Highly cross-linked self-assembled monolayer stationary phases: An approach to greatly enhancing the low pH stability of silica-based stationary phases

A new type of silica-based stationary phase with dramatically improved acid stability compared to any currently available silica-based stationary phase has been developed. Superior low pH stability is achieved by first self-assembling a densely bonded monolayer of (chloromethyl)-phenylethyltrichlorosilane (CMPES). The self-assembly step is followed by a Friedel-Crafts cross-linking of the reactive moieties with their neighbors, by addition of secondary, cross-linkable aromatic reagents, or by both. This phase is not endcapped. Elemental analysis data shows that an aluminum chloride catalyst is very effective at bonding aromatic cross-linking reagents, such as styrene heptamer and triphenylmethane, to the reactive CMPES monolayer. The stability of the retention factor of decylbenzene on the cross-linked self-assembled CMPES phases is compared to a sterically protected C₁₈ phase to illustrate its superior resistance to acid-catalyzed-phase loss. Inverse size exclusion chromatography and flow-curve comparisons of the cross-linked self-assembled CMPES and the sterically protected C₁₈ stationary phases illustrate their similar chromatographic efficiency.