The addition of toluene or methanol vapours did not produce useful results. The DC plasma-treated substrates were then either directly analysed by DESI imaging (Supplementary 3) or dissolved in 60 µL 30:70 water:methanol and analysed by direct infusion ESI-MS. Exposure of the standards to the DC argon plasma generated, in subsequent DESI-MS analysis, mass peaks which were consistent, unique, and spatially resolved; in this instance, an ITO slide was divided into four segments and standards were spotted onto each area -these segments were exposed for 0, 5, 60, or 240 seconds to the plasma before DESI analysis. Plausible structures corresponding to the peaks generated were not easily elucidated by MS/MS, but the data demonstrated plasma-mediated reactions with condensed-phase peptides producing unique products -it also demonstrated that increasing plasma exposure also increased peak intensity (Supplementary 3). To better understand the fragmentation mechanism and to elucidate the structures of identified peaks, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was added to the standards before exposure to the plasma. DMPO is a popular spin-trapping compound, typically utilised in Electron Paramagnetic Resonance (EPR) spectroscopy, which reacts selectively with molecules with unpaired electrons -most notably radical compounds, which we anticipated being generated with exposure to the DC argon plasma. The addition of an equal volume of DMPO to the standards before exposure resulted in the generation of spintrapped radicals, easily detectable by direct infusion (Supplementary 5). Analysis of the data demonstrated the presence of many peaks absent in the control sample but present in the exposed samples. The major peak at 667.35 m/z, generated by application of the plasma source to a leu-enkephalin-DMPO standard, was hypothesised to be an addition of the two compounds, joined at the phenolic hydroxyl group of the peptide via the Forrester-Hepburn mechanism (Figure ). An additional major peak expressed at 8.50-8.60 mins in the chromatogram, at m/z 263.14, was also resolved; this was annotated as the product of coupling between the leucine enkephalin [a1] ion and DMPO, followed by an oxidation. We intend to study the reactions of other compounds when exposed to the DC plasma; the minimal use of solvents and speed of reaction time may promise to be useful in an era of sustainable and efficient chemistry. In summary, the application of the direct current-generated argon plasma to peptides produces fragments which are spatially resolvable, consistent, and unique to the exposed peptide. Should a spin-trap compound like DMPO be added before exposure, the DMPO couples to the peptide to produce easily predictable fragments; this reaction also produces several sideproducts from either the breakdown of the DMPO-peptide conjugate, or the reaction of DMPO with the breakdown products of the peptide.