Shielded Stationary Phases Based on Porous Polymer Monoliths for the Capillary Electrochromatography of Highly Basic Biomolecules

A novel stationary phase for capillary electrochromatography has been prepared via photoinitiated grafting of two layers of polymer chains onto the pore surface of a porous polymer monolith. To achieve the desired retention, the original monolith with optimized porous properties was grafted with an "interior" layer consisting of the ionizable monomer, 2-acrylamido-2-methyl-1-propanesulfonic acid, followed by a "covering" layer of hydrophobic polymer chains. This technique affords monolithic CEC columns that facilitate electroosmotic flow (EOF) while preventing ionized analytes from interacting with the charged surface functionalities. Grafting of the second layer does not adversely affect the EOF. Grafting times of 30 and 60 s for AMPS and butyl acrylate, respectively, enabled the preparation of a monolith with full shielding of the analytes from the ionizable functionalities and excellent chromatographic performance. This approach allows for the first time the independent optimization of both electroosmotic flow and retention properties in CEC columns. The efficient isocratic separations of mixtures of peptides, including some that are highly basic and would be affected by unshielded charges, were routinely achieved in 40-90 s using a simple MS compatible mobile phase consisting of 20 mmol/L ammonium acetate in a 1:1 water-acetonitrile mixture.