2.5 µL of peptide standards leucine-enkephalin and bradykinin (Merck, 1 mg/mL in 30:70 water:methanol v/v) were applied in up to three separate areas of indium-tin oxide (ITO) slides (Merck, 70-100 Ω/sq), and allowed to dry. These standards were exposed to the DC-argon plasma at a distance of 12 mm from the corona tip to ITO slide for between 5 seconds and 4 minutes before analysis by DESI, or dissolved in 3x20 µL aliquots of 30:70 methanol:water and analysed by direct infusion.