We have our rat brain lysates stored at -80C and are about to start Westerns. We will need to thaw them for BCA quantification, but then want to have several aliquots of this available in the future for other western/phosphorylation analysis. Is it best to add sample buffer (containing SDS and DTT) and then store the aliquots in this, or just add the sample buffer separately to each aliquot as I pull them out of the freezer?
If you know for sure that you will only use the samples for SDS-PAGE, then it would be OK to store them in SDS-PAGE sample buffer. However, once you've added it, there is no going back - you won't be able to do anything else with the samples except SDS-PAGE.
Even if there is no problem in your tests in the future, I suggest you not to do this. Because it has happened to me many times that after adding loading buffer and keeping the sample at -80 degrees, when I have thawed the samples, the quality of the solution has been messed up and it has even become two phases and requires additional steps to change its quality back. However, sometimes the overall quality of the sample is also lost.
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