Evaluation of plasma induced liquid chemistry for bacteria treatment with a remote RF argon atmospheric pressure plasma jet

The inactivation of bacteria suspended in liquids by an atmospheric pressure plasma is a complicated and yet not well understood process, due to several constituents of the "plasma cocktail" in the plasma/gas phase which can contribute to bacteria inactivation and the transport and chemistry processes of the created species in the liquid phase. To investigate the main players in the bacteria inactivation processes we use a RF atmospheric pressure plasma jet which is used for remote bacteria treatment. The plasma jet is constructed to allow accurate power measurements. Mass spectrometry, optical emission spectroscopy as well as electrical diagnostics have been used to investigate the amount of charged and reactive species and UV-emission reaching the water surface. We show that with the chosen treatment conditions, reactive species (ROS and RNS) concentrations and probably (V)UV emission are high enough to induce chemistry in the liquid phase leading to bacteria inactivation, while the electric field and charged particles are too low to induce observed effects. To further determine the role of the reactive species interaction within the liquid, nitrite, nitrate and hydrogen peroxide concentrations have been obtained by ion chromatography and colorimetric methods. Combined with measured and estimated values of the reactive species in the gas phase we use a 0D-solution kinetics model to calculate other species expected in the liquid phase and important for bacteria inactivation, which have not been measured. We show that the obtained concentrations of HNO2, ONOO and H2O2 are in the same range as reported values in literature of the minimum inhibitory and bactericidal concentrations.