Via optical low-resolution long slit spectroscopy conducted at the 2.5 m INT in La Palma, He, O, N, Ne, A, and S abundances in 13 extended PN were calculated. The analysis of these results made it possible to infer that the nitrogen and oxygen abundances do not correlate, which implies that probably not all the nitrogen found was produced in the CNO cycle. Furthermore, the oxygen and neon abundances seem to correlate well enough, indicating a common origin of these two elements during stellar evolution. The values of He, O, N, and Ne abundances obtained, yield a gradient of those abundances with the position of the nebulae with respect to the galactic plane. This is consistent with the existence of different types of populations among the PN in the Galaxy.