8mm Screw Vial. Buying aijiren¡¯s 8mm hplc vial requires an 8-425 cap and 8mm septum. aijiren’s Caps are all made of high-quality polypropylene. You can choose whether to have a center hole cap or not. Septa uses a double-sided combination of PTFE and Silicone. The assembled cap and septum are very convenient for customers to use.