Effects of N2–O2Gas Mixture Ratio on Microorganism Inactivation in Low-Pressure Surface Wave Plasma

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In this study, the effect of N2/O2gas mixture ratio on low-pressure surface wave plasma inactivation of spore-forming bacteria was investigated. It was experimentally confirmed from the quadrupole mass spectrometry measurements that the spores were etched by atomic oxygen via converting the hydrogen atoms constituting microorganisms into H2O and the carbon into CO2. On the basis of results of plasma diagnostics by optical emission spectroscopy and the results of inactivation efficiency by colony-forming units and scanning electron microscope, we found that although there is the highest ultraviolet (UV) emission intensity in pure N2plasma and the highest etching efficiency in 90% O2/10% N2plasma, the inactivation rate of microorganisms was not so efficient. The best inactivation result was obtained in 30–80% O2gas mixture ratios after 60 s plasma irradiation. The present results indicated that more efficient inactivation is achieved by the synergetic effects between atomic oxygen etching and the vacuum ultraviolet (VUV)/UV emission by combining both effects via optimizing N2/O2gas mixture ratio.