Certified 9mm ND9 Screw Cap with PTFE/Silicone Septa, Pre-slit, 1.0mm thickness (For 2ML ND9 Vials) Certified 18mm ND18 Screw Metal Magnetic Cap with PTFE/Silicone septa, 1.5mm thickness (For ND18 Screw Headspace Vials) Certified 20mm ND20 Crimp Aluminum Cap with PTFE/Silicone Septa, 3.0mm thickness (For ND20 Crimp Headspace Vials)