how often do you replace buffers?

[lmh]

I'm sure this has come up loads of times but I couldn't think of a sensible search term... sorry!

I replace aqueous buffers every few days, and usually run them in gradients with pure methanol or acetonitrile, which I replace only when empty. Now I'm starting to run more methods where I'm making up aqueous-organic mixes with buffer, e.g. 1:4:5 50mM ammonium formate pH 3.2:water:acetonitrile. I'm using a hideously expensive grade of acetonitrile because Waters complain about background ions in LC-MS if I don't, and I don't want to throw away acetonitrile solutions.

How often do you recommend I change acetonitrile-containing buffers of ammonium acetate or ammonium formate? Sorry if this is a silly beginner question. Thanks for any help!

[Gerhard Kratz]

In that case you should filter your buffer every day, and adjust pH. Control the buffer composition by refractive index messurement and good luck. But in other cases buffer should be prepared fresh every day, latest every second day.
Godd luck

[Peter Skelton]

A 3:2 solution of aqueous:acetonitrile should be fairly resistant to any sort of bacterial or microbial growth, I'd feel comfortable keeping such a solution for at least a month, evaluating for any shift in RT or other undesired effect on a weekly basis by preparing a fresh solution and running the analysis in duplicate.

[lmh]

Thanks both! I'll watch RTs like a hawk and see what happens.

[artsjeroen]

Control the buffer composition by refractive index messurement

What do you mean by that?

We have problems with retention times at our laboratory, after filtration through a 0,45 um filter RT shifts are huge!
But this is filtration of buffer with organic phase (we use binary pumps)..