Optimization of the porous structure and polarity of polymethacrylate‐based monolithic capillary columns for the LC‐MS separation of enzymatic digests

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Abstract
The porous structure as well as the polarity of methacrylate ester‐based monolithic stationary phases has been optimized to achieve the separation of various peptides originating from enzymatic digestion. The porous structure, determined by the size of both pores and microglobules, was varied through changes in the composition of porogenic solvents in the polymerization mixture, while the polarity was controlled through the incorporation of butyl, lauryl, or octadecyl methacrylate in the polymer backbone. Both the morphology and the chemistry of the monoliths had a significant effect on the retention and efficiency of the capillary columns. The best resolution of peptidic fragments obtained by digestion of Cytochrome c with trypsin in solution was obtained in a gradient LC‐MS mode using a monolithic capillary column of poly(lauryl methacrylate‐co‐ethylene dimethacrylate) featuring small pores and small microglobules. Raising the temperature from 25 to 60°C enabled separations to be carried out at 40% higher flow rates. Separations carried out at 60°C with a steeper gradient proceeded without loss of performance in half the time required for a comparable separation at room temperature. Our preparation technique affords monolithic columns with excellent column‐to‐column and run‐to‐run repeatability of retention times and pressure drops.