Microfluidic Free-Flow Electrophoresis Based Solvent Exchanger for Continuously Operating Lab-on-Chip Applications.

For miniaturization and integration of chemical synthesis and analytics on small length scales, the development of complex lab-on-chip (LOC) systems is in the focus of many current research projects. While application specific synthesis and analytic modules and LOC devices are widely described, the combination and integration of different modules is intensively investigated. Problems for in-line processes such as solvent incompatibilities, e.g., for a multistep synthesis or the combination of an organic drug synthesis with a cell-based biological activity testing system, require a solvent exchange between serialized modules. Here, we present a continuously operating microfluidic solvent exchanger based on the principle of free-flow electrophoresis for miscible organic/aqueous fluids. We highlight a proof-of-principle and describe the working principle for the model compound fluorescein, where the organic solvent DMSO is exchanged against an aqueous buffer. The DMSO removal performance could be significantly increased to 95% by optimization of the microfluidic layout. Moreover, the optimization of the inlet flow ratio resulted in a minimized dilution factor of 5, and we were able to demonstrate that a reduction of the supporting instrumentation is possible without a significant decrease of the DMSO removal performance. Finally, the compatibility of the developed solvent exchanger for cell based downstream applications was proven. The impedimetric monitoring of HEK293A cells in a continuously operating microfluidic setup revealed no adverse effects of the residual DMSO after the solvent replacement. Our solvent exchanger device demonstrates the power of micro-free-flow electrophoresis not only as a powerful technique for separation and purification of compound mixtures but also for solvent replacement.

Temperature Gradient Elution and Superheated Eluents in Chip-HPLC.

Utilizing temperature as an active parameter for optimization in chip-based liquid chromatography is an important step toward high-speed and high-efficiency separations on the microscale. A device including a low thermal mass micro thermostat and a microfluidic glass chip as central elements were designed and evaluated for maximal heating performance of up to 4.7 °C s-1 at up to 200 °C. With this enabling technology, high-speed separations in temperature gradient mode were performed both in common reversed-phase eluents and environmental friendly ethanol-based alternatives.

Enantioselective reaction monitoring utilizing two-dimensional heart-cut liquid chromatography on an integrated microfluidic chip.

Chip-integrated, two-dimensional high performance liquid chromatography is introduced to monitor enantioselective continuous micro-flow synthesis. The herein described development of the first two-dimensional HPLC-chip was realized by the integration of two different columns packed with reversed-phase and chiral stationary phase material on a microfluidic glass chip, coupled to mass spectrometry. Directed steering of the micro-flows at the joining transfer cross enabled a heart-cut operation mode to transfer the chiral compound of interest from the first to the second chromatographic dimension. This allows for an interference-free determination of the enantiomeric excess by seamless hyphenation to electrospray mass spectrometry. The application for rapid reaction optimization at micro-flow conditions is exemplarily shown for the asymmetric organocatalytic continuous micro-flow synthesis of warfarin.

On-chip integration of organic synthesis and HPLC/MS analysis for monitoring stereoselective transformations at the micro-scale.

We present a microfluidic system, seamlessly integrating microflow and microbatch synthesis with a HPLC/nano-ESI-MS functionality on a single glass chip. The microfluidic approach allows to efficiently steer and dispense sample streams down to the nanoliter-range for studying reactions in quasi real-time. In a proof-of-concept study, the system was applied to explore amino-catalyzed reactions, including asymmetric iminium-catalyzed Friedel-Crafts alkylations in microflow and micro confined reaction vessels.

HPLC-MS with Glass Chips Featuring Monolithically Integrated Electrospray Emitters of Different Geometries.

We present and evaluate an approach for coupling liquid chromatography in glass chips with mass spectrometry via fully integrated electrospray emitters. We developed an instrumental platform which allows a robust and reproducible operation of high performance chip chromatography coupled to mass spectrometry. A comparison of differently shaped emitters, from flat over edged to pulled geometries, revealed that all types performed equally well for typical nano-HPLC flow rates. At very low flow rates below 50 nL·min(-1) very sharp, pulled nanospray emitters turned out to be mandatory for the generation of a stable electrospray.

High-performance liquid chromatography on glass chips using precisely defined porous polymer monoliths as particle retaining elements.

A stable and permanent integration of miniature packed bed separation columns into microfluidic systems is a major issue in nano liquid chromatography. Various approaches like differently shaped retaining elements or the use of key stone effect have been investigated. We show a flexible integration of miniature packed bed separation columns into microfluidic chips utilising common HPLC material achieved by laser-assisted generation of narrow, photopolymerised frits. The generated retaining elements serve as an in- and outlet frits for the columns. An optimised pre-polymeric solution, consisting of butyl acrylates and a porogen, allows a precise fabrication of frit-type structures with lengths of less than 100 m and the capability to withstand common slurry packing pressures of more than 250 bar. The separation of seven polycyclic aromatic hydrocarbons by pressure-driven, reversed-phase chromatography proves the high quality of the created chromatographic column inside a glass chip. Plate heights down to 2.9 were achieved and extremely fast separations with sub-second peak widths were performed in isocratic and gradient elution modes on very short columns (≤ 25 mm).