Surface Chemistry of the Base Material

Base material provides mechanically stable rigid porous particles (mostly spherical) for reversed-phase HPLC adsorbents. Particle porosity on the mesoporous level (30 to 500-A diameter) is necessary to provide high specific surface area for the analyte retention. Surface of the base material should have specific chemical reactivity for further modification with selected ligands to form the reversed-phase bonded layer. Base material determines the mechanical and chemical stability—the most important parameters of future (modified) reversed-phase adsorbent. 

2 Chemical Stability. Hydrolytic stability of base material is the most important parameter because most reversed-phase HPLC separations are performed in water/organic eluents with controlled pH. Selection of the mobile phase pH is mainly dictated by the properties of the ionizable analytes to ensure that they are in one predominate ionization state. 

Chemical modification of the adsorbent surface significantly alters practically all properties of the base material. Dense coverage of the adsorbent surface with inert ligands usually expands chemical stability of the packing material. 

1 Synthesis of Silica. Silica used in HPLC is an amorphous, porous solid, which could be obtained by different synthetic procedures. 

Colloidal Sol-Gel Procedure. Silica sol is passed through nonaqueous media, where it forms spherical droplets, which rapidly solidify into the hydrogel beads [19]. Solid beads are dried and calcinated at around 600-1000°C. This synthetic procedure usually gives spherical particles of silica with significant amount of impurities (Na, Fe, B, etc.) in the body and on the surface of material. These impurities can increase the acidity of the surface sUanols, thus lowering the pKa of the respective silanol groups. 

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