  The determination of heavy element abundances from planetary nebula (PN)
spectra provides an exciting opportunity to study the nucleosynthesis occurring
in the progenitor asymptotic giant branch (AGB) star. We perform
post-processing calculations on AGB models of a large range of mass and
metallicity to obtain predictions for the production of neutron-capture
elements up to the first s-process peak at strontium. We find that solar
metallicity intermediate-mass AGB models provide a reasonable match to the
heavy element composition of Type I PNe. Likewise, many of the Se and Kr
enriched PNe are well fitted by lower mass models with solar or close-to-solar
metallicities. However the most Kr-enriched objects, and the PN with sub-solar
Se/O ratios are difficult to explain with AGB nucleosynthesis models.
Furthermore, we compute s-process abundance predictions for low-mass AGB models
of very low metallicity ([Fe/H] =-2.3) using both scaled solar and an
alpha-enhanced initial composition. For these models, O is dredged to the
surface, which means that abundance ratios measured relative to this element
(e.g., [X/O]) do not provide a reliable measure of initial abundance ratios, or
of production within the star owing to internal nucleosynthesis.
