Chromatography Forum

Does anyone have a recommendation for a good buffer to use in the pH 10-12 range? The analytes in my separation contain two pKa values with pKa1= 3 and the second pKa ranging from 4-10. The column I am using can be used in the high pH range.

carbonate or phosphate.

Thanks Tom for your prompt reply!

Are there concerns with using carbonate buffers since they pick up carbon dioxide readily? If I use a carbonate buffer system, should it be a mixture of bicarbonate and carbonate to make a true buffer? Also, I am assuming it would be best to use an ammonium counterion i rather than sodium f I would use this for MS work.

*Any* high-pH buffer should be protected from the atmosphere, because they will all absorb CO2, so there is no difference there. If you're going to do MS, then phosphate is out in any case. The other possibility is something like methylamine, which has about the same pKa as carbonate (around 10.5).

Regardless of where you start, you will end up with a mixture of carbonate and bicarbonate ions in solution. I would think the easiest approach would indeed be to mix ammonium carbonate and ammonium bicarbonate solutions. For pH values above the pKa of carbonate, you would get to the same point by starting with ammonium bicarbonate and titrating upward with ammonium hydroxide. Below the pKa, you would have to start with the bicarbonate and blow CO2 into the solution (not the most practical! )

I do not recommend to use "ammonium carbonate". In reality, it consists of a mixture of ammonium bicarbonate and ammonium carbamate. We have always used buffers made from ammonium bicarbonate with the addition of ammonia. The advantage of this buffer is that it is completely volatile for MS without any ion suppression effects (or nearly without them). The buffers are rock stable in the pH 9 to 10 range.

Uwe,

Thanks for your response. If you look at the pKa of my analytes in my initial post. I need to use a buffer that is greater than pH 10. I do not have many choices at this point for an MS friendly buffer. Tom had suggested methylamine but I have not worked with this before and not sure of its buffering capacity.

As with many other cases, MS detection is the limiting item. For this reason, I recommend to stick with the ammonium bicarbonate, which you can get to ~ pH 10 with ammonia. Try it. If your analyte gives you symmetrical peaks, be happy. If you are concerned about buffering capacity and stability of retention, keep in mind that the pKa of your analyte changes with the organic solvent in the mobile phase. The pKa of bases shifts to lower values, while this buffer has a constant s/w pKa.