Geometric characterization of straight-chain perfluorohexylpropyl adsorbents for high performance liquid chromatography

The geometric characterization of porous adsorbents used as stationary phases in high performance liquid chromatography is a key aspect for the understanding and the description of retention mechanisms. Herein, we present a study aimed at describing the geometric characteristics of alkyl-perfluorinated silica stationary phases by employing chromatographic and non-chromatographic techniques. This study shows that, under typical chromatographic conditions, the perfluorohexylpropyl straight chains grafted to the silica surface assume a compact arrangement. As perfluorinated stationary phases are highly hydrophobic, their wetting behavior in water was investigated. To this end, a novel approach based on tracer pulse chromatography with pure water as the mobile phase and deuterium oxide as the labeled marker was employed. Experimentally it was found that, unless a high back-pressure is applied, water is expelled by the pores of the adsorbent. On the other hand, wetting can be significantly improved if 5–10% (v/v) of organic modifier is added to the mobile phase. The practical consequences of limited wettability are discussed, in particular with reference to fluorous-SPE (F-SPE) applications.