WHEATON® MicroLiter 20 mm Crimp-Top Headspace Vials, DWK Life Sciences. These headspace vials offer excellent performance. 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 well as low background peaks.