Chromatography Forum

Hello,

A HILIC article from U. Neue et al (J. Sep Sci. 2008, 31 1511-1518) describes the preparation of a ammonium acetate buffer at pH 6.8. I seem to recall that acetate buffering is best lower than this, but perhaps the ammonium ion allows this to be a adequate buffer at pH 6.8?

Your thoughts would be helpful.

Kind Regards.

Acetate has a pKa of about 4.7, so the "nominal" buffering range is 3.7 - 5.7. In practice, you can push up a coupe of tenths. Ammonium has a pKa of 8.2, which gives a nominal range of 7.2 - 9.2. That puts 6.8 on the very ragged edge.

The problem with pH 6.8 is that there is really no good non-volatile buffer there (volatility being critical for LC-MS). Ammonium acetate or ammonium formate are about as good as it gets.

in HILIC you can play around the conc. of the buffer and the pH in order to play around with the ionic characteristics of the compounds you are trying to separate
maybe this is why it was done this way in this article for this application.

pKa of ammonium is 9.2 so this is not a buffer at this pH

pH 6.8 ammonium acetate/formate usually siginfies the salt was simply dissolved in water and no pH adjustment was made. However, depending on the purity/age/storage conditons of the salt there's no guarantee dissolving it in water will give this pH so its always best to check.

It is not meant to be "buffer" but rather to provide some ionic strength and competing ions

Thank you for the responses. I was looking for a non-phosphate mobile phase at ~pH 7 and thought this ammonium acetate would do the trick. It has increased the retention of my basic component (which is what I wanted), but I did not feel comfortable calling it a 'buffer'. It sounds correct that the goal of such a mobile phase is for ionic strength, not a 'buffer' in the true sense.

Thanks again.