How to Seal a PCR Plate
Introduction
PCR plates, a staple of the laboratory for many years, are becoming even more prevalent in the modern setting as laboratories scale up their throughput and increasingly employ automation within their workflows. Achieving these objectives whilst preserving the accuracy and integrity of experiments can be difficult. One of the common areas where errors can creep in is with the sealing of PCR plates, with poor technique allowing evaporation of samples, altering the pH and therefore disrupting enzymatic functions, and inviting contamination. Learning how to seal a PCR plate correctly eliminates these risks and ensures reproducible results.
Find the Right Seal for your PCR Plate
Plate Caps vs. Film Seals vs. Lids
Caps are a good way to seal your plate with a tight seal, whilst still giving you the flexibility to very easily unseal and reseal the plate as you need without any waste. However, caps do have a couple of key drawbacks.
Firstly, you will have to buy the specific cap that is compatible, which makes them not versatile. You will have to make sure that the cap you choose fits the plate, which is dependent on its manufacturer, and choose either domed or flat based on the thermocycler you are using.
Secondly, applying the caps to the plate can be very repetitive and tedious, with the risk of cross-contamination if you put the wrong cap on the wrong well.
Whilst film seals are less flexible in terms of removing and replacing, they are very versatile as they will fit any type of PCR plate, with no regard to who the manufacturer is. They can simply be cut to size, making them very effective.
Another option is a plate lid. These offer less protection that caps and seals, and are mainly only used for short term cover to prevent contamination.
Optical vs. Foil Film Seals
Whether you require an optical, clear seal or an aluminium foil film to seal your plate is decided by your experimental format. Optical sealing films are transparent to allow you to observe samples, whilst still protecting them and preventing evaporation. They are also particularly useful in qPCR experiments which involve making highly precise measurements of fluorescence directly from the plate, in which case you will need a sealing film that filters out as little fluorescence as possible. It is important to make sure that the seal or cap you are using has a high enough level of optical clarity to ensure readings are accurate.
Foil films are appropriate for any samples that are light sensitive or are to be stored at below 80°C. For this reason, most samples destined for long term storage will require a foil film. Foil films are also pierceable, which is useful for either examining individual wells, or for the transfer of samples by needles. This can occur manually or as part of a robotic platform.
Also consider that aggressive substances that include acids, bases or solvents will require a seal that can withstand them, in which case a foil seal is likely more appropriate.
Adhesive vs. Heat Sealing Film
Adhesive film seals are very straight-forward and easy apply. All you need is for a user to apply the seal to the plate, and use a simple applicator tool to press down and form a tight seal.
Heat seals are more advanced, providing a longer lasting seal that has reduced evaporation rates compared to a conventional adhesive seal. This option is appropriate if you are looking to store samples in the long term, although this does come with an additional requirement for plate sealing equipment.
How to Seal a PCR Plate
Plate Sealing Method
Self-Adhesive
1. Make sure you are working on a flat and stable worksurface
2. Remove the film from its packaging, and remove the backing
3. Carefully place the seal onto the plate, whilst making sure that all of the wells are covered
4. Use an applicator tool to apply pressure across the plate. Start from one end and work your way over to the other, pressing evenly
5. Repeat this numerous times
6. Run your applicator around the outside wells, to make sure these are also properly sealed.
Heat Seals
Heat seals work by melting the film to the rim of each well, with the help of a plate sealer. To operate a heat sealer, refer to the instructions provided by the manufacturer of the equipment. Make sure that the manufacturer you source your equipment from is reputable, as it is very important that the seal is proper, effective and watertight.
Plate Sealing Top Tips
a. When applying pressure to the seal, go in both horizontal and vertical directions to ensure a proper seal
b. It’s always good practice to run a test run of whatever you are doing, and this is no different with plate sealing. Test with an empty plate before using one with samples.
c. When testing, take off the seal and look to see that the adhesive stuck down properly, with no gaps. There is a visual representation of this in the first reference document. If you have not sealed the plate properly, when you remove the seal there will be gaps where the adhesive has not made fully bond to the plate.
d. For shipping and transporting of samples, you may find it is helpful to apply a plastic seal on top of the foil seal for extra protection (especially from piercing).
e. Always make sure that there are no bumps or wrinkles when applying the film – these will cause leaks and evaporation
References
2. Complete Practical Guide to PCR Plate Sealing
3. How to use self-adhesive film with easy-to-follow steps
4. Sealing Options for PCR Plates / qPCR Plates - How to Find the Right One?