Ask LCGC: Capillary Electrophoresis (CE)

The following question and answer are taken from LCGC Europe’s “CE Currents” column (1).

Q: Does capillary electrophoresis (CE) sample solvent viscosity have an effect?

A: Viscosity has a profound effect on the amount loaded during pressure injection. There is a viscosity factor in the loading equation:

Volume injected = ΔPd⁴πt/128ηL (1)

where ΔP is pressure difference in millibar, d is capillary diameter in micrometers, t is injection time in seconds, L is capillary length in centimeters, and η is viscosity.It is essential to match the viscosity of the sample and standards otherwise differential loading amounts and incorrect quantitative results will occur. For example if the viscosity of samples solutions is higher than that of the standards then low assay results will occur. An increased sample solution viscosity can occur because of the nature of the sample under test, for example, tablets often contain cellulose which thickens solutions when dissolved.

Internal standards should be used whenever possible in CE to minimize sample effects and improve quantitative analysis performance.

Another approach to overcome viscosity issues can be the addition to directly mimic the sample. For example, in the determination of alginate in pharmaceutical preparations the authors added placebo tablets into standard solutions to ensure that the viscosity of the standard solutions exactly matched the samples — recoveries of 99.1% were obtained using this approach (1).

(1) N. Oztekin, S. Baskan, and F.B. Erim, J. Chromatogr. B, 850, 488–492 (2007).

Questions?

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