HFBA Solution Shelf Life

[thiazole]

How long do you normally keep an HFBA/water solution before tossing it. We put expiration dates on all our mobile phase solutions and I'm trying to figure out what to put on a new HFBA solution I just made (we usually just use TFA). Since we don't use it often, I want to preferably keep it as long as I can before dumping it out.

[HPLCaddict]

Unfortunately I don't have any experience with HFBA solutions in particular, but generally I wouldn't use aqueous solutions for more than one week.
To set up a proper expiry date, just try it. Prepare a fresh batch of mobile phase and perform a blank run (so see the occurence or change of system peaks) and some sort of reference run (to see retention time shifts) every day. Depending on the day-to-day changes you'll set the expiry date. And, as said above, even if it looks still good after one week, I'd suggest to dump the eluent then...

[thiazole]

Thanks. We never keep them for more than 2 weeks here, but at the company I used to work for (a biotech company with analytical supporting medicinal chemists - I was a medicinal chemist at that time) they'd use 5L bottles for the aqueous and it would take a good month to work through one of those, so I thought maybe that was common practice and that we were overly cautious where I work now.

[HPLCaddict]

One month? Oh my god! If you've ever seen a biotope growing overnight in your aqueous buffer, you get cautious
Of course, there is an extremely wide range of aqueous eluents out there and their susceptibility to microbial growth is extremely different, but why take a chance? If not premixed with methanol or acetonitrile, I'd rather make smaller amounts of aqueous eluents and change them more often. Might also prevent you from other headaches, such as slow pH-drift over time (especially when using pharmacopoeial highlights like phosphate "buffer" at pH 4.5).

[DJ]

I wouldn't let HFBA sit in organic for long. You can actually smell the ester in TFA/HFBA + propanol after a day or so.

I'm a huge fan of making only what you need.

Incidentally, I've distilled neat HFBA and it still did not get rid of the wavy baseline.