Surface modification based on Si‐O and Si‐C sublayers and a series of N‐substituted acrylamide top‐layers for capillary electrophoresis

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Abstract
Two approaches were used to prepare a series of surface‐modified capillaries. In the first, a sublayer was formed by coupling γ‐methacryloxypropyltrimeth‐oxysilane to the surface silanol groups forming an SI‐O bond; a top layer was then formed by polymerizing acrylamide in the capillary, which reacted with the sublayer. In the second approach, a sublayer was formed by silanol chlorination, followed by Grignard coupling of vinylmagnesium bromide to form an Si‐C bond at the surface; a top layer was formed by polymerizing either acrylamide (AA), dimethylacrylamide (DMA), N‐acryloylaminoethoxyethanol (AAEE), or N‐acryloylaminopropanol (AAP) onto the sublayer. The Si‐C‐poly(AA) capillaries were more stable and produced an approximately 10‐fold lower electroosmotic flow compared to the Si‐O‐poly(AA) capillaries. The Si‐C sublayer was used to compare the performance of all four top layers. Electroosmotic flow decreased in the order: Si‐O‐poly(AA), Si‐C‐poly(AA), Si‐C‐poly(AAEE), Si‐C‐poly(DMA), and Si‐C‐poly(AAP). Si‐C‐poly(AA) showed evidence of irreversible degradation at pH 9 already after 40–50 runs. Si‐C‐polyAAP‐coated capillaries demonstrated superior efficiency and migration time reproducibility for a number of alkaline proteins and for fluorescently labeled ovalbumin. Excellent performance was maintained, in the case of poly(AAP), for a least 300 runs (of 30 min duration) at pH 9.0.