There are three types of caps for sampling vials: crimp cap, snap cap, and screw cap. Each sealing method has its own advantages.
1. Crimp cap The crimp cap squeezes the septum between the rim of the glass vial and the crimped aluminum cap. The sealing effect is very good, which can effectively prevent the sample from evaporating. The position of the septum remains the same as the autosampler needle pierces the sample. Crimping vials with crimp caps requires the use of a crimper. For a small number of samples, a manual crimper is the best option. For large samples, an automatic crimper can be used.
2. Bayonet cover The bayonet cover is an extension of the sealing method of the jaw cover. The plastic cap that fits over the rim of the vial creates a seal by squeezing the septum between the glass and the stretched plastic cap. The tension in the plastic cover is caused by its attempt to return to its original size. This tension creates a seal between the glass, cap, and septum. The plastic snap cap can be closed without any tools.
The sealing effect of the bayonet cover is not as good as the other two sealing methods.
If the cap fits very tightly, the cap will be difficult to close and may break.
If it is too loose, the seal will be poor and the septum may come out of place.
3. Screw caps are universal. Tightening the cap applies a mechanical force that squeezes the septum between the glass rim and the aluminum cap. The screw cap provides an excellent seal against the septum mechanically during puncture sampling. No tools are required for assembly. The screw cap PTFE/silicone septa are attached to the polypropylene cap by a non-solvent bonding process. This bonding technique is designed to keep the septum and cap together during shipping and when the cap is attached to the vial. This bond helps prevent the septum from dislodging and dislodging during use, but the primary sealing mechanism remains the mechanical force applied when the cap is screwed onto the vial. The mechanism of cap tightening is to create a seal and hold the septum in place during needle insertion. It is not necessary to overtighten the cap as this will compromise the seal and cause the septum to fall out of place. If the cap is over-tightened, the septum can become cupped or dented.
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