Your Headspace Vial Cap is Crimping Tight Enough?
Headspace analysis is particularly sensitive to the quality of the sealing system used to accommodate the sample. The high pressures and temperatures in use require high performance and reproducible seals. New analyzers on the market continue to push the limits of these requirements. At high temperatures, high-quality seals are a prerequisite for reproducible analysis without siloxane contamination.
How the Crimper Works
Due to the stringent requirements of headspace analysis, crimp caps are generally preferred over screw caps. Crimp caps can be viewed as more reproducible and safer than screw caps, but the crimping process is critical. Pharmaceutical Packaging
Reproducible crimping can be even more important than analytical applications, and inspection methods exist to take the guesswork out of crimping. For example, Genesis Packaging Technologies manufactures a residual force tester used to measure the sealing force of pharmaceutical vials sealed with a stopper and aluminum cap. By comparison, the most common test in analytical laboratories is the "twist test". In this test, the chemist twists the cap to determine if it is tight enough based on his or her experience.
All crimping tools, even high-speed pharmaceutical crimping machines, work by forming the cap around the vial lip in a process that uses position to determine the end of the forming operation. The cap is crimped in the shape defined by the crimping tool. There is no way to evaluate the tightness of the cap on the crimping tool or on the machine itself. Figure 1 on the right shows the basic mechanism of a 4-jaw crimp and the final closed position. The tightness of the cap is determined by the final position of the central plunger piece but also depends on the thickness of the vial lip and the thickness of the silicone/PTFE or silicone/FEP seal in use.
Evidence of tightness
So, in the absence of an expensive residual force tester, how do you determine how tight a cap is?
The appearance of the seal after crimping provides good evidence of the amount of compression that has occurred.
The sides of a tightly crimped cap should be smooth, with no significant buckling or creasing due to excessive crimping, which could interfere with the autosampler mechanism. The seal shows a slight indentation in the center due to compression of the seal by the aluminum cap around the outer edge (shown in Figure 2 and 3). It might seem that the silicone elastomer could deform upward or downward as the outer rim is compressed, but the PTFE facings are pushed downward and pull the silicone portion.
How to Adjust the Crimper
First, the batch-by-batch variations in vials and seals must be taken into account. In our experience, variation within a package is small, but among many vials, considerable variation can creep in. That does not mean that there is a quality problem. Vial-to-seal variation.
Therefore, adjust the crimp from lot to lot and adjust the crimp when the appearance of the cap or seal changes significantly after crimping.
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