Effect of excited nitrogen atoms on inactivation of spore-forming microorganisms in low pressure N₂/O₂surface-wave plasma

Using a vacuum ultraviolet (VUV) absorption spectroscopy with a compact low pressure plasma light source, the absolute nitrogen atom density was measured to study its role in the spore inactivation with low pressure N 2 /O 2 gas mixture surface-wave plasmas (SWPs). Self-absorption effect of the resonance emission lines of nitrogen atoms near 120 nm was minimized by optimizing its discharge conditions of the plasma light source. Experimental results showed that excited nitrogen atom densities monotonically decreased with the decrease of N 2 gas percentage in N 2 /O 2 gas mixture SWPs, concomitantly with similar decrease of VUV/UV emission intensities of nitrogen atoms and molecules. In the pure N 2 gas SWPs, it was confirmed that a dominant lethal factor was VUV/UV emission generated by N 2 plasma, while spore etching occurred via physical and chemical interactions with nitrogen species. With an addition of O 2 gas, significant spore etching by excited oxygen atoms made it much easier for the VUV/UV photons emitted by nitrogen atoms, N 2 and NO molecules to penetrate through the etched spore coats to the core and cause the fatal DNA damage of the microorganisms. As a result, more rapid inactivation was achieved in the middle region of N 2 /O 2 gas mixture ratio, such as 30–80% O 2 gas addition, in the present N 2 /O 2 gas mixture SWPs.