Solving advanced micromachining problems for ultra-rapid and ultra-high resolution on-chip liquid chromatography

Abstract

High-performance liquid chromatography (HPLC) is one of the most versatile separation techniques available for the analysis of complex samples that are typically encountered in fields such as environmental monitoring, biology, pharmacy, biochemistry, etc. A distinction between different HPLC formats can occur in the shape of the stationary phase, which necessarily displays a selective interaction with the present analytes in order to establish a separation. The most prevailing format is the packed particulate bed, which generally consists of functionalized porous spherical particles that are randomly packed in a capillary. Monolithic media (polymeric or silica) have recently become popular because of the high permeability combined with reasonable mass transfer characteristics. In this frame it is important to stress that it is theoretically expected that disorder restrains the performance of a column. When conceiving the ideal chromatographic format, the achievement of more order is therefore an interesting route to pursue. A practical realization of this approach was first put forward in 1998 by prof. Fred Regnier, making use of a microfabricated array of pillars in glass in capillary electro-chromatography mode, replacing the random particles by very accurately positioned pillars. Recognizing the potential of the technique, Desmet and coworkers performed a number of computational fluid dynamics (CFD) to study the fluidic behaviour of such a pillar array in pressure-driven mode. As the flow-through pores can be chosen independently of the pillar diameter, optimal designs can be tailored to provide the optimal combination of flow resistance and plate height. A first device containing non-porous silicon pillars was then characterized by De Pra et al. in 2005 under non-retaining conditions, achieving a minimal reduced plate height of 0.2 in a 40 % porosity pillar bed consisting of 10 μm pillars, in agreement with the CFD predictions. Even though this work was an important trigger in generating interest in this novel format, no separations were demonstrated, hence keeping the more relevant fluidic behaviour under retentive conditions in the dark