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Sample bottles come with three types of caps: crimp caps, snap caps, and screw caps. Each sealing method has its own advantages.
Crimp caps compress the septum between the bottle rim and the crimped aluminum cap. This provides excellent sealing, effectively preventing sample evaporation. The position of the septum remains unchanged when the automatic sampler's needle pierces for sampling. A crimping tool is required for sealing crimp cap sample bottles. For a small number of samples, a manual crimping tool is suitable, while an automatic crimping tool is more efficient for a large number of samples.
Snap caps are an extension of the crimp cap sealing method. A plastic cap, fitted over the bottle rim, forms a seal by squeezing the septum between the glass and the stretched plastic cap. The tension in the plastic cap arises from its attempt to return to its original size, creating a seal between the glass, cap, and septum. Snap caps can be applied without any tools.
- The sealing effect of snap caps is not as good as the other two sealing methods.
- If the fit of the cap is very tight, it can be challenging to put on and may risk breakage.
- If too loose, the sealing effect is compromised, and the septum may dislodge.
Screw caps are versatile. Tightening the cap applies mechanical force, compressing the septum between the glass rim and the aluminum cap. The sealing effect of screw caps is excellent during the needle-piercing sampling process, mechanically holding the septum in place. No tools are needed for assembly.
The PTFE/silicone septum in screw caps is fixed to the polypropylene cap using a solvent-free bonding process. This adhesive technology is designed to ensure that the septum remains attached to the cap during transportation and cap assembly onto sample bottles. While this bonding helps prevent septum detachment or misplacement during use, the primary sealing mechanism is still the mechanical force applied when screwing the cap onto the sample bottle.
The mechanism of screwing the cap tight involves forming a seal and keeping the septum in the correct position during the insertion of the sampling needle. It is essential not to overtighten the cap, as this can affect the seal and lead to septum detachment. If the cap is overtightened, the septum may become cup-shaped or exhibit indentations.