Characterisation of inner surface and adsorption phenomena in fused-silica capillary electrophoresis capillaries

Method validation in capillary electrophoresis (CE) is substantial for the routine use in quality control. Thus all phenomena that can influence results in CE must be carefully characterised. Especially the inner surface of the fused-silica capillaries plays an important role. Sometimes there is a big difference in the properties of capillaries from different batches. Inner capillary surface can be pictured by scanning electron microscopy (SEM). Nature, pattern and number of cracks and other surface defects altered from batch to batch. These surface defects were no artefacts of sample preparation, which could be excluded by comparing different preparation techniques and results from literature. Treatments with hydrofluoric acid cannot completely remove the surface cracks and defects in subsurface areas. Hydrofluoric acid treatment not only changes the surface roughness but also the inner capillary diameter. A correlation between etching time and increase of diameter is investigated. SEM also pictures adsorption phenomena, e.g. proteins are not adsorbed as a homogenous layer. Clusters at different regions are observed. Thus the zeta potential differs considerably with space, leading to losses in separation efficiency. SEM can readily be used to analyse the effectiveness of rinsing procedures. There are residuals of adsorbed compounds even after using aggressive rinsing reagents.