CHARACTERIZATION OF A RANGE OF ALKYL-BONDED SILICA HPLC STATIONARY PHASES - XPS AND TOF-SIMS STUDIES

A series of alkyl-bonded silica HPLC stationary phases with a mean particle diameter of 5 mum has been prepared using a variety of dimethylalkylsilyl chlorides (i.e. monofunctional bonding agents) with alkyl chain lengths varying from Cl to C18, including phenyl and cyanopropyl groups. The stationary phases have been characterized using XPS and time-of-flight (ToF)-SIMS. Elemental analyses by XPS have identified trace quantities of iron, sodium and chlorine at the silica/dimethylalkylsilyl chain interface, and aluminium was detected by ToF-SIMS. A comparison of percentage carbon obtained by XPS with bulk carbon-loading analyses suggests that the alkyl chains are bonded to the surface of the porous silica particles. Molecular ions characteristic of the dimethylalkylsilyl groups were identified in the ToF-SIMS data together with ions attributable to these alkylsilyl groups attached to fragments of silica, the latter indicating covalent bonding to the silica surface. In addition, a series of low-mass signals corresponding to alkyl chain fragments and silyl/silica-based ions was observed. The value of high-resolution ToF-SIMS data for ion assignments is well demonstrated. The relative ion intensities of several of these low-mass ions were studied as a function of changing alkyl chain length. In general, as the alkyl chain length increased, the abundance of the silicon-containing fragments decreased, whilst the abundance of the alkyl-based fragments increased, within the ToF-SIMS spectra.