After following P.K.E. readings downstairs, Egon Spengler, Ray Stantz, and Peter Venkman sighted Ectoplasmic Residue on the drawers of a card catalog. At Egon's request, Peter attempted to gather a sample of the residue in a plastic petri dish. Peter didn't have the easiest time but gathered enough and returned the dish to Egon, just as a bookshelf almost fell on the trio. It seems likely the sample contributed to the research done during the Library ghost encounter and helped the Ghostbusters to figure out how to capture and store ghosts.