Figure 4.

Comparison of the peak-normalized mass spectra including the signals assigned to (blue) H₂O⁺ and H₃O⁺ and (red) H₂¹⁸O⁺ and H₃¹⁸O⁺. The red line is background-corrected to exclude the background of the H₂¹⁸O⁺ peak centered at 20 u/e. This is shown in Figure S6 in the Supporting Information. See also the text for details on possible small ¹⁷O backgrounds.