# Multimodal Comparison of Cold Atmospheric Plasma Sources for Disinfection
## 4 Results and Discussion
The results of the chemical characterisation are further confirmed by emission spectroscopy data, with the interpolations of rotational and vibrational temperatures of nitrogen reported in Figure 4. In argon plasma, with heavier atoms and colder electrons with respect to helium one [41; 42], vibro-rotational relaxation is faster, leading to lower vibrational temperatures (below 2000 K) and slightly higher rotational ones (between 500 and 1000 K). In helium plasmas, oppositely, rotational temperatures remains below 500 K while vibrational temperatures can reach more than 3000 K. In defining the equilibrium between oxygen and nitrogen species, a key role is played by the reaction N\({}_{2}\)(\(\
u\)) + O \(\longrightarrow\) NO + N, whose rate depends on the vibrational excitation of nitrogen molecules. At higher vibrational temperatures as in helium, therefore, the reaction is boosted, increasing the production of NO and, through HNO\({}_{2}\) and HNO\({}_{3}\), of nitrates and nitrites in water [43; 44]. Moreover, the reaction consumes part of the available atomic oxygen, competing with the chemistry of oxygen species and resulting in a reduction of peroxides. Together with other reactions, the equilibrium is expected to move towards nitrogen chemistry at higher vibrational temperature: this prediction matches the results from the chemical characterisations, which reported a shift of the chemistry from peroxides towards nitrogen species when switching from argon to helium. For the air jet, since nitrogen is directly involved in plasma formation, both rotational and vibrational states are highly excited, resulting in a predictable increase in nitrates and nitrites. The chemical measurements described above can also be used to estimate the pH of the solution. Test strips have been preliminary used, sensing a neutral pH in all the solutions. Figure 4: Rotational and vibrational temperatures of nitrogen molecule extracted from optical emission spectra fitting the second positive system, for the five configurations. Performing a second characterisation of the chemical composition of the treated samples one hour after the treatment, concentrations compatible with those measured immediately after the treatment are found. It is known that, in acidic environments, the balance between nitrates and nitrites is expected to drift over time [11]. The absence of such drift provides further confirmation that the treated solutions are almost neutral. It should be noted that most of the characterisations reported in the literature measured an acidification of the samples after the treatments, while a neutral pH is seldom found [45], confirming again the uniqueness of each experimental setup and the complexity of the involved chemistry. To further improve the characterisation of the treated samples, the oxidation-reduction potential was measured. Starting from an initial value of \(320\pm 10\) mV, all the sources showed an increase in potential, reaching \(439\pm 4\) mV for the micropulsed helium source. A similar value was measured in the micropulsed argon source (\(429\pm 2\) mV), while values lower than 400 mV were detected in the radiofrequency source. Notably, the Miblet-treated samples, with their high nitroxide content, led to a high value (\(577\pm 11\) mV), far surpassing the other sources. It is worth mentioning that NO\({}_{3}^{-}\) is a strongly oxidising agent. It is therefore not surprising that the ORP measurements follow in trend the nitrate concentration. Previous studies have attempted to mimic the effect of plasma treatments using chemical solutions. The synthesis of mock plasma-treated solutions typically focuses on reproducing the pH and the concentrations of NO\({}_{2}^{-}\), NO\({}_{3}^{-}\), and H\({}_{2}\)O\({}_{2}\); however, the results are inconclusive, reporting biological effects both different from [46, 47] and comparable to [14, 48] those of the corresponding plasma-treated water. Most of the studies, however, report significant acidity, while the solutions considered in this work are neutral.