  During the million years of evolution, gas dust and ice in protoplanetary
disks can be chemically reprocessed. There are evidences that the gas-phase
carbon and oxygen abundances are sub-solar in disks belonging to nearby star
forming regions. These findings have a major impact on the composition of the
primary atmosphere of giant planets (but it may also be valid for super-Earths
and sub-Neptunes) as they accrete their gaseous envelopes from the surrounding
material in the disk. In this study, we performed a thermo-chemical modelling
analysis with the aim at testing how reliable and robust are the estimates of
elemental abundance ratios based on (sub-)millimeter observations of molecular
lines. We created a grid of disk models for the following different elemental
abundance ratios: C/O, N/O and S/O, and, we computed the line flux of a set of
carbon-, nitrogen and sulphur-bearing species, namely CN, HCN, NO, C$_{2}$H,
c--C$_{3}$H$_{2}$, H$_{2}$CO, HC$_{3}$N, CH$_{3}$CN, CS, SO, H$_{2}$S and
H$_{2}$CS, that have been detected with present (sub-)millimeter facilities
such as ALMA and NOEMA. We find that the line fluxes, once normalized to the
flux of the $^{13}$CO $J=2-1$ line, are sensitive to the elemental abundance
ratios. On the other hand, the stellar and disk physical parameters have only a
minor effect of the line flux ratios. Our results demonstrate that a
simultaneous analysis of multiple molecular transitions is a valid approach to
constrain the elemental abundance ratio in protoplanetary disks.
