Characterisation of a dc plasma-on-liquid system.

An atmospheric pressure plasma jet is created in helium gas flowing from a grounded capillary tube. The tube is in close contact with the surface of a liquid, in which a positively biased titanium electrode is immersed and so the liquid itself acts as a second electrode. The practical interest is in the chemistry effects within the bulk liquid. However here the electrical characteristics and spatially resolved emission spectra associated with the plasma are examined and interpreted in the context of the basic physics and chemistry of the discharge. Spectra are examined for solutions of distilled water, phosphate buffered saline and ethanol and provide information about the spatio-temporal nature of the plasma and the species present. These and the electrical characteristics (e.g. breakdown voltage) of the microplasma are found to be dependent on the He gas flow, the distance of the capillary from the surface of the liquid and the solution composition. 1