I have about 2mL of Pen-Strep left in a centrifuge tube that doesn't freeze even after having being stored in -20 for more than a month. It just remains in its liquid form. Has anyone faced a similar situation before? Any ideas on what's causing it?
Hi Romel - that does sound odd. Maybe your pen-strep stock is at a higher concentration, but ours usually freezes solid. I would trash that pen-strep and confirm that your -20 C is working well.
Hi Romel, pen/strp is usually very stable at any temperature unless it has been kept in light and is dark yellow...
Lots of people prepare antibiotics stock solutions in 50% ethanol, which would explain why it doesn't freeze.
It's easier because you don't have to wait for it to thaw.
Thank you all for your responses. As Chelsea suggested, I placed the tube in another freezer that was set to -30. The pen-strep solution froze completely after about 30 minutes inside the freezer. I then moved it to the old freezer I was previously storing it in, and it stays frozen solid. I am not sure what to make of this, as all the other reagents, including vials of Pen-Strep (from the same container) were always completely frozen when stored in the -20 freezer, except for this one tube.
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