# Multimodal Comparison of Cold Atmospheric Plasma Sources for Disinfection
## 4 Results and Discussion  
The biological effects, reported in the first column of Table 1 with the label _direct_, appear to be very different for the five configurations. The micropulsed source when operating in helium is very effective, inducing an abatement larger than 6 orders of magnitude, which corresponds to the upper detection limit of the method. The same source, when operated in argon, shows a significantly reduced effectiveness, reaching \(3.5\pm 0.2\) logarithmic abatement. In contrast, the radiofrequency source seems not to be effective at all, whether using helium or argon. Finally, the Mibjet source is observed to reduce the bacterial load by slightly more than one order of magnitude. The differences in the biological performance are evident. It is therefore worthwhile to further analyse the factors involved in the interaction, to track down their possible responsibility in determining such a huge gap. The phenomena having a role in plasma-biological interaction can be categorised in two classes: immediate, with time scales spanning in the range ns-us-ms, and long-term, with time scales from seconds and higher. The first class includes UV radiation, electric fields, local heating of the substrate, and short-lived reactive chemical species. UV radiation, besides its importance in the chemistry of the gas phase and in the interaction with air molecules to create radicals, is generally not strong enough to penetrate the water layer and induce significant damage to the cell membrane [34]. On the other hand, electric fields can play a role: especially when the jet reaches the water, on the tip of the plasma, local high fields are reached, and it is proven that high electric fields can induce lethal electroporation [35; 36]. _A priori_, heat can also play a role in the bactericidal processes. Even if the bulk water has been observed not to heat, in fact, local temperature peaks can be reached in the contact point between plasma jets and the water surface. The sensitivity to temperature changes is highly dependent not only on the bacterial species considered, but also on the specific strain and substrain. The bacteria used in this work are resistant up to 50 \({}^{\circ}\)C, while they die in several minutes above 54 \({}^{\circ}\)C [37]. Finally, short-lived species like O\({}_{2}^{-}\), atomics ones like O and H, and molecular radicals as OH and NO, are known to be highly cytotoxic [38]. In this class, we are referring here to species directly generated by the plasma or in the gas-liquid interaction, and not to the tertiary species which can be generated in further reactions between long-lived species, which instead belong to the long-timescale chemistry. The second class, in fact, collects all the reactive oxygen and nitrogen species which accumulate in water, as for examples nitrates, nitrites and peroxides. Bacteria are exposed for many minutes to these species, and to the environment--in terms, for example, of pH--that they create. \begin{table}
\begin{tabular}{l c c} \hline  & \multicolumn{2}{c}{**Abatement [logAbat]**} \\  & **Direct** & **Indirect** \\ \hline Micropulsed/He & \textgreater{}6 & \textgreater{}6 \\ Micropulsed/Ar & \(3.5\pm 0.2\) & \(3.2\pm 0.4\) \\ Radiofrequency/He & \textless{}0.5 & \textless{}0.5 \\ Radiofrequency/Ar & \textless{}0.5 & \textless{}0.5 \\ Mibjet & \(1.3\pm 0.2\) & \(2.9\pm 0.5\) \\ \hline \end{tabular}
\end{table} 
Table 1: Measured logarithmic abatement of _E. coli_ for the five configurations, in direct and indirect treatments. A fast and reliable way to split the effects of the two classes is to perform indirect treatments. With indirect treatments, immediate effects are excluded, being the bacteria still not present in the water, and only long-term effects can have a role. The logarithmic abatements obtained, reported in the last column of Table 1 with the label _indirect_, turn out to be compatible with the values of direct treatments.