Tungsten is a chemical sample found in Cambridge Polymer Labs, initially found as unidentified sample 65. Once the sample has been loaded into one of the polymer coating applicator's sample to holder, the option 'scan loaded reagents' needs to be selected from its terminal in order to identify the sample by its chemical name.
Other than when placed in the polymer coating applicator, tungsten cannot be removed from the player character's inventory.
Tungsten can only be found in Cambridge Polymer Labs. It is located in the room on the ground floor, directly opposite the clean room on the counter behind the Isotope Containment terminal.