The alien sample spawns in the following locations:
To activate the sample, press 'E' (or the keybind for 'use'). It has one use after which it will disintegrate. Use the ignition for slowing down zombies on key choke points in the map. It takes around 10 seconds for the sample to extinguish and will ignite zombies for 30 seconds, so it's quite lengthy and as such effective when used in corridors where zombies cannot escape the flames (for instance in front of the final room on Ultimate).