Novel approaches to the use of coupling agents for grafting nonpolar molecules onto silica particles in reversed-phase HPLC columns — ASN Events
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Novel approaches to the use of coupling agents for grafting nonpolar molecules onto silica particles in reversed-phase HPLC columns (#44)

Genhai G. Liang 1
  1. Australian Pesticides and Veterinary Authority (APVMA), Kingston, ACT, Australia

Reversed-phase HPLC columns are commonly packed with micro-sized hydrophilic silica gel particles to which a brush-like monolayer of nonpolar molecules are chemically attached. The conventional method using octadecylsilyl chloride (ODS-Cl) or dimethyloctadecylchlorosilane (DMODS-Cl) cannot fully endcap the residual polar silanols on the chemically-treated silica surface, peak tailing and poor reproducibility may occur[1].

This paper proposes 3 novel methods for grafting the acid-treated silica surface using Silane, organometallic chemicals (Titanate and Aluminate), and Chrome complex as coupling agents. These coupling agents can act as hydrophobic segments (designated R below) that may improve endcapping capability.

Silane molecules R-Si(OCH3)3 can be hydrolysed and condensed to form oligomers which then react with hydroxyl groups on the treated silica surface to form a network of chemical bonds [silica-Si-O-Si(-O-)2R] and hydrogen bonds (see figure). This network could sterically hinder the hydrophilic silica surface from interacting with polar solutes. The need for endcapping may be eliminated and the column life-span extended due to stronger chemical bonds and/or physical bonds.

The second group of coupling agents are the organometallic chemicals, which include Titanate coupling agent, CH3O-Ti(COOR)3. These coupling agents can theoretically form 3-time more compact layers of hydrophobic molecules than the ODS-Cl method, thereby significantly reducing the chance of the solute molecules penetrating into the silica surface.

The final example is the first-generation coupling agent, Chrome complex chemical. Each molecule of the fatty acid chrome complex has a bulky reactable head that can react and sterically shield several molecules of silanols, thus it may be a potential candidate for endcapping silanols on the silica surface.

If necessary, hydrophobicity of the nonpolar molecule can be further increased by fluorination, or decreased by adding several desired hydrophilic molecules using a reversible addition–fragmentation chain transfer (RAFT) agent.

These potential approaches to the use of coupling agents may lead to chromatography with enhanced peak characteristics and improved repeatability.

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  1. Y. Sudo, Journal of Chromatography, A 737 (1996) 139.