Stationary Phase Gradients in Liquid Chromatography

Stationary phase gradients have considerable promise in liquid chromatography. When used in conjunction with mobile phase gradients, they have been shown to improve selectivity and hence the resolution of a separation. Two approaches have been used to create such gradients. The first involves serially connecting columns with different stationary phases together with low dead volume connectors. The second involves creating the stationary phase gradient in a single housing. The latter is similar to a mixed-mode stationary phase, with the exception that the relative placement of the functional groups varies from one end of the column to the other. Unlike the serially connected columns, the continuous stationary phase gradient has the potential for improved selectivity due to neighboring ligand effects since the different functional groups are in proximity to each other, in contrast to being in separate columns. While very promising, such continuous, mixed-mode stationary phases are more challenging to make. In this presentation, we will describe the fabrication of phenyl-modified stationary phase gradients using two different approaches. One involves adding the phenyl group to a bare silica column, while the other involves removing the phenyl group on a uniform column, both in a gradient fashion. In both cases, the columns are chromatographically characterized with test mixtures and with either a mixture of amphetamines or sunscreen chemicals, and comparisons to uniformly modified phenyl columns are made.