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Electrospray from hot embossed polymer microfluidic chips formed using laser machined and electroformed tools

The aim of this project was to develop high performance polymer microfluidic chips with reduced complexity for Electrospray Ionization Mass Spectrometry (ESI-MS) analysis. This paper presents the fabrication and testing of developed hot embossed open channel polymer microfluidic chips for ESI-MS. Hot embossing was done using a laser machined steel tool and an electroformed nickel tool on polystyrene (PS) and polycarbonate (PC) substrates. Stable electrosprays were generated at microchannel exits of replicated microchips without cover using a high voltage difference between a positive stainless steel electrode in the reservoir and a negative aluminum plate. Electrospray parameters such as; nozzle tip distance from counter electrode, ESI onset potential and duration were investigated. For open channel systems, the results show that the electric field for stable ES directly relates to the distance between the channel tip and counter electrode, onset potential applied and to the flow velocity of the test solution in the microchannel. Fluid is delivered as a result of electroosmosis due to an applied electric field and capillary action, thereby eliminating the need for external pressure devices. From experimental results, for an open-channel of 100μm width, 100μm depth, length 12.5mm attached to an open reservoir of diameter 2 mm, the optimum distance between the channel exit tip and counter electrode is 1.2 mm for initiation of electrospray at voltage of ~2000 volts. The laser machined steel tool was found to be more durable than the nickel tool for PS/PC microstructure fabrication.