  The ALICE muon trigger (MTR) system consists of 72 Resistive Plate Chamber
(RPC) detectors arranged in two stations, each composed of two planes with 18
RPCs per plane. The detectors are operated in maxi-avalanche mode using a
mixture of 89.7% C$_2$H$_2$F$_4$, 10% i-C$_4$H$_{10}$ and 0.3% SF$_6$. A number
of detector performance indicators, such as efficiency and dark current, have
been monitored over time throughout the LHC Run2 (2015-18). While the
efficiency showed very good stability, a steady increase in the absorbed dark
current was observed.
  Since the end of 2018, the LHC has entered a phase of long shutdown, during
which the ALICE experiment will be upgraded to cope with the next phase of data
taking, expected in 2021. The MTR is undergoing a major upgrade of the
front-end and readout electronics, and will change its functionalities,
becoming a Muon Identifier. Only the replacement of the most irradiated RPCs is
planned during the upgrade. It is therefore important to perform dedicated
studies to gain further insights into the status of the detector. In
particular, two RPCs were flushed with pure Ar gas for a prolonged period of
time and a plasma was created by fully ionizing the gas. The output gas was
analyzed using a Gas Chromatograph combined with a Mass Spectrometer and the
possible presence of fluorinated compounds originating from the interaction of
the plasma with the inner surfaces of the detector has been assessed using an
Ion-Selective Electrode station.
  This contribution will include a detailed review of the ALICE muon RPC
performance at the LHC. The procedure and results of the argon plasma test,
described above, are also discussed.
