Effect of pH and sodium dodecyl sulfate concentration on the analytical window in the direct-injection analysis of plasma samples by capillary electrophoresis.

Plasma samples can be analyzed with direct injection onto a capillary electrophoresis system if interactions between the plasma proteins and capillary walls are minimized. This can be achieved using micellar electrokinetic capillary chromatography with a surfactant such as sodium dodecyl sulfate. The surfactant complexes with the proteins, giving them a net negative charge, and thus causes them to be repelled from the negatively-charged fused-silica capillary walls. Migrating as anions, the complexed proteins appear late in the electropherogram. In such analyses, it is important to manipulate the mobility of the analyte(s) of interest such that they migrate in the useful analytical time window before the plasma proteins. In this article, a study of the effect of buffer pH and surfactant concentration on the width of the migration time window is reported. It is shown that at pH 6 the migration time window is wide, but reduced electroosmosis can result in inordinately long run times; pH 7 gives an acceptable analytical window with acceptable run times over a wide range of sodium dodecyl sulfate (SDS) concentrations; pH 8 and higher are most useful when higher concentrations of surfactant (> 50 mM SDS) are employed.