Is Ammonium bicarbonate a better buffer for high pH reverse phase gradient HPLC applications than other ammonium salts such as Ammonium acetate or Ammonium formate? If so, why?

Hi Don,

According to my "Practical HPLC Method Development": "Use organic, citrate, and borate buffers to minimize silica support dissolution (avoid phospate, ammonium, and carbonate, if possible)." Also, don't know what kind of detector you'd have, but UV absorbance for a 10 mM soln of ammonium acetate is 1.88 at 200nm, 0.53 at 210, and 0.15 at 220. Couldn't find info on formate. The buffer recipe for high pH listed in the back of the book combines 0.2M solns of glycine and NaOH for pH 8.6 to 10.6.

I'm sure a more enlightened member of the forum can give more specific info.

Regards,
Russell Rondeau

A buffer is a buffer if it buffers the pH. Ammonium bicarbonate does this in the pH 8 to 10 range. Acetate and formate buffer in the acidic range, not in the alkaline range. Ammonium buffers in the alkaline range, but it is a component of the ammoniumbicarbonate anyway. Ammonium formate and ammonium acetate are salts at neutral pH, not buffers.

I mentioned the choice of ammonium counterion because of a citation in the June 2001 issue of LCGC titled 'Buffers and Baselines'. The exact quote is: "Some workers have found ammonium bicarbonate to be especially useful at high pH levels." The cited author is Uwe Neue. I understand that in the vicinity of pH 9.3 it is only the ammonium / ammonia equilibria that creates a buffering effect. Is there a reason that ammonium bicarbonate was recommended exclusive of its buffering characteristics?

The primary reason for using ammonium bicarbonate at alkaline pH is the volatility of such a buffer for LC/MS applications. Of course, other volatile counterions to the ammonium ion would work equally well. Ammonium formate or acetate in the weakly alkaline range are quite suitable as well. If one does not care about volatility, many other choices are possible for the weakly alkaline pH range.