  The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 years
along its journey through the Solar System between 3.8 and 1.24~au. Thanks to
the high resolution mass spectrometer on board Rosetta, the detailed ion
composition within a coma has been accurately assessed in situ for the very
first time. Previous cometary missions, such as $\text{Giotto}$, did not have
the instrumental capabilities to identify the exact nature of the plasma in a
coma because the mass resolution of the spectrometers onboard was too low to
separate ion species with similar masses. In contrast, the Double Focusing Mass
Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and
Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode,
outperformed all of them, revealing the diversity of cometary ions. We
calibrated and analysed the set of spectra acquired by DFMS in ion mode from
October 2014 to April 2016. In particular, we focused on the range from 13-39
u$\cdot$q$^{-1}$. The high mass resolution of DFMS allows for accurate
identifications of ions with quasi-similar masses, separating $^{13}$C$^+$ from
CH$^+$, for instance. We confirm the presence in situ of predicted cations at
comets, such as CH$_m^+$ ($m=1-4$), H$_n$O$^+$ ($n=1-3$), O$^+$, Na$^+$, and
several ionised and protonated molecules. Prior to Rosetta, only a fraction of
them had been confirmed from Earth-based observations. In addition, we report
for the first time the unambiguous presence of a molecular dication in the gas
envelope of a Solar System body, namely CO$_2^{++}$.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics