Cavity ring-down spectroscopy of the spatial density distributions of H2O2 in the effluent of the COST-Jet and the kINPen-sci.*

Cold atmospheric plasma jets (CAPJ) operated with a helium feed gas containing small admixtures of water vapour are excellent sources of H₂O₂ for localized treatment of heat-sensitive surfaces and biological tissue. A detailed understanding of the complex plasma chemistry inside the plasma as well as in the effluent is required to be able to tailor the composition of reactive species, in particular reactive oxygen and nitrogen species (RONS), to specific applications. Direct, sensitive and selective measurements of key RONS are important for this but remain challenging. This study uses continuous-wave cavity ring-down spectroscopy to determine the spatial distribution of the H₂O₂ molecules in the effluent of two widely used CAPJs:  kINPen-sci and COST-Jet. The measured H₂O₂ density close to the jet nozzle was found to be 2.3×10¹⁴ cm⁻³ for the kINPen-sci and 1.4×10¹⁴ cm⁻³ for the COST-Jet. In the effluent, up to 35 mm from the nozzle, the density in the kINPen-sci is roughly a factor two higher than in the COST-Jet. For the COST-jet, the distribution of H₂O₂ in the effluent up to 15 mm is initially highly uniform, suggesting negligible mixing of the plasma with ambient air. At greater distances, there is strong mixing and the H₂O₂ density is rapidly diluted. For the kINPen-sci jet, there is a more pronounced radial profile close to the nozzle, but a more gradual mixing of ambient air with distance. This work highlights the differences between the spatial distribution of H₂O₂ in the effluent of both types of plasma jet as a result of the source design and ambient mixing.  For applications this means that a detailed understanding of the H₂O₂ production in the plasma source, as well as of the transport of H₂O₂ to the substrate through the effluent, is required to optimise the intended effects in a treatment.