PTFE; allows direct on-column injection at the center of filter disc at top of column without stopping solvent flow. Top thread is 5/16 -24 UNF for use with injection-port cap that holds septum. Side female thread is 1/4 -28 UNF to accommodate tubing adapters or other miniature plumbing systems. Bottom male thread refers to Ace-Thred number on columns. Complete item is supplied with one PTFE-coated silicone-rubber septum. Not suitable for larger columns. Order paper filter discs separately with column end-fitting adapters.