van Krevelen scatter plots of hydrogen to carbon versus oxygen to carbon (left panels) and versus nitrogen to carbon (right panels) in the three residues analysed in negative ESI mode in the mass ranges 50–300 (light-blue dots) and 150–400 (blue dots). Panels A and B: 1:1:1 + 29 eV/16 u residue; panels C and D: 3:1:1 + 67 eV/16 u residue; panels E and F: 1:1:1 + 98 eV/16 u residue. The size of the dots is given by the normalized intensity. Grey dashed lines delimit the zone of highest density of ions observed in the residues analysed by Danger et al. (2016). Arrows show the H/C, O/C and N/C weighted averages reported in Table 2 for the range 50–300 m/z (cyan arrows) and 150–400 m/z (blue arrows).