This paper shows that in situ micromachining can be used to simultaneously position and define (i) support particles, (ii) convective transport channels, (iii) an inlet distribution network of channels, and (iv) outlet channels in multiple chromatography columns on a single quartz wafer to the level of a few tenths of a micrometer. Stationary phases were bonded to 5 x 5 x 10 microns collocated monolith support structures separated by rectangular channels 1.5 microns wide and 10 microns deep with a low degree of deviation of channel width between the top and bottom of channels. High aspect ratio microfabrication can only be achieved with deep reactive ion etching. The volume of a 150 microns x 4.5 cm column was 18 nL. Column efficiency was evaluated in the capillary electrochromatography (CEC) mode using rhodamine 123 and a hydrocarbon stationary phase. Plate heights in these columns were typically 0.6 micron in the nonretained and 1.3 microns in the retained modes of operation. Columns were designed to have identical mobile-phase velocity in all channels in an effort to minimize outgassing during operation. When the total lateral cross-sectional area of channels at all points along the separation axis is identical, linear velocity of the mobile phase in a CEC column should be the same. Columns were operated at atmospheric pressure.

Mixing confluent liquid streams is an important, but difficult operation in microfluidic systems. This paper reports the construction and characterization of a 100-pL mixer for liquids transported by electroosmotic flow. Mixing was achieved in a microfabricated device with multiple intersecting channels of varying lengths and a bimodal width distribution. All channels running parallel to the direction of flow were 5 microm in width whereas larger 27-microm-width channels ran back and forth through the parallel channel network at a 45 degrees angle. The channel network composing the mixer was approximately 10 microm deep. It was observed that little mixing of the confluent solvent streams occurred in the 100-microm-wide, 300-microm-long mixer inlet channel where mixing would be achieved almost exclusively by diffusion. In contrast, after passage through the channel network in the approximately 200-microm-length static mixer bed, mixing was complete as determined by confocal microscopy and CCD detection. Theoretical simulations were also performed in an attempt to describe the extent of mixing in microfabricated systems.

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