Combination of large-volume sample stacking with an electroosmotic flow pump with field-amplified sample injection on cross-channel chips.

A combination of two online sample concentration techniques, large-volume sample stacking with an electroosmotic flow (EOF) pump (LVSEP) and field-amplified sample injection (FASI), was investigated in microchip electrophoresis (MCE) to achieve highly sensitive analysis. By applying reversed-polarity voltages on a cross-channel microchip, anionic analytes injected throughout a microchannel were first concentrated on the basis of LVSEP, followed by the electrokinetic stacking injection of the analytes from a sample reservoir by the FASI mechanism. As well as the voltage application, a pressure was also applied to the sample reservoir in LVSEP-FASI. The applied pressure generated a counter-flow against the EOF to reduce the migration velocity of the stacked analytes, especially around the cross section of the microchannel, which facilitated the FASI concentration. At the hydrodynamic pressure of 15 Pa, 4520-fold sensitivity increase was obtained in the LVSEP-FASI analysis of a standard dye, which was 33-times higher than that obtained with a normal LVSEP. Furthermore, the use of the sharper channel was effective for enhancing the sensitivity, e.g., 29 100-fold sensitivity increase was achieved with the 75-μm wide channel. The developed method was applied to the chiral analysis of amino acids in MCE, resulting in the sensitivity enhancement factor of 2920 for the separated d-leucine.