Capillary zone electrophoresis with fluid-impervious polymer tubing inside a fused-silica capillary

A fused-silica capillary was used as a mold in the in situ formation of, and as a sheath for, a highly crosslinked poly(styrene-divinylbenzene) inner tubing having 3-5 microns wall thickness. This 'tube-in-the-tube' construction with the thin-walled, fluid-impervious polymer inner tubing precluded contact between the aqueous buffer solution and the inner wall of the fused-silica capillary. As a result, this structure withstood long-term treatment with highly alkaline solutions without deterioration. In order to hydrophilize the inner surface of the polymer tube, a polyoxyethylene oligomer was grafted to its inner wall and subsequently crosslinked. The inner tube with such hydrophilic coating was also stable to hydrolytic attack by 1 M NaOH. Although the nonpolar polymeric inner surface generated electrosmotic flow as if it had some fixed negative charges, the flow velocity became almost negligibly small once it had been hydrophilized. As illustrated by electropherograms, the hydrophilization of the polymeric inner tube greatly facilitated the CZE of basic proteins in the pH range from 3 to 6 without the need for additives in the electrophoretic medium to mask the silanol groups at the surface of quartz capillaries.