The flexible sealing lip inside the cap ensures a positive leakproof seal under even the most adverse conditions. This deep internal lip fits snugly against the interior wall of the tube preventing the contents from coming in contact with the threads, thus reducing the chances of sample contamination. Caps are supplied with attachment loops in order to prevent contamination and mix-up. Cannot be autoclaved since closure is made of high density polyethylene. Can be centrifuged up to 17,000g.