## 3 Results  
### Chemical Composition of PAW  
Water samples were plasma treated for 5, 10, or 15 min in atmospheric air to generate a range of PAW. The PAW were chemically characterised immediately after plasma treatment (Day 0) and 7 days (week 1) post generation. The pH of both types of PAW decreased, from neutral to acidic, to levels of 3.03 and 2.7 after 5 min exposure to plasma for spark discharge (SD) PAW and glow discharge (GD) PAW, respectively, and remained stable after 1 week of storage. There was little further decrease with additional treatment time up to 15 min (Figure 2A). The conductivity of SD and GD PAW increased with plasma treatment time, up to 2800 \(\upmu\)S for both types of PAW and remained stable after 1 week (Figure 2B).  
Hydrogen peroxide was only detectable for SD PAW and not for GD PAW. Measurements on the day of PAW generation showed that H\({}_{2}\)O\({}_{2}\) for SD PAW plasma treated for 10 min increased up to approximately 1500 \(\upmu\)M and after 15 min plasma treatment, the concentration plot seemed to approach a plateau (Figure 2C). Similar concentrations of peroxides (measured by KI with phosphate buffer or water) for the same samples of SD PAW were measured on Day 0, indicating that the peroxide was almost exclusively hydrogen peroxide (Figure 2D,E). After 1 week, hydrogen peroxide and overall peroxide concentrations remained at comparable levels for most SD PAW samples. As previously reported for this plasma set-up, no hydrogen peroxide was detectable in GD PAW by TiOSO\({}_{4}\), but reaction with potassium iodide indicated the presence of other peroxides/oxidative species. After treatment for 5 min no detectable concentration of peroxides was observed, whereas 938 \(\upmu\)M of peroxides were detected after 15 min exposure to plasma (Figure 2D). After 1 week of storage, GD PAW 10 and 15 had lost more than half of the peroxide concentration compared to Day 0 (Figure 2D). Concentrations of peroxides were also measured by KI with addition of water instead of phosphate buffer solution in order to measure peroxides in nonbuffered environment (Figure 2E). Comparison between Figure 2D,E shows that SD PAW concentrations of peroxides in buffered and nonbuffered conditions were similar, whereas GD PAW measured by KI with water had about three times higher concentration of peroxides than KI with phosphate buffer solution. This indicates that there is a higher amount of oxidative species in nonbuffered GD PAW and suggests that neutralisation of the pH by phosphate buffer leads to loss of some plasma reactive species. After 1 week of storage, both measurements showed substantial peroxide loss for GD PAW.  
Nitrites were only detected for GD PAW with concentrations up to 254 \(\upmu\)M for GD PAW 15 min (Day 0) (Figure 2F). Measurements after 1 week showed that nitrite had decreased for all GD PAW samples (50 \(\upmu\)M). Nitrate concentrations increased in both SD PAW and GD PAW with plasma activation time up to 5.9 and 6.7 mM, respectively, and after 1 week, no significant change in concentration of nitrates was observed for all PAW (Figure 2G).  
Figure 2: Chemical composition of plasma activated water (PAW) made by spark and glow discharges on the day of generation (Day 0) and after 1 week storage. (**A**) pH; (**B**) conductivity measurements; (**C**) hydrogen peroxide measured by TiOSO\({}_{4}\); (**D**) peroxides measured by KI buffered with PB; (**E**) peroxides measured by KI and water; (**F**) nitrite concentration; (**G**) nitrate concentration (data shown as mean \(\pm\) SD, \(n=3\) experiments with measurements performed in triplicate).