A Dual-Mode Microwave Resonator for Liquid Chromatography Applications

This work presents a microwave microfluidic sensor for high performance liquid chromatography (HPLC) applications. The sensor is based on a modified square ring loaded resonator (SRLR), where a transmission line and a ring are electrically shorted with a center gap. A microfluidic channel is bonded above the gap for liquid-under-test (LUT) measurement. When the dielectric constant of LUT is above a threshold value, two degeneration modes of the resonator are separated, resulting in two transmission-zero frequencies. The threshold dielectric constant can be easily tuned by the gap size. High sensitivity is achieved when LUT dielectric constant is close to the threshold value. These features enable the proposed resonator to be optimized for different microfluidic applications. To validate the design, three resonators with 10 μm, 30 μm and 90 μm gap sizes are built and tested with water-methanol solutions in various volume fractions. Additionally, the sensor is connected in series with HPLC system for caffeine and sucrose detection. The detection linearity is characterized by measuring water-caffeine samples from 0.77 ppm to 1000 ppm. A 0.231 ppm limit of detection (LOD) is achieved, revealing a comparable sensitivity with commercial ultraviolet (UV) detectors. The compatibility of the proposed sensor to gradient elution is also demonstrated.