Fabrication of a Microchip Device for Liquid Phase Ion Mobility Spectrometry

A polymer based microfluidic chip has been developed to generate and detect ions from different chemical samples for liquid phase ion mobility spectrometry. This is a device which integrates both the ionization source and the detection part in the same module for lab-on-a-chip applications. The fluidic part consists of a nozzle (12 μm deep) and a drift channel (400 μm deep), which are embedded into the chip made of polydi-methyl-siloxane (PDMS). Its (PDMS) transparency, insulating properties, and chemical inertness make it indispensable as the device material of electrospray ionization ion mobility spectrometry. Soft lithography and oxygen plasma were used for channel fabrication and bonding respectively. The electronic part consists of high voltage power supplies, current detection system, and a series of platinum microelectrodes embedded in the nozzle and drift channel. Platinum microelectrodes, being inert to chemicals and bio-molecules, become essential components in microfluidic devices. The biggest challenge of laying down platinum into a deep channel has been overcome by developing a new fabrication procedure. This approach, included in details in this study, can be used for both low and high aspect ratio channels and avoids hazardous chemicals. The depth of the Pt-electrode was measured as 300-400 nm with a surface roughness of ±50 nm. The continuity of microelectrodes, ionization and detection performance of the developed microchip were investigated. A non-polar liquid, benzene, and a polar liquid sample, 90:10 Methanol:Water, were ionized separately in the microchip. The maximum ion current measured by the detector was approximately 0.7 nA due to sample ionization. It was also observed that benzene was not ionized. In both cases the electrodispersion ionization voltage was 1250 V/mm and the drift (electric) field was 50 V/mm. This ensures the continuity of microelectrodes and satisfactorily demonstrates the ionization and detection of ions. Negative and positive ions were detected by the microchip. It was also observed that the increase in drift field increased the ion current probably because the loss of some ions at the channel surface was avoided at a higher drift field.

Al Mamun, Nazmul Huda

Washington State University




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