## 1 Introduction  
Atmospheric pressure plasmas have been shown to exhibit beneficial outcomes for biomedical applications in wound healing [1], cancer [2], and other diseases. These outcomes are attributed to direct or indirect exposure of tissues to plasma-generated reactive oxygen and nitrogen species (RONS) regardless of the application method [3], i.e., directly as in dielectric barrier discharge (DBD) [4], indirectly via atmospheric pressure plasma jet (APFJ) [5], or through plasma-conditioned medium [6]. Investigations correlating the possible biological responses induced by plasma treatment to plasma parameters and components are typically limited to the source used. The configuration, discharge parameters, and environment influence the plasma components that interact with the cells and, thus, the contribution of each component to the cellular response [7; 8; 9; 10; 11]. However, direct cause-effect correlations have not yet been defined. Understanding how plasma generated RONS, UV, and electric fields affect cells will allow devices to be tailored for specific biological applications and eventually guide the community toward the determination of the \"effective plasma dose\" of each device. Plasma-generated hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) is theorized to be a major effector of tumor cell killing [12; 13]. However, a study comparing the nanosecond pulsed floating-electrode DBD (nspDBD) and the kINPen (an APPJ) showed similar in vitro reductions in the viability of the CT26 colorectal cancer cells at operating parameters where the kINPen produced twice as much H\({}_{2}\)O\({}_{2}\) as the DBD [14]. These observations indicate that biological responses are not solely induced by long-lived, aqueous H\({}_{2}\)O\({}_{2}\), and that plasma components other than H\({}_{2}\)O\({}_{2}\) contribute to cell death [14]. The relative contribution of each component may be different, partly due to the differences in amounts of UV exposure, electric field, and short-lived and charged species in each plasma [4]. Recent literature supports these observations whereby exogenously added RONS are unable to replicate a biological effect observed subsequent to plasma exposure [15]. Therefore, changes in cell viability and other biological responses may not be the result of just RONS but of synergy from all components of plasma discharges where each plasma source produces a unique mix of these effectors. Yet comparisons between devices have traditionally relied on the chemistry produced to explain the observed differences in biological responses [16; 17; 18]. This limited chemistry set does not represent the chemical and biochemical processes possible during plasma treatment. For example, UV exposure and applied electric field are individually capable of inducing biological responses and enhance the observed effects of plasma-generated species [15; 19; 20]. In summary, there is a dire need for indicators that accurately reflect the magnitude of the biological response in correlation to all components of plasma treatment. Recent studies have explored the use of cysteine as such an indicator to trace the short and long-lived species in the liquid phase and their potential biological impacts [11; 21; 22]. The different modifications infer the presence of specific molecules from short-lived species like OH to end-products like H\({}_{2}\)O\({}_{2}\). Therefore, modifications to molecules like cysteine, the cysteine-containing glutathione (GSH) [23], or phenol [24] could be reflective of the dominant chemical species from plasma treatment that influence cellular outcomes. The amounts and types of modifications measured could help to optimize plasma treatment regimens, as well as plasma sources for specific applications. This would also provide a potential method for comparison of plasma sources and their potential for biological applications without the need for cell culture. However, the correlation between molecular modifications and specific cellular responses must be established first.