Septa for 13-425 Thread Caps provided by STE-Mart is made of PTFE/silicone fusion, it has strong resealability to offer good close performance after repeated punctures and can withstand strong acids and alkalis. The PTFE/Silicone/PTFE septum is recommended for the most critical applications such as ultra trace analysis or where there is a longer time between injections or for internal standard methods.