Approximately half of the elements with atomic number Z > 30 are created by slow neutron-capture process in the stellar interior of low- and intermediate- mass stars on the asymptotic giant branch (AGB), during the phase of thermal pulses. These elements, collectively called s-process elements, are dredged up onto the surface of the star and later ejected into the medium by stellar winds, giving us the opportunity of observing them in planetary nebulae. In this work we study the chemical content of the planetary nebula NGC 3918 through deep, high-resolution (R~40000) spectrophotometric data taken with UVES at VLT. We identify and measure more than 750 emission lines, some of which are faint lines of the s-process elements Rb, Kr, and Xe. The abundances of these elements are calculated with unprecedented accuracy, thus constraining the efficiency of the s-process and convective dredge-up in the progenitor star of NGC 3918.