Headspace analysis is particularly sensitive to the quality of the sealing system used to contain the sample. The high pressures and high temperatures in use require high-performance and reproducible seals. Newer analyzers coming on the market continue to push the envelope on these requirements [See, the Shimadzu HS-20, the Agilent 7697A (See Configuration with Tray), and the Thermo Scientific TriPlus for examples of headspace samplers with oven temperatures up to 300°C]. At the higher temperatures high quality seals are a condition for reproducible analysis free of siloxane contamination [See CRS Clean Seals for some information about siloxanes in vial septa].
Background – How a Crimper Works:
Because of the stringent requirements of headspace analysis crimp caps are generally preferred over screw caps. While crimp caps can be assumed to be more reproducible and more secure than screw caps, the crimping process is important. In pharmaceutical packaging
reproducible crimping can be of even more significance than in analytical applications, and inspection methods exist to take the guess-work out of crimping. Genesis Packaging Technologies, for example, manufactures a Residual Force Tester that is used to measure the sealing force in pharmaceutical vials sealed with stoppers and aluminum caps. In analytical labs by comparison the most common test is a “twist-test”, in which the chemist attempts to twist the cap to determine if it is tight enough from their own experience.
All crimping tools, even the 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 with a geometry defined by the crimping tool. The crimping tool or machine itself has no way to assess the tightness of the cap. In Figure 1 at the right, the basic mechanism of the 4-jaw crimper is laid out, showing the final closed position. The tightness of the cap is determined by the final position of the central plunger piece, but it 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 a high-priced residual force tester – how do we determine the tightness of the cap?
The appearance of the seal after crimping provides good evidence of the amount of compression that has occurred.
A well-crimped cap has smooth sides with no major buckling or creases that come from over-crimping and which might interfere with autosampler mechanisms. The seal shows a slight depression in the center that comes from the compression of the seal by the aluminum cap around the outer edge (shown in figure 2 and 3). One might assume that the silicone elastomer might deform either upwards or downwards when the outer edge is compressed, but the PTFE facing is pushed downward and pulls the silicone portion with it.
What About the Twist-Test?
This is a matter of opinion, and I would guess that this blog is not going to settle this issue. The twist test seems pretty inconsistent to me – it depends highly on the condition of the interface between the very-slippery PTFE facing and the smooth (we hope) vial lip. Once the twisting has started, it seems to become easier to twist again, and different people have dramatically different hand strength – the distribution of responsibility for opening jars of jam in various households demonstrates this issue exactly. Still, the twist test is here to stay. I would argue, however, for allowing a bit of slippage if the seal form is good and the cap form is good.
How to Adjust the Crimper
First – you need to allow for some batch-to-batch variation in vials and seals. Our experience is that within a package the variation is low, but between lots of vials some substantial variation can creep in. That isn’t to say that there is a quality issue – to get a good crimp allowance has to be made for normal variation in the vials and seals.
So – adjust the crimper between lots, and adjust the crimper when the appearance of the cap or seal after crimping changes noticeably.
Electronic Crimping Tools over Manual Crimping Tools
Electronic crimpers deliver highly reproducible performance. The final crimp position (the position of the plunger) is determined within microns by the motor controller. The electronic crimpers do rely on the opposing force of the crimping to come to a complete stop, however, so repeatedly crimping the same cap on a vial should not be attempted. (It can also lead to interference between the jaws and the vial sides in some cases). Electronic crimpers have adjustment buttons, plus and minus, that allow the settings to easily change when needed.
Manual crimpers provide a manual stop that can be adjusted to provide a reproducible endpoint when the handles are closed. It is important to make sure that that stop is reached when a manual crimper is used.
Unfortunately for some manual crimpers, this can require additional squeezing against the stop to make sure that it has been reached. Some manual crimpers have the stop clearly visible on top of the tool to help with this, but in some cases the stop is hidden in the handle. Obviously it is not a good idea to rely on feel to determine if a crimp is good enough.
Steel and Magentic Caps
The Combi-PAL and other autosamplers made by CDC require magnetic crimp caps. 2-part crimp caps are available with magnetic tops and aluminum sides, and all-steel caps are also commonly used. The 2-part caps can be handled just like aluminum caps. The steel caps are more economical, but they really need to be crimped with a high-power electronic crimper if they are to be used in quantity. The force required to crimp an all-steel cap is very high, so arm fatigue and even injury is a concern. The commonly-used battery-powered electronic crimpers do not have enough power to crimp steel caps, but wall-powered versions are available that are easily capable of delivering very reproducible crimps of solid steel caps.
For more information on crimping tools, vials, or if you have any additional questions, contact firstname.lastname@example.org or call us at 502-491-6300.