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:[log in to unmask]] On Behalf Of Ngo
Duc Tri
Sent: Thursday, December 27, 2007 1:35 PM
To: [log in to unmask]
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