  The determination of chemical abundances in ionized gas nebulae is based on
the detection of auroral emission lines, from which it is possible to deduce
the line temperatures of several ions. These lines are weak in low excitation
objects, so its measurement is only possible in great aperture telescopes. The
GTC would allow to plan a comprehenssive study of chemical abundances in such
objects from mid-high resolution spectra, what would give more accurate
observational restrictions to models of chemical evolution.
