18mm 20ml Screw Headspace Vial – hplcvials.com 1. 20ml precision screw headspace vial, 18mm, round bottom, top space vial, strong adsorption. 2. The 20ml precision threaded headspace vial is used for the threaded vial (10ml+20ml) and magnetic screw cap of CTC and TriPlus autosampler. 3.