Probing the inner surface of a capillary with the atomic force microscope

Abstract

The inner surface of a fused silica capillary for zone electrophoresis was probed by the atomic force microscope (AFM) in the contact mode. Only uncoated surfaces were analyzed, after a simple washing cycle with detergent, NaOH and HCl, and final equilibration in distilled water. Three progressively larger surface areas were probed: 0.5 x 0.5 micron, 2 x 2 microns and 4 x 4 microns. In all cases, it was found that the surface is remarkably smooth, with a median height increasing from 1.3 to 5.6 nm; mean height, from 1.3 to 5.8 nm; root-mean-squared roughness, from 0.35 to 1.5 nm; and average roughness, from 0.28 to 0.67 nm (the lower values referring to the smaller area, the larger to the largest area probed). The lowest "peaks" detected are of the order of 1-2 nm; some scattered peaks as high as 16 nm are occasionally found. It is concluded that the inner surface of a capillary is not a serrate or notchy structure, but is indeed quite smooth. Since the average roughness is comprised within the thickness of the diffuse double layer (> 10 nm) existing on the silica wall as a result of silanol ionization (and, in fact, it is on the average considerably smaller), it is concluded that it cannot possibly influence peak shape and contribute to peak decay in an electrophoretic run.

Comparison of capillary zone electrophoresis performance of powder‐blasted and hydrogen fluoride‐etched microchannels in glass

“…From this paper it can be concluded that the timewidth increase of eluted zones shows a 6th-power dependence on fluctuations in capillary radius. For conventional electrophoresis on fused-silica capillaries this effect is insignificant, the reported average roughness of 0.28-0.67 nm [24] on a nominal capillary diameter of 20-100 mm would lead to a dispersion increase of 0.04% at the most. However, the situation may be different on microfabricated channels.…”

Fused-silica capillaries for capillary electrophoresis and gas chromatography: Inner surface corrosion, within-batch differences, and influence of drawing parameters studied by atomic force microscopy

“…Here it has been suggested that structures within the range of the electric double layer would not influence the z-potential [8]. In order to confirm this assumption, a standard method was used to check the influence of the surface on the separation efficiency [16].…”

“…Barberi et al [8] have found heights of between 1.3 nm and 5.6 nm for fresh capillaries. They reported roughness values in the range between rms = 0.35 nm and rms = 1.5 nm.…”

Topographical properties of polymer films deposited in capillaries for electrophoretic separations of large organic molecules

“…Therefore, we have selected two coating procedures found previously to yield the best results, studied the thickness and topography of the films thus obtained, and related them to the electrophoretic performance of the capillaries. Atomic force microscopy has primarily been used as it is a potent technique for characterization of bare/etched silica surfaces [16,17,23] and of adsorption of proteins on bare silica [18 -20] and on silica coated with polymers [21,22] or surfactants [24].…”