M.J.J. Hetem

Characterization and stability of silanized and polymer-coated octadecyl reversed phases

Three different reversed phases for high-performance liquid chromatography obtained with monofunctional octadecylsilane or with cross-linked polymethyloctadecylsiloxane with and without trimethylsilane precapping of the substrate were synthesized on the same batch of Nucleosil silica. After extensive characterization, these phases were subjected to artificial ageing under simulated routine conditions. Changes in properties of the bonded-phase packings were quantified with chromatographic techniques, solid-state 29Si NMR and elemental analysis. These changes are correlated with lipophilic and polar selectivity, accessibility of silanol groups at the surface, loss of silanes/siloxanes, separation performance and silica degradation of the stationary phases. Improved surface shielding properties and stability towards eluents of high pH are considerable advantages of cross-linked polymer-coated and octadecyl stationary phases. Precapping of the substrate with trimethylsilanes prior to polymer coating reduces the concentration of silanol groups to less than 50% and yields stationary phases which exhibit further decreased silanol interactions because the residual silanol groups are shielded in a double manner by the trimethylsilyl groups and by the polymer layer on top of it. This also leads to an increase in stability. The non-precapped polymer-coated stationary phase showed an advanced cross-linking of the polymers after ageing experiments with high pH buffer solutions, resulting in improved silanol shielding and large shifts in selectivity. After precapping with trimethylsilane-enolate the polymeric methyloctadecylsiloxane coating proved to be more stable than the non-precapped phase. Also, the precapped phase showed hardly any chemical bonding with the silica surface; only Van der Waals and other dispersive interactions with the anchored trimethylsilanes at the surface exist. The high mobility of the polymer coating at the pore surface of the substrate is retained for the precapped stationary phase, also after simulated ageing experiments. This precapped polymer coated stationary phase showed an almost consistent separation performance after ageing experiments with high pH buffer solutions, but a small shift in selectivity was observed.

Study of the changes in mono-, di- and trifunctional octadecyl-modified packings for reversed-phase high-performance liquid chromatography with different eluent compositions

Two different types of silica substrates uniformly modified with reversed-phase high-performance liquid chromatographic (RP-HPLC) phases obtained with mono-, di- and trifunctional octadecylsilanes were subjected to artificial ageing under simulated routine conditions and were subsequently analysed and evaluated. Changes in the properties of the bonded phase packings were characterized and quantified with chromatographic techniques, solid-state NMR and elemental analysis. These changes were correlated with selectivity, loss of silanes, gain in silanol content and rearrangement of the silica to silane bonding. When eluents of extremely high pH were used, the multifunctional octadecylsilane stationary phases showed a higher resistance towards ligand stripping. The substrate shielding of difunctional octadecylsilanes showed superior characteristics due to direct surface attachments. The rigidity of the silica substrate used for modification influences the stability of the RP-HPLC phases, especially for the monofunctionally modified octadecylsilane stationary phase. With the use of multifunctional C18 silanes for modification, the influence of the characteristics of the substrate on the stability of the RP-HPLC phases was reduced compared with monofunctional C18. In contrast to monofunctionally modified phases, the polar selectivity of the multifunctionally modified phases remained more or less constant after intensive use with aggressive eluents. A chromatographic characterization method for obtaining information about the stability, capacity, selectivity and silica degradation of different RP-HPLC stationary phases is also presented.

Deactivation with polymethylhydrosiloxane : a comparative study with capillary gas chromatography and solid-state 29Si nuclear magnetic resonance spectroscopy

Deactivation of fused-silica capillary columns and vitreous silica surfaces with polymethylhydrosiloxanes (PMHS) at temperatures between 240 and 360°C was investigated by gas chromatography (GC) and solid-state 29Si NMR spectroscopy. The influence of temperature, amount of water at the surface and film thickness on the stability and nature of the polymer deactivation layer was studied by solid-state 29Si NMR of the model substrate Cab-O-Sil, a fumed silica. These NMR measurements match the GC results and offer additional information relating to the nature of the deactivating layer at surfaces silylated under various reaction conditions. Differences in the nature and structure of the deactivating film occur when the silica surface is coated with a different thickness. Optimum deactivation is achieved when a few monolayers of densely cross-linked polymers anchored to the silica surface are formed. On top of this thin, rigid layer, longer mobile polymer chains containing unreacted silicon hydride groups are present. After optimum deactivation, the fused-silica capillary column wall shows no interaction with various critical test components as used in GC practice. The deactivating films exhibit good thermal stability and solvent resistance.

Effect of acid pretreatment of the silica substrate on the stability of octadecyl modified reversed phases

SummaryTwo stationary phases, identically modified and derived from the same experimental Chromspher substrate, but one of which has been pretreated with a hydrofluoric acid solution, are compared after ageing with various eluents under simulated routine conditions. The hydrothermal hydrofluoric acid treatment of the silica substrate prior to modification with dimethyloctadecyl-silane improved both the surface coverage and the stability of the resulting stationary phase compared to those of the original substrate. The stability of the silica-to-silane bonding after modification was characterized by means of chromatographic techniques and elemental analysis. Changes in the rigidity of the substrate were studied by means of solid state29Si NMR. The effect of the hydrothermal acid treatment on relevant substrate parameters, including formation of crystalline regions at the surface, is also discussed.