Figure: Figure 2. DFMS mass spectra collected on 15 March 2016 around m/z equal to 62 (a) and 61 (b). The measured intensities per pixel are plotted with black squares, in arbitrary units (a.u.), and with their statistical uncertainties. The peaks are fitted with Gaussian functions. We show the individual Gaussians in gray, highlighting those representing the relevant S-bearing species in magenta, and the overall fit in purple. The exact mass positions of the different chemical species are indicated by dashed lines. The standard deviation of the fitted peak position is ≈6 × 10−4.]

< 3% variation. In addition, the instrument sensitivity changes as a function of mass and varies for the different species. We accounted for the mass-dependent sensitivity change according to the description, e.g., in Le Roy et al. (2015). As the species-dependent sensitivities are not known, it is not possible to derive the signal strength as number of ions per time, and we show it in arbitrary units (a.u.). However, N2 gas was used to calibrate the DFMS detector and the established conversion factor was applied to allow a quantitative interpretation of the resulting numbers as N2+ ion equivalents. The estimated errors in the signal intensities of the CnHmS (n and m being the stoichiometric coefficients of C and H, respectively) species targeted in this work are ≈15%. This estimation for the overall errors includes: (1) a statistical error (negligible for the signals under consideration except for the very low abundant ones like C2H3S), (2) a fit error, and (3) an error related to different signal gains of the individual pixels. Errors related to the detector gain cancel out as all species were measured on the highest gain step. The non-negligible statistical error for low-abundant species increases the respective overall error to be 20%. 20% overall error was also estimated for CH3S on m/z=47. While the counting statistics is good for CH3S, its exact mass is very close to 13C16O18O, an isotopologue of the abundant cometary volatile CO2, and no distinction by fitting is possible. The observed signal was hence corrected by subtracting 1.2% of the signal of 12C16O18O on m/z=46 based on the carbon isotope ratio measured in CO2 (H¨assig et al. 2017). The latter is separable from CH2S by fitting. The C2H4S signal on m/z=60 has an even larger overall error of 50% as this species is fully in the slope of the large peak of the main COS isotopologue which increases the fit’s uncertainties considerably.
3. RESULTS AND DISCUSSION The fits presented in Figure 2 and discussed above allow unambiguous association of the observed signals to C2H6S and C2H5S, respectively, and show that the contribution of the pure hydrocarbon species (C5H2 and C5H) are negligible. The corrected observed intensities (in a.u.) are now compared to the reference fragmentation patterns from NIST. Figure 3 shows the observed intensities of organosulfur species between m/z=45 and 62 and how they compare to the expected intensities (Steins 2018) for the possible combinations of parent molecules. The most striking difference between the two isomers, DMS and ethanethiol, manifests itself with regard to the C2H5S signal at m/z=61, corresponding to M-H of both candidates. While fragmentation of DMS explains the observed C2H5S signal within 1σ error margins (top panel), the expected ethanethiol fragment intensity is off from the observed signal by more than 6 H¨anni et al.