Underivatized silica surface

The results of detailed studies require a partial relaxation of the hopeful assumption that all protons of underivatized silica systems are at the surface [18]. Recent NMR studies indeed show that moderately intense D2O exchange conditions leave about 3% of the original proton content in a silica gel sample. This residual 3% is attributable to trapped OH groups. On the basis of extensive... 

A major advantage of underivatized silica for LC-MS(/MS) is the absence of ligands that may detach from the surface and show up as spurious peaks in the mass spectra. Recently introduced bonded phase materials seem to have addressed this issue, so that functionalized silica gels are as common today as bare silica in HILIC-MS applications. 

Because traditional HILIC columns such as underivatized silica, cyano-, amino-, and diol-bonded phases have hydrophilic surfaces, they are incapable of separating small molecules via hydrophobic interaction. By comparison. 

The retention factor imder linear conditions is related to the initial slope of the isotherm k = Fdqf dC). It is easy to check that this relation applies when the isotherm coefficients are calculated. Deviations from this relation are not unusual, and can often be ascribed to the heterogeneity of the adsorbent smface that can be explained, for example, by a small concentration of underivatized sUanol groups on the surface of chemically bonded silica. It may be that an additional Langmuir isotherm term would be necessary to account for the contribution of these groups, term that could become saturated at very low concentrations [172]. 

Alumina is the second inorganic oxide in use in HPLC. However, it is by fai not as widespread as silica, and there are fewer types commercially available The reason is primarily that a simple, high-qu ty surface derivatizatioi technique akin to the silanization of silica is not available, which restrict applications largely to the use of underivatized alumina. 

Underivatized CPG beads can be derivatized by reaction with 3-amino-propyltriethoxysilane, in the same way as silica gel, to create an aminopropyl linker arm (cf. Fig. 1). The aminopropyl group can then be coupled directly to a p-nitrophenyl ester of a nucleoside-3 -succinate (20). The amount of nucleoside loading achieved is strongly dependent on the pore size of the support, since the surface area decreases with... 

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