Chromatography Forum

Is it possible to use ammonium salts of other organic acids than formic and acetic acid as buffers in positive ESI-MS?

I am of course fishing after the higher pKa's of e.g. citric and succinic acid.

Not being an LC person and not seeing another posting, I'll make a guess - in the event it will help: Acetic acid and formic are volatile, so they will not accumulate in the front end of your MS. Citric and succinic acids will accumulate.

They are not completely non-volatile, boiling point of succinic acid is 235 C. But since no-one is using them in mobile phases for MS, I guess they don't work.

I am currently using 0.2% oxalic acid in my aqueous phase in a gradient that goes from 92% to 45 % aqueous. So far I have not noticed any adverse effects on the MS ,excluding the fact that the entrance cone gets somewhat dirty in 50 or so injections. But that can be due,at least in part, to the nature of my samples: crude extracts of biological tissues.

Hope it helps

I couldn't find the boiling point of oxalic acid, but it should be lower than for succinic acid. Apparantly it works! I will not help me though, since pKa of oxalic acid is 4.3 (i.e. not a buffer at pH 6-7)

A quick google confirmed what I sort of remembert of oxalic. It doesn´t have a normal (atmosphric pressure) bp, it sublimes at 157°. The melting points vary, apparently depending on hydration, decomposition sets in under some melting conditions.
I imagine this interesting applicatiion of carras (any refs?) might be due to oxalic decomposing in the MS.
There have been some discussions on buffers for pH ~ 7.

What pH are you looking for? Ammonium bicarbonate is good for higher pHs. Bicarbonate/carbonate pKa is 10.3, bicarbonate/carbonic acid is 6.4. Of course, a buffer near the lower pKa will evolve CO2 over time, particularly if you are on the acidic side of 6.4, so you will probably want to evaluate the shelf life of your mobile phase.

Not being an LC person and not seeing another posting, I'll make a guess - in the event it will help: Acetic acid and formic are volatile, so they will not accumulate in the front end of your MS. Citric and succinic acids will accumulate.

I've actually used 1mM citric acid, pH adjusted to 11 with TEA. I had to bump the desolvation temp up to 600/650 in order to get rid of an accumulation on the curtain plate. I don't know if it was related to the citric acid or the matrix, but it was resolved at higher temperatures.

The highest pKa of citric acid is 6.4. It has no buffering capacity at a pH of 11. The triethylamine pKa is 10.75, so it is a buffer at that pH 11. Just leave out the citric acid as the only thing it is doing is contributing some ionic strength. If you want the ionic strength, use ammonium formate/acetate.

I have tried citric a couple years ago - it did not work. It leaves film on the spray insides and ion transport capillary. You can measure for some time, but as soon as you stop HPLC flow/spray for a moment - further measurement is not possile. You can not get any ions into MS - have to shut down instrument and clean source.

Hi, everybody!

In answer to HW Mueller's inquire about references for the use of oxalic acid, here's the one in which our method is based:

Granelli,K. et al, Analytica Chemical Acta, 637, 87-91,2009.

it is about multiresidue analysis of antibiotics in muscle.

I started using what most people would use I guess, unbuffered ammonium acetate (pH 6.9). The separation looks OK, but I have no idea what happens with pH of this solution when exposed to air (or if using different suppliers of NH4OAc)?

I tried ammonium citrate pH 6.8, but the background in MS was so bad that I couldn't even see my main peak in the TIC.

(for clarification, this has nothing to do with the other dicussion on the LC page)