Wavelength versus purity of the atomic lines selected over an arbitrary wavelength range for a Sun-like (left) and an Arcturus-like (right) spectrum at R = 20 000 (grey filled circles). The size of the points is proportional to the strength of the line, i.e. ~1 − core_flux. The subsample of the grey points that are selected as reliable lines for spectral synthesis from Heiter et al. (2021) for the Gaia-ESO survey (synflag=‘Y’) are highlighted in colour. Orange circles are associated to iron-peak lines, red to even-Z elements, blue to odd-Z elements and green to neutron-capture elements. We can see that the GES has selected lines that have, on average, a purity value greater than 0.8, with an overall greater purity for the Sun than for Arcturus.