Plasmonic excitations in pristine and air-exposed graphene-Ru and graphene-Pt contacts have been investigated by using high-resolution electron energy loss spectroscopy. Loss measurements show that the effects of air exposure are very different in these two systems. While in graphene-Ru contacts, plasmons are completely quenched, plasmons in the graphene-Pt interface show only a frequency shift together with an overall intensity attenuation. These results pose significant questions as regards the stability in an ambient air atmosphere of graphene-based plasmonic devices and indicate a suitable choice of graphene-metal contacts.