Screw-top Vials and caps are easy to use and require no special tools. The unique thread design provides a consistently secure seal, preventing evaporation. Screw Thread Vials are available in 8-425 (8mm), 9-425 (9mm), 10-425 (10mm) and 13.425 (13mm) neck finishes. But the most commonly used is 9mm hplc vial.