Chromatography Forum

Hi all,

Quick question for anyone who has experience working with high pH mobile phases. Any thoughts on how frequently a 5mM pH 9 - 9.5 NH4 bicarb buffered mobile phase lasts before it's no longer useful -- how long it stays good in other words in your experience? Obviously NH4 bicarb buffered mobile phases have a pretty limited shelf life because it's a fairly volatile buffer. If it stays good for up to a week or two that might be acceptable -- if it's only good for a day or two, however, at that point NH4 acetate (the buffer I normally use for that pH) is preferable.

Any thoughts?

Thanks,
WCH

The stability of the mobile phase should be proved during method validation (1, 2, 3, 4, 5 days). In order to avoid 483 sins the AMV protocol should have a 'Stability of Solutions' section.

Speaking as a chemist: I wouldn't expect the stability of an ammonium carbonate to be much different from ammonium acetate (with ammonia added) at the same pH and similar concentrations.

Also, does anyone uses the same aqueous buffer for up to 2 weeks?

Hello

I'm using Ammonium bicarbonate buffer for chiral methods and it is ok for month or even longer. For other methods I prepare fresh batch every time (every sequence)

However my opinion is that all depends on method - some method will tolerate buffer for longer time than others.

Regards

Tomasz Kubowicz

The stability of the mobile phase should be proved during method validation (1, 2, 3, 4, 5 days). In order to avoid 483 sins the AMV protocol should have a 'Stability of Solutions' section.

Technically, I'd say that HPLC Chemist's response is correct. I'd say that USP/FDA expects you document to validate the stability of EVERYTHING, or just make it fresh.

I like the phrase in the Volumetric Solutions section of USP for sodium hydroxide, which states "standardize frequently", like we're supposed to know what it means.

In the consumer products industry, we used USP actives in consumer products, so we had to validate our own procedures. We did do validation/stability studies of active standard solutions stored in a refrigerator over time. But no other stability studies of chemical solutions, there was just not enough time.

No FDA auditor at manufacturing ever questioned anything like that, typically were fine that there was a validation report.

Thanks, helpful.

Also, re....

Also, does anyone uses the same aqueous buffer for up to 2 weeks?

Obviously not straight aqueous buffer -- you can have problems with microbial growth. But I never use straight aqueous buffer for that reason, 5-10% methanol in my experience is usually enough to prevent aqueous growth. I agree that long-term re-use of an aqueous buffer is unwise but as long as it contains some organic (preferably methanol) to prevent growth no reason not to re-use it a few times over a short period of time (e.g. 1-2 weeks).

... but with caution if you're doing electrospray MS, where, depending on your target compound, you may find hydrogen adducts in a fresh buffer and sodium adducts in a buffer that's sat overnight in a glass bottle.