Description. Thermo™ 20mL Screw Top Headspace Vial, w/ Round Bottom, 100-pk. Unlike crimp cap vials, Thermo Scientific™ National 18mm Screw Top Headspace Vials require no tools for closure. Manufactured with superior quality (Type 1, Class A) glass, they are available with a round bottom in sizes with usable volumes of 10mL or 20mL.