Sometimes I aliquot liquid into 2 mL microcentrifuge tubes to store at -20C. The aliquot may be 50 or 100 uL. I will check hours or days later and some have not frozen. It is an antibiotic dissolved in PBS. I know that I can throw them into the -80C, but why don't they freeze at -20C? Larger aliquots (1mL) do.
This also occurs with larger amounts, however. Sometimes I will store 1 mL of DMEM from cells and store at -20. They may freeze, but then after thawing and putting them back in, they will remain liquid for days. Sometimes a few will freeze and others will not, even though they are side-by-side! Is is the freezer or the tubes? Are the contents still good?
Thank you.
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