## 1 Introduction  
Plasma is the fourth state of matter besides solid, liquid, and gas. An ionized gaseous substance includes reactive substances, free radicals, and photons. It is worth noting that 99% of visible matter in the universe exists in plasma form [1]. One particular type of plasma, known as cold atmospheric plasma (CAP), has garnered attention in the biomedical field due to its energetic property and low gas temperature. CAP contains reactive oxygen and nitrogen species (RONS), including reactive oxygen species (ROS) and reactive nitrogen species (RNS). These species have shown promising potential in producing a range of biological effects such as antimicrobial, anti-infective, and anticancer effects [2; 3; 4; 5; 6]. When plasma is used to treat a liquid, it produces a solution called plasma-activated water (PAW), a new type of electrochemical biocide [7]. PAW has also demonstrated anticancer properties, which makes it a valuable area for further research [8]. The biomedical applications of PAW depend mainly on the RONS generated therein [9], which are the result of reactions in plasma chemistry [10; 11]. The two most used plasmas in biomedical applications are dielectric barrier discharge (DBD) plasma and atmospheric pressure jet plasma, which are generated by their corresponding generator devices [12]. Dielectric barrier discharge (DBD) devices consist of two parallel electrode plates, where one electrode is connected to a high-voltage source and the other is grounded [13]. For some specific clinical treatments, such as wound treatment and tumor treatment, the target to be treated cannot be placed between the two electrode plates, making the application of DBD devices limited. Plasma jet devices deliver the working gas from one end of the jet tube, ionize the gas inside the jet tube by designing a suitable electrode structure, and then spray the resulting discharge plasma into the air from the other end of the jet tube [14]. This design is more advantageous than the DBD device. Moreover, when treating the sample, the energetic particles generated by the jet plasma will have a direct chemical reaction with the active molecules in the air, generating a large amount of RONS and transporting them to the sample. Therefore, this study chose the plasma jet device as the CAP generation device. Following CAP treatment, the PAW contains ROS and RNS [15], which are general terms for a group of substances that are not a single species. ROS includes several species such as singlet oxygen \({}^{1}\)O\({}_{2}\), superoxide anion radical \({}^{\bullet}\)O\({}_{2}^{-}\), hydroxyl radical HO\({}^{-}\), superoxide radical HOO\({}^{-}\), ozone O\({}_{3}\), hydrogen peroxide H\({}_{2}\)O\({}_{2}\), etc. [7; 10; 16; 17]. However, singlet oxygen \({}^{1}\)O\({}_{2}\) and hydroxyl radical HO\({}^{-}\) have a short lifespan in water. Ozone O\({}_{3}\) reacts quickly with nitrite NO\({}_{2}^{-}\) due to its high oxidation. Superoxide anion radical \({}^{\bullet}\)O\({}_{2}^{-}\) and superoxide radical HOO\({}^{-}\) undergo disproportionation reactions at a fast rate in PAW, and superoxide anion radical \({}^{\bullet}\)O\({}_{2}^{-}\) can also undergo bimolecular self-reactions. Although hydrogen peroxide H\({}_{2}\)O\({}_{2}\) reacts with other reactive substances, it has a long lifespan and can be detected. On the other hand, RNS comprises nitrite NO\({}_{2}^{-}\), nitrate NO\({}_{3}^{-}\), nitric oxide NO, nitrogen dioxide NO\({}_{2}\), peroxmitrite ONOOH and OONOOH, and so on [7; 16; 18]. Nitric oxide NO and nitrogen dioxide NO\({}_{2}\) have a short lifespan in water and undergo a series of reactions to produce the more stable nitrite NO\({}_{2}^{-}\) and nitrate NO\({}_{3}^{-}\). Peroxynitrite ONOOH and OONOOH are unstable in water and decompose rapidly. Nitrite NO\({}_{2}^{-}\) reacts with oxidizing species in the PAW but has a longer lifespan, whereas nitrate NO\({}_{3}^{-}\) is highly stable in water.