Freeze Thawing Chemicals - (Aug/29/2012 )
This maybe a stupid question but I'm going to ask anyways.
I've gotten into the habit of making small aliquots of chemicals if they need to be at -20 just because it takes less time to thaw and I have this nagging voice in the back of my head that freeze thawing anything more than a couple times is not a good idea for any chemical.
I guess my question is: What types of chemicals should one be cautious about freeze thawing? If the insert says nothing about freeze thawing issues should I not worry about it?
I'd say usually in all macromolecules where also secondary structures might be important, I'd avoid frequently freeze thawing (e.g. proteins, DNA, RNA).
All the small molecules (most buffer substances, salts) it doesn't matter.
Not sure about lipids and bigger carbohydrates. Other chemicals usually have a label how to store them.
I am personally obsessed about freeze-thawing: dNTPs and T4 DNA ligase buffer (contains ATP)
I also avoid freeze thawing dNTPs, but what about primers?
With primers, I was told not to freeze-thaw them too many times. But I fail to see why. Maybe I am a bit stupid, but for me DNA (unless it is genomic DNA that breaks when you freeze-thaw it) should be stable enough. On the other hand, how many times are you actually reusing the same set of primers? (unless it is the colony PCR primer for a vector which I take care of). I usually do my PCR once, and this is it with that set of primers.
Andreea
I only aliquot those that specifically require so, dNTPs, some buffers, qPCR probes, sequencing master mix and all fluorescently labeled NA in general, proteinase K,..
On the other hand I'm a friend with fridge, so I rather keep stuff at +4 than freeze-thaw them all the time when I need them within several weeks.
Aliquoting everything really isn't possible, we have freezers full already. Sometimes I aliquot gDNA when there is a large amount, but that is mostly because it thaws quicker when I need it.
Yeah my problem is also that our freezers are full and i dont have enough space for aliquoted reagents and chemicals
Sometimes clearing them out helps...samples from people who left ten years ago (depending which regulations you have to keep this stuff), the 100 tubes each of PCR, restriction buffers and magnesium chloride etc, all the collected PCR tubes and plates with old reactions, people keep for whatever reason, everything which is unlabelled or unreadable labelled and much other junk...
It's surprising how much space you finally have 
Well yes, samples from people who left ten years ago... but will someone continue with those expeirments? Are there some patient samples, that should be generaly preserved? Isn't there a plasmid you were looking for last three years for a chance? Should PI decide what to do with them? -> nothing gets cleared
Option two - important looking samples noone can tell who it belongs to, you tell everyone to cheki it and tell it's not theirs, before deciding what to do with them -> nothing gets cleared
Option three - you get impatient and just throw away those unnecessary looking tubes that stayed for years unnoticed on the bottom of the tray covered in snow -> week later someone starts to complain about his missing samples
Result: You just don't have enough freezers, that's all 
I agree with hobglobin. I have only one -20 shelf (most of my other labmates have at least 2) and I am very ok with it because I clean it every half a year. Moreover, we do not allow people to leave without clearing their freezer spaces and pass on everything: box from their hand to the hand of the PI. If you are nasty and impose rules, you can have clean freezers. But... Trof, you are also right, depends on the research area: human samples need lots of space. But as a solution to: I have thrown these old samples with 3 cm of snow on top of them: send an email around the department stating you found these samples labeled ...smth-smth... you will throw them in a week if there is no reply. If nobody replies, though luck! you did more than enough to warn them.
Andreea