Volatile Buffers in Neutral pH range

[adam]

Hi

Can anyone recommend buffers that are both volatile and have buffering capacity in a fairly neutral range?

I've just gone through all my references, and it seems like nothing quite meets both requirements.

Thank You!

[Multidimensional]

Perhaps something simple such as Carbonic Acid?

Here is a link to a table with more options.

"Popular LC/MS and HPLC Volatile Mobile Phase Buffers"; https://hplctips.blogspot.com/2014/06/p ... phase.html

[Rndirk]

Getting close to pH 7, and staying there, with MS-compatible additives is not straightforward. You can make a solution of ammonium hydrogen carbonate (=bicarbonate) giving you a pH close to 8. At this point in preparing a proper buffer, you add the conjugate acid to lower the pH. The problem is that the conjugate acid is carbonic acid which is not stable (decomposes to water and carbon dioxide).

You can lower the pH of an ammonium bicarbonate solution with other acids (acetic, formic,..) but it's not a buffer system. It's unstable and not very reproducible, forming CO2 bubbles as you go down in pH...

I don't know why some tables list carbonic acid as a useable buffer/agent, it's not. You don't want to use sparkling water as a mobile phase.

[Andy Alpert]

You could try the acetate or formate salt of ethylenediamine. Its pKa's are 10.7 and 7.6, so it will buffer in the neutral range. Its acetate and formate salts are volatile. Don't try the trifluoroacetate salt, though; it acts like an ionic liquid.

[adam]

Thanks guys. This is great.

Two questions for Andy and Rndirk.

Do you have recipes for these buffer systems (even if it's just rough outline of the steps).

And how low in pH can you go with the ammonium bicarbonate buffer, before you start to see carbon dioxide gas forming? Is this buffer system OK to use at and above pH 7?

Thanks very much

[Andy Alpert]

Addressing just the preparation of a buffer with ethylenediamine:

I'd start by adding a known quantity of the acid (formic or acetic) to water. Insert a pH electrode and proceed to add ethylenediamine dropwise until you reach the pH you want. Prepare a stock solution this way (filtering it through a 0.45-µm porosity nylon filter if particulates are a concern) and dilute it as needed. If this is for a mobile phase, then use HPLC-grade acid and the purest grade of ethylenediamine you can find (and use a freshly opened bottle to minimize the accumulation of colored oxidation products).

[Rndirk]

You can try to make a 10mM ammonium hydrogen carbonate solution and adjust the pH down to where you want with formic acid. If I recall correctly, it takes approximately 20µL of formic acid to adjust 500mL 10mM down to pH ~ 7. It's best to test it yourself.

The lower you go, the less stable it will be. Since the pKa carbonic acid = 6.3, going down below 7 is not recommended. I found that a freshly prepared mobile phase of pH ~7 takes less than 1 day to increase above 8 by standing on the instrument, which can be risky for your columns. If you want stable retention times, I don't recommend to use this kind of mobile phase.

[adam]

Thanks a lot for the feedback guys. This is great.

Let me ask one other thing (and this is more general, and not necessarily pertaining directly to the initial focus of this thread).

In some cases, when I cannot find a recipe, I figure out how to make the buffer the old fashioned way. And when I do this, I use the Henderson-Hasselbach equation. My question is: Would this be a "good enough" way of doing it? I know that some 'hard core' types would say this does not account for the actual activity of the components involved.

I guess I'm wondering: How critical is this? Is Henderson-Hasselbach good enough, or is there another equation that I should be using.

Thank you in advance

[Andy Alpert]

That's fine for estimating the composition of the reagents you'll need, but I'd still stick a pH electrode into my solution while I'm adding the acid or base to insure that I get exactly the pH I want.