A New Concept of End-capping for Reversed Phase Silica Material: Conversion of Silanol Groups to Siloxane Bond by Heating

Tuesday, March 03, 2020: 10:00 AM - 12:00 PM

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Description

A reversed phase column like C18 has been widely used in various fields such as pharmaceutical, medical, environmental, industrial and food area. Especially silica based C18 column has showed high performance although an end-capping must be done enough in order to decease a peak tailing of a basic compound caused by a residual silanol group on the silica surface. An End-capping has been recognized to be an important factor for a silica based reversed phase column. In the beginning residual silanol groups after C18 bonding were reduced by bonding with trimethylsilyl reagents (TMS). This end-capping method has been improved by various end-capping reagents with not only a monofunctional group but also a difunctional group, a high reaction temperature or a long reaction time. In the past ten years some column manufacturers have developed the unique method that positive charge added on the silica surface suppressed influence of residual silanol groups against a basic compound. In this study, not only bonding with an end-capping reagent but also conversion of silanol groups to siloxane bond by heating were tried as an end-capping. After bonding with C18 on a silica gel, a difunctional end-capping reagent like hexamethyldichlorotrisiloxane was used as a first end-capping, then trimethylchlorosilane was used as a second end-capping. These two end-capping reactions were done at over 200 degree Celsius under a nitrogen atmosphere. The C18 with the above end-capping was more hydrophobic and showed a longer retention time higher than the C18 with the above end-capping done at 40 degree Celsius lower temperature although both C18s showed a good peak shape for a basic compound. Conversion of silanol groups to siloxane bond by heating made hydrophobicity high. The peak shape of a basic compound and stability under both acidic and basic pH conditions were evaluated.