This is a tough question. Certainly, in an ideal world (and also in the world that e.g. Hampton or Emerald salespeople would like us to live in) you should replace expired reagents. The reality of course is that it's sometimes quite costly to do so. It is also expensive (in terms of time and effort) to not be able to reproduce your results - so there's always a balance between price in dollars and price in human work.
Here are my two cents on this subject: Any screens containing MES should be carefully checked. MES is notorious for turning 'scary yellow' with time, and this yellowing is accompanied by a pH shift. This is especially prominent if the solution was exposed to light. Any screens containing PEGs should be checked, and the stock PEG solutions should also be tested for pH change. Fresh PEG solutions have almost no buffering power (they should have none, but there are always small amounts of impurities) and their pH should be around 6 (or even better, 7). With age PEGs tend to go acidic, see an earlier post on this subject. Anything that contains organic material and "not too much salt" should be looked at - bacterial or fungal contamination is always possible. At 4C it's more likely to grow yeasts or molds, whereas at r.t. bacterial and fungal growths are equally possible. Swirl the tubes - if you see something float up - discard the tube. Relatively few screening solutions are colored (iron, Ni, Cu, jeffamine, and several other ingredients confer color). If a colorless solution went colored - discard it. On the other hand, stock solutions of inorganic, and some of the organic salts are pretty stable, even at 5-year mark. With those, you have to watch out for slow evaporation of water, because water can slowly migrate through plastic. If your solutions have crystals in them, it's likely that they have evaporated "somewhat". What you want to do with these is your choice - you can attempt to dilute with ddH2O to the original volume, or you can relabel as "saturated" (a problem, since at different temperatures the concentration of stuff in solution changes). Finally, it's good to remember that acetic acid and ammonia are volatile and therefore ammonium salts and acetates can change with time in several unpredictable ways. Solid NH4OAc or NH4Cl can (and do) sublime away right from their jars (although NH4OAc tends to suck up water instead and become acidic). Artem -----Original Message----- From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On Behalf Of Ngo Duc Tri Sent: Thursday, December 27, 2007 1:35 PM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] Expired Crystallization Screening? Dear CCP4 users, I'm sorry for non-ccp4 question. My lab is using protein crystalization screening from Hampton and Emerald. However I noticed the Expires Date was 05/2002. That means we are using the too old chemical. Could I ask your experience about this problem? Should I need to remove these expired screening because we still have many stocks. Thank you for your reply. My best regards, TriNgo PhD Student, Sungkyunkwan University
