WHEATON® MicroLiter Crimp-Top Headspace Vials, 20 mm, DWK Life Sciences. These headspace vials offer excellent performance. Caps are blue polypropylene. Butyl stoppers can be used when PTFE is not required. Butyl/PTFE septa are an economical choice with low background peaks. Molded PTFE/butyl septa offer the tightest seal of all PTFE items, as