# Multimodal Comparison of Cold Atmospheric Plasma Sources for Disinfection
## 4 Results and Discussion
Again, the radiofrequency source is not efficient at all while the micropulsed one outperforms when operated in helium, reaching more than 6 logarithmic reduction. An exception is represented by the MibJet, whose result for indirect treatments is measured higher in abatement with respect to the direct ones. This further confirms the role of long-living species, considering that while in direct treatment the sample is exposed to rising concentrations, in the indirect one the source is exposed from the beginning to the final maximum concentrations, resulting for a given treatment time in a larger integrated effect. These results identify the long-timescale chemistry as the main responsible for defining the plasma-biological interaction, and it is particularly interesting in the framework of designing efficient and _ad hoc_ plasma sources. It is important to emphasize that this result is limited to the sources considered here and should not be generalised. Other plasma sources, in different setups and operating conditions, proved to be effective only in direct treatments [39]. In the reported setup, however, due to the geometry of the sources, the electric field applied to the substrate is expected to be relatively modest, and the water is expected to substantially screen the UV flux, leaving to the chemistry a leading role. Having proven the fact that the interaction between plasma and bacteria is mainly chemical, it becomes of fundamental interest to measure the concentration of the chemical species which are formed in the treated samples. Figure 3 reports the concentration measured by spectrophotometry in water for the different sources. The immediate outcome is that, while the biological effects span more than six orders of magnitude, the chemical concentrations are measured to be all on the same order. Moreover, while the micropulsed helium source outperforms in the biological treatments, this uniqueness is not reflected in the chemical characterisation. Both the micropulsed and the radiofrequency sources, when operating in argon, show an improved production of hydrogen peroxide, which is present in concentrations substantially higher than in the helium-operating ones. The increased presence of peroxide in argon plasma, however, does not correspond to an improvement in the abatement power. Moreover, in previous studies, higher concentrations of hydrogen peroxide are reported to be required to induce bacterial inactivation: the H\({}_{2}\)O\({}_{2}\) minimal concentration to  
Figure 3: Measured concentration on species in treated samples, for the five configurations. achieve 1 logAbat in 10 min treatment is reported to be around 40 mg/L [40], more than double with respect to the \(18.7\pm 1.1\) mg/L measured in the micropulsed helium source. Hydrogen peroxide can be therefore excluded from having, individually, the main role in bacterial abatement. A similar conclusion can be drawn regarding nitroxides: the Miblet, in fact, is measured to be very efficient in the production of nitrates, and even for nitrites the obtained values are slightly above those of the micropulsed helium source (\(68\pm 3\) vs. \(13.1\pm 0.6\) for NO\({}_{3}^{-}\), \(14\pm 2\) vs. \(10\pm 2\) for NO\({}_{2}^{-}\)). Again, this does not correspond to an higher abatement power. Moreover, literature studies reported lower inactivation rates with higher NO\({}_{2}^{-}\) and NO\({}_{3}^{-}\) concentrations (as an example, less than 1 log-reduction for 54 mg/L of NO\({}_{2}^{-}\) and 41 mg/L of NO\({}_{3}^{-}\) in [14]). Similarly to peroxides, therefore, nitrates and nitrites cannot be individually responsible for the biological effects.