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. PTFE/silicone septa are most popular for HPLC and IC applications. Pre-slit septa are easier to pierce