  The spectral signatures associated with different negatively charged exciton
complexes (trions) in a WS$_2$ monolayer encapsulated in hBN, are analyzed from
low temperature and polarization resolved reflectance contrast (RC) and
photoluminescence (PL) experiments, with an applied magnetic field. Based on
results obtained from the RC experiment, we show that the valley Zeeman effect
affects the optical response of both the singlet and the triplet trion species
through the evolution of their energy and of their relative intensity, when
applying an external magnetic field. Our analysis allows us to estimate a free
electron concentration of $\sim 1.3 \cdot 10^{11}$ cm$^{-2}$. The observed
evolutions based on PL experiments on the same sample are different and can
hardly be understood within the same simple frame highlighting the complexity
of relaxation processes involved in the PL response.
