Crimp caps provide the best quality seal for the longest duration, but this is true only if the Crimping Tool that is used to create the seal is calibrated properly and the user performs a quality crimp. To maintain adequate calibration for crimping tools, you should re-calibrate every 10-15 crimps and verify a good seal is produced every time you crimp.
This can be inconvenient and time consuming so many users will choose the screw caps for their applications. Screw caps provide a good compromise of secure fit as well as convenience when high quality vials and caps that are matched to each other in their design are used. The threads of the vial and cap must match for optimal performance. Plus the screw cap must be applied properly. Click HERE to watch a short video How to Tighten a Screw Cap.
Neither cap types will vent most gases due to the rubber / Teflon septa barrier, with a good seal. Silicone rubber is thicker and reforms better than natural rubber and therefore maintains a tighter seal especially after initial Injection.
Snap Caps are not recommended for preventing gas from escaping an Autosampler Vial. The design of Snap Caps are not made for this purpose.