Your accounts/procurement people are almost certainly saying push caps and your quality people are telling you to get screw caps. So you may need to take other factors into account.
- One of the most important things you need to assess is how often you are likely to want to access the sample. Push on caps are easy to apply (you just push them on….) but they are not so easy to remove; you can purchase gizmo’s that can prise out the individual caps but they are fiddly to use at best, and as for automated instruments that remove 96 at a time – we have yet to see one that works reliably for these caps. We have heard horror stories involving caps being fired across the lab as they are being removed – so also don’t forget about the potential cross contamination issues.
- Rather than remove a push cap, it is also possible to pierce them with a needle to extract liquid samples from the tube. But once again there are some potential complications with this, such as how to prevent the whole tube and cap being taken with the needle as it is removed (you will need a method to hold down the tube) and using a needle that won’t push the cap into the tube or pull it out as it exits. Also, don’t under-estimate the possibility of creating a small vacuum within the tube when withdrawing sample – you may find that the amount transferred may not be as large as you thought it was.
- There are a number of instruments on the market that will reliably remove screw caps and these can usually also replace the cap (and form a good seal in the process).
- Take account of the temperature at which you plan to store your valuable samples. Push caps don’t perform well below -20⁰C, because of the material they are made from and if you are going below -80⁰C, then don’t even think about using them.
- As indicated above, from a financial point of view, push caps are the best choice. They can also be applied easily and will form a good seal at normal laboratory and standard refrigerator temperatures. Screw caps are more expensive, and are more difficult to manually place onto the tube, but they will form a reliable seal that can work down to liquid nitrogen temperatures.
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