Monolithic columns with a gradient of functionalities prepared via photoinitiated grafting for separations using capillary electrochromatography

Stationary phases for capillary electrochromatography with a longitudinal gradient of functionalities have been prepared via photoinitiated grafting of polymer chains onto the pore surface of a porous polymer monolith. In order to achieve the desired retention and electroosmotic flow, the hydrophobic poly(butyl methacrylate-co-ethylene dimethacrylate) monolith with optimized porous properties was grafted with a layer of ionizable polymer, poly(2-acrylamido-2-methyl-1-propanesulfonic acid). A moving shutter and a neutral density filter were used to control the dose of UV light received at different locations along the monolith in order to create the longitudinal gradient of functionalities. Formation of the desired gradients was confirmed using electron probe microanalysis of different locations along the column. The preparation technique significantly affects performance in the CEC mode as demonstrated on the separations of a model mixture using columns both with homogeneous distribution of grafts and with a gradient of functionality. Columns grafted with the gradient of functionalities were found superior to those functionalized uniformly. A comparison of the performance of the gradient column with another containing evenly distributed functionalities showed the performance benefits of the "gradient" column.