Same as Fig. 3, but for 18–19 u q⁻¹. Stacked spectra in low resolution (top panel) and HR at 18 u q⁻¹ (middle) and 19 u q⁻¹ (bottom). Isotopic ions have been indicated for information. As one may see, the DFMS mass-per-charge resolution cannot separate: neither H¹⁷O⁺ and DO⁺ from H₂O⁺, nor H₂ ¹⁷O⁺ and HDO⁺ from H₃O⁺, and the individual contributions have to be fitted numerically as done in the neutral mode (cf. Altwegg et al. 2015; Schroeder et al. 2019). In addition, the peak in LR at 19 u q⁻¹ is at the edge of the detector such that it is only partially resolved.