Plug. PEEK, 1/4-28, ARE-Applied Research brand. Bulk Price.
Additional Info:
Knurled for a tight finger grip and also has "flat" surfaces for using a wrench to quarter tighten for best seal or to loosen if plug gets stuck. Screw thread for holes that are 1/4 - 28 inches.
Also useful with Mobile Phase Delivery Caps 58820-06, to prevent air / oxygen from entering the bottle while one of the holes are not in use.
Click HERE for economical pack of 10.