Septa for 8-425 Screw-Thread Vial Caps, Thermo Scientific Supplier: Thermo Scientific Septa for 8-425 screw thread caps are constructed of PTFE, PTFE/rubber, or PTFE/silicone to suit a variety of HPLC, GC and IC applications. PTFE Disk septa are chemically inert PFTE red rubber septa are economical for many GC applications