Low-power microwave-generated helium microplasma for molecular and atomic spectrometry

Atmospheric pressure microplasmas are a promising technology for low-power optical emission spectroscopy for chemical detection. In this work, we examine a microstrip split-ring resonator (MSRR) discharge operating at 1.8 GHz in helium as an excitation source. The source can sustain a plasma with as little as 0.2 W of microwave power, and can be operated continuously with no electrode damage. With a compact 156 mm focal length spectrometer system, we determined detection limits on the order of 1 ppm for methane, n-butane, carbon dioxide, and hydrogen sulfide with plasma powers of both 0.3 and 1.0 W. With appropriate choice of the monitored emission lines, the detection system is robust to small additions of air. We also demonstrate the applicability of the MSRR as a sensor for gas chromatography.