Integration of uniform porous shell layers in microfabricated pillar array columns by electrochemical anodization

Electrochemical anodization has been applied to grow 300 nm thick porous shell layers in 5 μm diameter silicon pillars. These pillars are positioned in a triangular grid with 2.5 μm spacing to form a pillar array columns (PAC) for liquid chromatography. The uniformity of the porous layer was assessed by determining local plate heights along the column length, which appeared to be constant. Minimal plate heights between 4 to 6 μm were obtained at optimal flow rates and for different retention factors. Using the retention surface involved in retention as an indicator of the available surface, an increase in surface by a factor of 30 was found when comparing to non-porous columns. A commercial capillary LC instrument was modified to perform 2 nl sized injections which allowed more than 80% of the on-chip generated performance (expressed in theoretical plates) to be maintained when coupling a 1m long column to an external UV-detector. As the corresponding pressure drop for optimal operation is typically around 10 bar per m and the maximal pressure that can be applied is in the order of max of 350 bar, very long PACs, well capable of delivering much more than 1 million theoretical plates are within sight.