24mm White Screw Closures--Aijiren Vials for HPLC/GC. The 24mm Screw Cap produced by Aijiren is often paired with 24mm Sample Storage Vial. This type of Vial is used to store samples and sometimes used in autosamplers. For example, 40ml Vial is often called EPA Vial, and the center hole on the Cap is here.