Study of an RF cold atmospheric pressure plasma jet: mass spectrometry and bacteria inactivation

In the past few years, the bactericidal effect of atmospheric pressure plasmas have gained significant attention in dermatology and for application like the sterilisation of heat sensitive medical devices such as endoscopes, dentist and surgical tools. Previous research performed at the TUe by van Gils et al has shown that plasma induced liquid chemistry is important in the inactivation of bacteria in a liquid. A chemical model was developed to predict the time depend chemical compounds induced by the plasma due to a constant production of nitric oxide (NO), ozone (O3) and hydroxide (OH) in the liquid phase. This model was fitted to species measured in the liquid phase. HNO2, ONOO − and H2O2 were found in quantities that could explain the bactericidal effect of a plasma treatment. This motivated a study of bacteria inactivation by a plasma jet at different axial positions, aiming to find the necessary threshold concentrations of plasma produced species. The gas phase reactive neutral species, liquid species, UV and ion fluxes are measured in order to find a correlation between these parameters and the bactericidal effect of a plasma. This work has focussed on two issues: (1) the determination of the absolute surface densities of plasma produced NO and O3 species to remove the necessity of the assumptions in the model, and (2) the distance dependent bactericidal effect of the plasma treatment. A molecular beam mass spectrometry study has been performed to measure the absolute NO and O3 surface densities. A procedure has been developed to circumvent the large changing background signal of the mass spectrometer which complicates absolute calibration of NO under the experimental conditions of the RF plasma jet. This method enables the MS to detect sub ppm NO and O3 concentrations. A systematic study of the production of NO and O3 shows that an increase in plasma dissipated power and admixing air results in an increase of the NO density in the plasma jet. The admixing of O2 decreases the NO production. The O3 production of the plasma jet is increased by admixing O2 and air into the plasma, increasing the power and decreasing the duty cycle. To explain the bactericidal effect of a plasma, distance dependent biological measurements are combined with diagnostics of NO, O3, H2O2, UV and ion flux, and the pH in the liquid. O3 and NO surface densities and ion fluxes are obtained with the mass spectrometer. H2O2 densities in the liquid are measured with a colorimetric method. The UV flux is measured with a spectrometer. It is shown that a synergistic effect of either H2O2 and pH and/or the pH dependent HNO2 concentration is responsible for the distance dependent effect of a bacteria inactivation with a plasma jet.