Standard solution integrity

[ChristineRG]

I'm curious- a former analyst created a new GCMS analysis method for Opiates and then left for a new position. My labmats and I are new to this type of method development, and I have a question: The new internal standard (which uses D6 internal standards vs the old method of using D3 internal standards) degrades MUCH more quickly (about 3 months vs 9-12 months.) We refrigerate them (in MeOH) but we have the capability of placing them in the freezer. My supervisor does not want to do this because she believes that they will degrade even faster by pulling them out (about every other week) to come to room temp from a freezer temp vs a refrigerated temp. Has any research been done on this area? We don't have the time to re-do the validation by changing the standard prep, but re-making the internal standard every 2-3 months is equally unacceptable. Any suggestions?

[Ron]

The best way is to test this by taking a split of the solution next time and storing it in the freezer. Pull it out periodically and see how long it takes to degrade. My guess is that the solution will take much longer to degrade in the freezer, good lab practice requires that the solution be allowed to warm to room temperature before use, so it will be warming coming from either type of storage.

I know one group that did stability studies comparing a regular freezer to a -40C freezer, and they stopped the study after about 3 years when there was no sign of degradation in the -40C freezer.

[HW Mueller]

One does this by dividing up the sample and taking out only one of these as needed, the others stay in the freezer until needed later. Freezing is always better unless you have a problem with crystallization, for instance cells can lyse when freeze-thawing. Some proteins precipitate and stay precipitated after thawing.

[Jade.Barker]

Freezing is always better unless you have a problem with crystallization, for instance cells can lyse when freeze-thawing. Some proteins precipitate and stay precipitated after thawing.

Exactly, well said.

[Ron]

That is a hard concept for some people to grasp, unfortunately in many cases the people in positions of authority are difficult to convince to change the way things have always been done.

[sam.pedraglio]

The suggestions both from HW Mueller and from Ron are the right way to proceed.
During a GLP method validation you have to check both these kinds of stability:

-Freeze and thaw: prepare a solution and inject it; keep one sample of the solution in the fridge (-20, -40, -80 or which ever temperature you prefer), leave it at least 8hrs, take it out, make an aliquot and analyze it; the remaining solution must be freezed again; follwo this procedure at least 3 or 4 times;

-Cold storage: prepare some aliquots of your solution and store them in the fridge; after, for example 1week, take an aliquot, analyze it and compaire the response vs. a freshly prepared solution; repeat the procedure along the time (i.e. 2weeks, 4weeks, 2months and so on).

The theory says you could consider your sample stable if you have test it; outside this time you must throw it away.


Good luck.