Pressure-Driven Reverse-Phase Liquid Chromatography Separations in Ordered Nonporous Pillar Array Columns

Abstract

Building upon the micromachined column idea proposed by the group of Regnier in 1998, we report on the first high-resolution reversed-phase separations in micromachined pillar array columns under pressure-driven LC conditions. A three component mixture could be separated in 3 s using arrays of nonporous silicon pillars with a diameter of approximately 4.3 microm and an external porosity of 55%. Under slightly retained component conditions (retention factor k' = 0.65-1.2), plate heights of about H = 4 microm were obtained at a mobile phase velocity around u = 0.5 mm/s. In reduced terms, such plate heights are as low as hmin = 1. Also, since the flow resistance of the column is much smaller than in a packed column (mainly because of the higher external porosity of the pillar array), the separation impedance of the array was as small as E = 150, i.e., of the same order as the best currently existing monolithic columns. At pH = 3, yielding very low retention factors (k' = 0.13 and 0.23), plate heights as low as H = 2 microm were realized, yielding a separation of the three component mixture with an efficiency of N = 4000-5000 plates over a column length of 1 cm. At higher retention factors, significantly larger plate heights were obtained. More experimental work is needed to investigate this more in depth. The study is completed with a discussion of the performance limits of the pillar array column concept in the frame of the current state-of-the-art in microfabrication precision.