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... 

An additional corroboration of the formation of Si-C bonds on silica surface is provided by the data on the hydrolytic stability of organosilicas produced as a result of the solid-phase hydrosilylation reaction. The treatment of these organosilicas with water vapours in the presence of pyridine [141] does not lead to any variation in the character of their IR spectra (Fig.10, spectra 1- 4). At the same time in the case of methoxysilica under similar conditions one can observe the destruction of Si-O-C bonds and formation of silanol groups (Fig. 10, spectra 5,6) which is evidenced for by the decrease in the intensity of the absorption bands in the region of 3000 - 2800 cm attributed to ether groups, and by the appearance of the band 3750 cm" A... 

Bonded phase chromatography (BPC) takes place either under normal phase or reverse phase conditions. In reverse phase mode the stationary phase is non-polar while the eluant is polar, e.g. methanol or acetonitrile with aqueous buffers. Bonded phase packings have superceded the classical packings where the stationary phase was distributed over the surface of the support particles and bound simply by physical forces of attraction. However, due to the problems of solvent stripping and limited hydrolytic stability, these classical systems, though developed for a few specialised applications, have been replaced by organo-bonded stationary phase materials. 

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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