Frost is Bad, Isn’t It?
October 14, 2016
Frost is generally a sign of poor sample handling and/or controls. Frost accumulates from samples (vials, boxes) being exposed to an ambient environment where moisture in the air condenses on them and freezes to frost when back in the freezer. Some frost is nearly unavoidable (unless you are in a hermetic environment), but excessive frost is normally a sign of samples being out of the freezer too long or with -80°C ULT freezers, the door being open too long. In both circumstances, innocent samples are warming! Frost can cause a few other problems, for example in fully automated systems frost can prevent reading barcode labels and may cause the vials to freeze to trays causing picking problems.
So all frost is bad…?
We are continuing to run exposure tests (to above Tg) on stem cells in our lab where original testing showed 50% reduction in cell recovery under these conditions. During these experiments we have one vial that is fitted with a thermocouple so we can measure temperature to know when to return the samples back in the freezer. We have noticed over time that we have to leave the samples out of the LN2 freezer longer to warm to approximately -110°C. This summer though, the exposure time really jumped and the vials had a lot of frost accumulation. After hypothesizing what may be causing this (room temp, air flow, HVAC, conduction from something) we thought, could it be the frost?
We moved the vials to a CryoPod and wearing cryo gloves, easily wiped the frost off the vials. Also of note was a frost bed in the bottom of the cryobox of about 1cm deep which we dumped out. We then put the vials and cryobox back into the LN2 freezer and after allowing a day to re-equilibrate, monitored the next several warming exposures and the results were surprising.
The frost actually slowed the warming rate of the samples!
Why is this? The frost is snow/ice, and when exposed to the ambient room it absorbs some of the energy (heat) keeping it from entering the sample vial. Of course the sample will still warm and risk crossing Tg, but at a slightly slower rate.
Bottom Line – We do not advocate allowing frost to accumulate on your sample vials, we actually recommend you do what is possible to minimize frost as frost means exposures (warming!). We just thought this is interesting information to pass along.