Porous silicon as a stationary phase for shear-driven chromatography

We report on the possibility to strongly increase the mass loadability and retention capacity of shear-driven chromatography (SDC) channels by growing a thin porous silicon layer on the stationary wall part. The thickness of the produced porous silicon layers was found to increase linearly with the anodisation time, and could easily be varied between 50 and 300 nm. Combining these layers with sub-μm thin flow-through channels, we believe it is the first time a sub-μm on-chip LC system with a phase ratio similar to that in packed column HPLC (i.e., Vs/Vm≅1.5) is obtained. The chromatographic performance of the produced channels has been tested by separating binary mixtures of coumarin dyes under RP-LC conditions. The plate height measurements, yielding Hmin≅0.5 μm (corresponding to more than 2×106 plates/m) for a retained component with k″=3, showed good agreement with the theoretical expectations. Due to the presence of some macroscopic defects in the prepared layers, the quality of the separations could however only be maintained over a few millimeters of the channel length. This length was however more than sufficient to separate the coumarin mixture, given the extremely small plate heights of the system.