1/4" x 3.5" Glass Tube with no end caps. PTFE Microcaps are placed on each tube to keep debris from entering the sorbent during transit. Microcaps are not meant to seal or stop sampling. Manufactured by Camsco in the USA. 1/4" glass ATD sorbent tubes come in 3.5” length and are standard for PerkinElmer™, Markes™ and Shimadzu™ instruments.