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Spontaneous injection in microcolumn separations.

The phenomenon of spontaneous (ubiquitous) injection in microcolumn separations has been characterized to improve quantitative precision for ultramicrosampling and to enhance separation efficiency. By combining fluorescence imaging, video microscopy, and measurements from capillary electrophoresis, we demonstrate that spontaneous injection is caused primarily by an interfacial pressure difference formed at the inlet of the capillary. This complex injection mechanism has been modeled with some simple assumptions based on fluid dynamics. In particular, studies showed that extraneous injection is reduced up to 12-fold by etching the capillary inlet or by using a thin-walled capillary. Variations by a factor of 2 in the injection length can result from delays between sample introduction and reinsertion into the inlet vial if the timing is not controlled precisely. Evidence is presented that evaporation of buffer from the inlet can reduce the injection length by more than 1 order of magnitude.