  We present a novel classification scheme for stars evolving in the transition
phase between the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN)
stage based on the results obtained with ISO-SWS. With the better sensitivity
and higher spatial resolution of Spitzer this analysis can now be extended to a
larger number of sources located in the Galactic Bulge, in the Magellanic
Clouds and in other Galaxies of the Local Group, offering an excellent
opportunity to study the validity of the proposed scheme in environments which
are free of the distance scale bias that hinder the observations made with ISO
on galactic sources located at uncertain distances. The new observations will
be used to test the current evolutionary models which predict the dredge-up of
processed material to the surface of low- and intermediate-mass stars as a
function of the progenitor mass and of the metallicity.
