Caps: 18mm, Headspace, Screw Thread, 100/PK -. These magnetic screw caps have been developed for the CTC instrument but are compliant with CTC Combi Pal, Varian, Aijiren, Gerstel, Shimadzu, Perkin Elmer and Thermo autosamplers. Caps are built with numberous threads to force the septa against the vial to seal it to be gas tight.