After analysing the tissue, several peaks present only in plasma-exposed samples -but not in plasma-naïve sampleswere observed. To probe these further, we subjected haemoglobin (1 uL of 1 mg/mL in 50% methanol) to the RF-argon plasma, then dissolved and analysed by electrospray ionisation as described earlier. Here, again, several new peaks were observed; cross-matching these peaks with the results from the tissue analysis suggested several candidate reporter ions for haemoglobin, such as at m/z 306.19 and 258.19 (Supplementary 10). Many other peaks without counterparts in either the mNeonGreen or haemoglobin standards were also observed, reflecting the huge diversity of proteins in a typical tissue sample. Finally, we compared the spatial resolution of mouse kidney samples subjected to tryptic digest compared to those subjected to plasma for one minute by MALDI imaging with a pixel size of 10 µm. Using the ubiquitous haem peak at m/z 616.18 as an exemplar, the resolution of the plasma-treated samples enables clearer differentiation of the tissue fine structure, including not only identifying marks of the outer medulla but also the inner medulla (figure ).