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

Area of Science:

• Analytical Chemistry
• Surface Science
• Physical Chemistry

Background:

• Fused silica capillaries are widely used in zone electrophoresis.
• The surface characteristics of these capillaries can influence separation efficiency and peak shape.
• Previous assumptions suggested a rougher inner surface.

Purpose of the Study:

• To quantitatively assess the surface topography of uncoated fused silica capillaries.
• To determine if capillary inner surface roughness affects electrophoretic performance.
• To investigate the relationship between surface roughness and the diffuse double layer in electrophoresis.

Main Methods:

• Atomic Force Microscopy (AFM) in contact mode was employed.
• Analysis of uncoated fused silica capillary inner surfaces after standard cleaning protocols.
• Probing of three distinct surface areas: 0.5x0.5 µm, 2x2 µm, and 4x4 µm.

Main Results:

• The inner capillary surface was found to be remarkably smooth across all probed areas.
• Median height ranged from 1.3 to 5.6 nm, mean height from 1.3 to 5.8 nm.
• Root-mean-squared roughness varied from 0.35 to 1.5 nm, and average roughness from 0.28 to 0.67 nm.

Conclusions:

• The inner surface of fused silica capillaries is not serrated but is exceptionally smooth.
• The average surface roughness is significantly smaller than the diffuse double layer thickness (>10 nm).
• Capillary surface roughness is unlikely to influence peak shape or cause peak decay during electrophoretic runs.