ulfur 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.

3σ (bottom panel). The same findings result from the investigation of additional mass spectra collected on the same day (15 March 2016) at different times as well as two days later (17 March 2016). In the appendix, Figure 4 plots the additional data and Table 1 lists the derived C2H5S/C2H6S ratios which are compatible, within error limits, with DMS but not with ethanethiol. The observed C2H3S signal on m/z=59, corresponding to approximately 8% of the C2H6S signal on m/z=62, is only partially due to the M-3H fragment of DMS or ethanethiol, respectively. The unexplained intensity most likely indicates the presence of an additional low-abundant parent with a molecular sum formula of C2H4S and a nominal mass of 60 Da that produces a C2H3S (M-H) fragment on m/z=59. Unfortunately, the C2H4S signal is hidden below the flank of the strong COS signal and can only be retrieved from the data with an uncertainty as large as 50% (hatched bar in Figure 3). Attributing the C2H4S signal to either thiirane or thioacetaldehyde (or a mixture of both), the two structural isomers of C2H4S, likely explains the C2H3S intensity yet unexplained by DMS or ethanethiol. While the thiirane fragmentation pattern from Steins (2018) is compatible with the residual intensities in Figure 3 (notably DMS residual intensities seem to fit better than those from ethanethiol), the fragmentation pattern of thioacetaldehyde is unknown. Similar to aldehydes, preferred loss of the aldehyde hydrogen atom is likely for thioacetaldehyde, which might lead to strong M and M-H signals too. It has to be stated clearly that, due to the large error margins of the signals and the missing fragmentation information for thioacetaldehyde, no conclusions can be drawn from the C2H3S signal as to which of the two C2H6S isomers produces a better match. Alternatively, heavier organosulfur species might yield C2H3S as a fragment, see further discussion of heavier organosulfur species below. Last but not least, also the observed signals on m/z=45-47 favor the presence of DMS over that of ethanethiol. While these signals are mostly attributed to methanethiol (CH4S) and its fragments (gray bars in Figure 3) and contributions of the C2H6S isomers are minor, the CH2S fragment on m/z=46 leaves sufficient residual intensity after subtraction of the methanethiol contribution to be distinctive: Again, the DMS contribution matches the missing intensity almost perfectly (< 1σ difference) while the ethanethiol contribution does not (approximately 2σ difference).

[Figure: Figure 3. Measured versus expected intensities for the two combinations methanethiol + DMS (top) and methanethiol + ethanethiol (bottom). Color-coded are the individual contributions in percent of the observed intensity. Reference spectra have been taken from NIST (Steins 2018) and were scaled to C2H6S on m/z=62 for both DMS and ethanethiol and to CH4S on m/z=48 for methanethiol as those species can be unambiguously assigned to the respective molecular ions. The hatched contribution to C2H4S on m/z=60 can be assigned to thiirane and/or thioacetaldehyde (parent), fragments of which we do not indicate as no reference spectrum is available for thioacetaldehyde. Details are given in the main text.]