(350a) Analytical Model of Concentration Boundaries in Single Interface Isotachophoresis

Single interface isotachophoresis has been used as a preconcentration technique to achieve up to million fold increase in sample concentration [1].  It employs an electrolyte system in which the mobility of the sample analytes is bound by mobility of the leading (LE) and the trailing electrolytes (TE). The sample ions are mixed uniformly in the trailing electrolyte, and not injected as a finite plug between the LE and TE as in conventional isotachophoresis. An initial interface between the LE and TE is created in simple-cross microchip geometry as illustrated in Fig 1. On applying an electric field, a steady diffused boundary forms at the LE and TE interface [2]. The sample ions in the TE overspeed trailing ions and accumulate in this moving concentration boundary [3].

The typical concentration of LE in the experiments is approximately 1 M and analyte concentration is 1 nM or less. The thickness of the diffused boundary is  few microns and the channel length is order 10 cm. So, the concentration and length scale ratios involved in this problem are respectively order 10⁹ and 10⁴ or greater. This makes the problem numerically stiff and difficult to resolve fully.  We present a simplified theoretical model to predict sample concentration growth and the characteristics of the moving concentration boundary. We perform controlled experiments to validate the theoretical model. The effects of varying current, leading and trailing ion concentration on the same concentration and moving boundary are investigated experimentally and compared with the theoretical model.  The model is useful for experiment design and optimization of stacking and separation conditions.

Reference:

[1]   Jung B, Bharadwaj R, Santiago JG, Analytical Chemistry, 2006, 78, 2319-2327

[2]   Saville DA, Palusinki OA, AICHE Journal, 1986, 32, 207-214.

[3]   Ludmila K,Bocek P, J. Chromatography, 1997, 689, 13-24.