Integrated microchip device with electrokinetically controlled solvent mixing for isocratic and gradient elution in micellar electrokinetic chromatography

A monolithic micromachined device is presented which allows on-chip adjustments of the content of organic modifier in the run buffer for fast, efficient MEKC separations. Isocratic and gradient conditions are controlled by proper setting of voltages applied to the buffer reservoirs of the microchip. The precision of this control is tested for gradients of various shapes (linear, concave, convex) by mixing pure buffer and buffer doped with a fluorescent dye. The effect of isocratic and gradient solvent changes on the MEKC separation of a mixture of coumarin dyes is demonstrated using methanol and acetonitrile as modifiers. Separations were carried out using a column length of 25 mm and a field strength of 660 V/cm with high resolution. Analysis times were as short as 33 s for methanol and under 22 s for acetonitrile. Gradients with both modifiers were executed within 10 s. Of the two modifiers, acetonitrile proved to have a more pronounced impact on the elution pattern of the test mixture. Only slight band broadening is observed for gradient runs as compared to isocratic runs using methanol. On the other hand, in the case of acetonitrile gradients, some of the peaks exhibit a focusing effect (as observed in HPLC gradients), yielding up to 100 000 plates.