ESI-MS compatible permanent coating of glass surfaces using poly(ethylene glycol)-terminated alkoxysilanes for capillary zone electrophoretic protein separations

Thin poly(ethylene glycol) silane (PEG-silane) coatings formed from JV-(triethoxysilyl propyl)-O-poly(ethylene oxide) urethane with different chain lengths of poly(ethylene glycol) (MW 750 and 4000-5000) are used to modify glass microfluidic channels and fused-silica capillaries for electrophoretic separations of proteins. These coatings combine three important properties, which make them favorable for proteomic analyses including reduction of protein adsorption, compatibility with mass spectrometry due to their stability, and the ability to control the magnitude of electroosmotic flow (EOF). The coatings have been successfully used in microfluidic chips and fused-silica capillaries for separation of protein sample mixtures under low EOF conditions. The long-chain and mixed PEG-silane coatings suppress electroosmotic flow by more than 90%, whereas the short-chain PEG silane suppresses EOF by 65-75% at pH values of 3-9. The long-chain and mixed PEG-silane coatings are suitable for low EOF applications or for cases where negative effects of EOF are to be minimized. Efficient separations of unlabeled basic proteins at low pH and FITC-labeled proteins at high pH were achieved, as well as excellent stability for at least 200 electrophoretic runs. Additionally, these covalent coatings produce no detectable background ions in ESI-MS, making them compatible with on-line mass spectrometry.