18mm Magn Screw Cap & Silicone/PTFE Septa, Kit, round bottom . Home → Gas Chromatography (GC) → Vials → Vials, 20ml Headspace vial w/ 18mm Magn Screw Cap & Silicone/PTFE Septa, Kit, round bottom, 100/pk. CANNABIS TESTING. pesticides, cannabinoids, terpenes and residual solvents for safer, higher quality cannabis products. Gas Chromatography.