Hi Jerome,
-I have heard that PEG solutions can become unstable in light. We usually store our block in the fridge, where photons are scant anyway. For any stocks that I prepare, I wrap the tube/bottle in aluminum foil. I'm not sure about freezing them.
-Some labs (not ours) evidently prepare buffered stocks of PEG solutions, as their pHs tend to shift with time. This is something I've been meaning to try. Of course, you may need to worry about buffer components that are incompatible with crystal hits.
