Transport Number Mismatch Induced Stacking of Swept Sample Zones for Microchip-Based Sample Concentration

In this presentation, we report a hybrid technique for on-chip sample preconcentration. Our technique is based on the observation that when a buffer of lower cationic transport-number is electrokinetically introduced into a second buffer of higher cationic transport-number, but the same conductivity, an expanding zone of lower conductivity is formed. Ionic components of the sample will experience field amplification effects in this expanding zone. This phenomenon was combined with SDS sample sweeping to carry out concentration of hydrophobic analytes on a microchip. Sample concentration enhancement effects using this technique were tested with a number of hydrophobic fluorescent dyes, and signal enhancements of up to 560-fold were achieved for Rhodamine dyes in only 10 mm of channel length. This sample concentration technique can be conveniently integrated into Lab-on-a-Chip devices to achieve enhanced detection sensitivity for hydrophobic and ionic analytes.