Traditionally in chromatographic separations the composition of the mobile phase is manipulated to optimize selectivity. We investigate the alternate paradigm of stationary phase gradients (SPGs), in which ligand density is varied along the length of the stationary phase to produce novel selectivities. A SPG that utilizes multiple ligands has the exciting possibility of neighboring ligand effects that may produce more unique selectivities. Recently, we reported an amine gradient on a silica monolith, which was the first tunable and continuous SPG for liquid chromatography. Precise control over the creation of the gradient is required because it allows for the chromatographer to have direct control over the column’s selectivity. This is achieved using a technique we developed denoted controlled rate infusion (CRI). The focus of our most recent work is the fabrication of a phenyl gradient on a bare silica packed column using CRI. In this method, a solution containing phenyltrimethoxysilane is infused into the packed column in a time dependent fashion so that one end is more heavily modified than the other. These gradient columns, as well as uniformly modified columns, are chromatographically characterized under various mobile phase conditions using probe analyte mixtures that are comprised of benzene derivatives and amphetamines. Chromatographic characterization of the gradients demonstrates the change in selectivity of the gradient stationary phase relative to the uniform counterpart. To characterize the extent of ligand modification with distance (e.g., gradient profile), the column is divided into sections and each section evaluated via Raman spectroscopy. Expanding on this work, phenyl-amine gradients will also be prepared and studied. This will be used for furthering the investigation of the neighboring ligand effects that may arise from having multiple ligands on the same stationary phase, which may enable the separation of a wider range of compounds.

Phenyl and amine stationary phase gradients on packed columns for high-pressure liquid chromatography

254th National Meeting (2017)

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