18 mm Magnetic Screw-Thread Headspace Caps and Blue PTFE/Silicone, 1.5 mm thick Septa for SPME, 100-pk. 18 mm Magnetic Screw-Thread Headspace Caps and Red PTFE/Silicone, 1.9 mm thick Septa, 100-pk. 18 mm Magnetic Screw-Thread Headspace Caps and PTFE/Red Chlorobutyl Septa, 100-pk. 23087. Headspace Vial.