A Tunable High-<i>Q</i> Microwave Detector for On-Column Capillary Liquid Chromatography
Yan Cui, Aijun Ge
Abstract
We report a novel approach to ultrahigh quality factor (Q) in the split-ring resonator with an active feedback loop, which utilizes a phase shifter to tune equivalent negative resistance for accurate loss compensation. Based on the proposed technique, a microwave detector is designed for on-column liquid chromatography. With lossy solvents loaded, the measured Q is as high as 4.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> at ~ 3.89 GHz. We demonstrate the highly sensitive and contactless measurement of glucose-water solution in a capillary tube under stop-flow and plug-unplug modes, respectively. With the stop-flow mode, different concentrations between 40 and 150 mg/dL are measured, which covers the normal blood-glucose levels. The minimum detectable quantity of glucose is ~18.8 ng with an effective sample volume of ~47 nL. The plug-unplug mode provides fast and flexible sampling and analysis. Further investigations are required for quantification of material-under-test permittivity and promotion of sensitivity.