20 mm GC Grade Soil Septum for Headspace (100/pk) This is a 20 mm version of our high-quality seal for soil sampling. Suitable for your most sensitive analysis; it is pre-conditioned to be free from volatiles. A layer of PTFE on silicone; 3 mm total thickness. Packaged: 4 in. x 6 in. plastic bag; Weight: 122.5 g.