Imaging Measurements of the Spatial Distributions of OH, H₂O₂, and HO₂ in the Effluent of the COST Reference Source

Non-thermal plasmas (NTP) for medical applications have been gaining interest in the scientific community due to their controlled delivery of reactive oxygen and nitrogen species (RONS) that have been shown to produce immunostimulatory effects. However, the complexity of possible chemical pathways has made it difficult to differentiate the importance of specific RONS for this positive effect. It is therefore of interest to not only determine the overall concentration of RONS in the effluent, but also their spatial distributions. We combine laser-induced fluorescence (LIF) and photofragmentation laser-induced fluorescence (PF-LIF) techniques to image the spatial distributions of three key reactive species - the hydroxyl radical (OH), hydrogen peroxide (H₂O₂), and the hydroperoxyl radical (HO₂) - in the open effluent of the Capacitively Coupled Atmospheric Pressure Microplasma Jet (COST-jet). Approaches to quantifying the LIF and PF-LIF signals and differentiating between PF-LIF signals from H₂O₂ and HO₂ will be discussed. We consider several helium-based admixtures (He, He+O₂, He+H₂O, and He+N₂+O₂) that have been used in previous studies of the COST jet, and we compare results with the jet issuing into nitrogen and air atmospheres to separate possible chemical pathways. We observe the formation of chemical reaction fronts that indicate a significant increase in HO₂ production where the effluent of the jet mixes with oxygen from the surrounding air. This research contributes to our understanding of the formation, consumption, and transport of OH, H₂O₂, HO₂ in the jet effluent. Ultimately, a detailed account of these complex plasma chemistry interactions is needed to advance medical applications and tailor the deliverable densities of key reactive species.