237508 - 20 mL, 76x23 mm Clear vial. Precision-Thread vials are an alternative to crimp top headspace vials. The design allows a leak tight seal with moderate twisting force. Since they do not require crimping, samples can be taken and sealed in the field without the need for sample transfer in the lab. Vials can be used either for traditional headspace analyses or SPME. Vials have rounded bottom and 18 mm diameter necks. 320126 - 18 mm Pre-Assm Silver Screw Cap w/Sil White/PTFE Blue Septa. Magnetic Precision-Thread Screw Caps present a convenient alternative to crimp caps designs. Caps come with 18 mm liners already installed. The liners are thinner than typical crimp top headspace liner seals, so they may be more easily pierced by SPME sampling systems. The cap has a 8 mm center hole also suitable for SPME.