Characterization of Salt-Modified Silica Gel Coated with Disodium Hydrogenphosphate as an Adsorbent for Gas-Solid Chromatography

Abstract

A simple, effective modification of the surface properties of an adsorbent for gas-solid chromatography (GSC) has been studied by coating the adsorbent surface with various inorganic salts.1-13 Although most authors have focused their interest on effect of the nature of the salt used as a modifier on the retention volume, they have rarely done so concerning influence of the amount of salt coated. In previous studies14,15 we elucidated dependence of the retention volume of a given solute on the amount of loaded salt (the salt-loading) on the basis of a mixed retention mechanism. Reasonable values could be calculated regarding the distribution constants of the adsorption equilibria on the subsurfaces formed on the surface of a salt-modified adsorbent. In the present study we examined the same approach as that used for characterization of a salt-modified silica-gel adsorbent coated with disodium hydrogen phosphate as a less reactive salt than the previously used potassium fluoride. ExperimentalSilica gel (Fujigel KC 5) was dried at 120° C for 15 h after sizing into a 60 -80 mesh fraction. Crystalline disodium hydrogenphosphate (Na2HPO4 12H20) was heated at 120° C for 5 h to be converted to an anhydrous salt prior to weighing. Na2HP04-coated silica-gel beads were prepared by depositing salt on silica gel by evaporating water, and packed into a stainless-steel column (100X0.3 cm i.d.) by tapping. The packed column was conditioned at 150°C for 15 h in a nitrogengas stream at a flow rate of 40 cm3 min' and at 120° C for 3 h in a helium-gas stream at the same flow rate as that before use. A Hitachi Model 263-30 gas chromatograph, equipped with a thermal-conductivity detector, was used along with a Hitachi Model D-2500 Chromatointegrator.Chromatographic measurements were made at 60 and 120° C using helium as the carrier gas at a flow rate of 40 cm3 mine. The reduced retention volume (VNR) and salt-loading (XC) were calculated by dividing the net retention volume of each solute by the weight of the salt-free silica gel packed into the column, and as a weight ratio (the anhydrous salt loaded/the saltfree silica gel), respectively. The specific surface area of salt-coated silica-gel beads (As; on the basis of unit weight of the salt-free silica gel) was measured by a BET method of nitrogen adsorption. Resultsand Discussion