Buffer range 6-8 compatible LC-MS

[razabal]

Hi all!
I'm actually doing developpement for a new method of purity for a new product in reverse phase and I have to find a buffer for my mobiles phases useable between pH 6-8 MS compatible and easy to prepare (for routine analysis) and mixeable with MeCN.
Do you have any idea?

Thanks a lot

Raza

[gstaepels]

Hi Raza,

You could consider using ammoniumcarbonate. This buffer is also used with ELSD.

[Uwe Neue]

Around this pH, I have found stability problems with this buffer. If you prepare it, measure it fresh, and again a few hours later. I found a pH shift into the alkaline, which I attribute to loss of CO2.

[razabal]

Is it really no other buffer MS compatible useable at this range, excepted ammonium carbonate?
Do you have got some usefull links about it for me?
Thanks gstaepels and M.Neue for your answer!

Razabal

[tom jupille]

Is it really no other buffer MS compatible useable at this range, excepted ammonium carbonate?

A buffer is at it's most effective when the pH is at its own pKa, and the capacity decreases in a logarithmic function as you move away, so that at 1 pH unit on either side of the pKa, you only have about 10% of the capacity remaining. Buffer pH ranges are thus usually given as the pKa +/- 1 unit.

Carbonic acid <-> bicarbonate has a pKa of about 6.4 which places it in about the right pH range but, as Uwe pointed out, the carbonic acid has a tendency to revert to CO2 and disappear. Methylamine ("aminomethane") is shown as having a pKa of 8.3, so it overlaps the upper end of the 6-8 range, but again, the un-ionized compound is quite volatile and tends to "go away" (and it smells bad!).

[dr_Pyrex]

In our lab "we love" 0.01-0.05 M ammonium acetate or ammonium formate (pH about 6.8 ).

[Uwe Neue]

I am aware that many people use ammonium acetate at neutral pH for manipulating the ionic strength. But it is not a buffer at pH 7. With respect to buffering capability, it is about as good as table salt, NaCl.

[Noser222]

Are you sure you need a buffer? Sometimes it's easier to go with 0.1% formic or acetic acid.