Dielectric analysis of microfluidic systems with an integrated microwave resonant device

Herein we present a device for performing non-contact dielectric spectroscopy upon liquids in a microfluidic environment. The device is comprised of a compression-sealed, micromilled polytetrafluoroethylene (PTFE) chip with an embedded GHz-frequency coaxial resonator. The resonator is overmoded, allowing dielectric measurements at six discrete frequencies between 1 and 8 GHz. A novel electromagnetic coupling structure allows transmission measurements to be taken from one end of the resonator, which yields a large dynamic range; provides coupling to all modes, and allows easy integration with the fluidic circuitAn optimised microchannel design maximises sensitivity and repeatability. A simple `fingerprint´ method for identifying solvents is demonstrated, whereby a sample is characterised by air-referenced changes in centre frequency and bandwidth of the first six modes of the device. Complex permittivity values are also obtained from these measurements according to a perturbation theory-based inversion, and are quantified for a variety of common solvents. A combination of experimental and simulated results is used to characterise the device behaviour, limits of operation and measurement uncertainty. The error of the fingerprint method is five orders of magnitude lower than the measured changes in frequency, and the uncertainty of the complex permittivity values is <; 2%. The high stability of temporal measurements, coupled with the robustness of the design, make this device ideal for analytical chemistry and industrial process control.