  The infrared (IR) range is extremely useful in the context of chemical
abundance studies of the gas-phase interstellar medium (ISM) due to the large
variety of ionic species traced in this regime, the negligible effects from
dust attenuation or temperature stratification, and the amount of data that has
been and will be released in the coming years. Taking advantage of available IR
emission lines, we analysed the chemical content of the gas-phase ISM in a
sample of 131 Star-Forming Galaxies (SFGs) and 73 Active Galactic Nuclei
(AGNs). Particularly, we derived the chemical content via their total oxygen
abundance in combination with nitrogen and sulfur abundances, and with the
ionisation parameter. We used a new version of the code HII-CHI-Mistry-IR v3.1
which allows us to estimate log(N/O), 12+log(O/H), log(U), and, for the first
time, 12+log(S/H) from IR emission lines, which can be applied to both SFGs and
AGNs. We tested that the estimations from this new version, that only considers
sulfur lines for the derivation of sulfur abundances, are compatible with
previous studies. While most of the SFGs and AGNs show solar log(N/O)
abundances, we found a large spread in the log(S/O) relative abundances.
Specifically, we found extremely low log(S/O) values (1/10th solar) in some
SFGs and AGNs with solar-like oxygen abundances. This result warns against the
use of optical and IR sulfur emission lines to estimate oxygen abundances when
no prior estimation of log(S/O) is provided.
