Dynamics of Field Amplified Sample Stacking
Abstract We present an analysis and an experimental study of field amplified sample stacking. The analysis consists of a one dimensional, unsteady electromigration model of the concentration fields in sample stacking. The model predicts that the sample concentration develops as a non-linear concentration wave with a peak concentration increasing with time that reaches a maximum value determined by ratio of electric fields in the sample and the buffer region. The experimental work is a preliminary study of the effects of electroosmotic flow on sample stacking for a single fluid-fluid interface within a glass microchannel. Using a simple cross-channel configuration a high-gradient initial step-function in the buffer concentration field was established which causes an unsteady sample concentration process. The experimental results qualitatively confirm the wave-like profile of the stacked analyte predicted by the analytical model. Experiments also provide evidence of internal pressure generation due to mis-matched electroosmotic velocities during stacking. The effect of electric field strength and sample-to-background buffer concentration ratio, γ, are also presented. Stacked sample concentration at γ = 4 is found to initially increase with increasing electric field but saturates at higher electric fields. Stacking rates at higher γ ratios indicate that concentration enhancement is quickly limited by dispersion due to internally generated pressure gradients.