ion-pair agents+mass spec

[olivia]

Hi, can perfluoropentanoic acid be used in buffer for mass spec method. I believe it is a volatile ion-pair agent
Many thanks
Liv

[Kostas Petritis]

Yes, you can actually use up to perfluorooctanoic acid (up to 5 mM). Although these perfluorocarboxylic acids are volatile in their acid form they are not volatile if you put anything else than H as countair ion, even if that is ammonium...

For more info check out the paper below:

Volatility evaluation of mobile phase/electrolyte additives for mass spectrometry
K. Petritis, C. Elfakir, M. Dreux
LC-GC Europe 15 (2002) 98-102

[seaclimb2001]

I have successfully used n-alkyl primary amines such as hexylamine as ion pair reagents for anionic analytes on Evaporative Light Scattering Detectors (ELSD). The requirements of ELSD and MS for volatility are similar.

The concentrations needed tend to be much higher than if one were using a quaternary ammonium salt. And, needless to say, the pH of the mobile phase must be such that the ion-pairing agent is ionized, as well as the analyte.

Good luck.

[Mary Carson]

The perfluoroacids can be used as an ion-pair for MS, with some caveats. The biggest caveat is that they hang around the system for a long time and kill the negative ion response of the mass spec. If you are going to use them, it is really nice if you can dedicate an MS and LC for that use, and keep them away from your other systems. This goes for TFA as well as the longer chain acids.

[Kostas Petritis]

Although I agree about the negative ion response (which in most cases is not used along with negativly charged ion pairing reagents) I personally find it easy to clean the system (LC + MS) from them so I do not really agree that they hang around the system for long time...

[Mary Carson]

What is your cleaning regimen? I heard once that one had to actually swap out the membranes in the LC degasser to completely remove residual heptafluorobutyric acid from the system....

[Kostas Petritis]

In terms of mass spectrometry the perfluorocarboxylic acids were not "sticky" and can be removed in general with a thorough cleaning up to the orrifice or Q0 (if necessary, i.e. if you need to work on negative ion mode). Even in the case of more persistant ions such as triethylamine, leaving the LC-APCI overnight at high temperatures should remove everything from the source...

In terms of chromatography, I have in the past investigated columns before and after the use of different perfluorocarboxylic acids. Tanaka tests before and after the use of IPC came out exactly the same and I was not able to see any appreciable ion exchange capability for these regenerated columns. Cleaning protocol in these cases was just ACN and methanol and in the case only of pentadecafluorooctanoic acid I was using THF as well.

You can refer to the article below for details on the work done:

Ion-pair reversed-phase liquid chromatography for the determination of polar underivatized amino acids using pefluorinated carboxylic acids as ion pairing agent K. Petritis, P. Chaimbault, C. Elfakir, M. Dreux Journal of Chromatography A, 833 (1999) 147–155

I was not using an online degaseur for that work so I do not know if there are specific problems with that (I doubt that there are in the case of TFA). I will be doing actually some work in the next 6 months that will probably involve the use of HFBA with online degaseur as well coupled to ion traps and I could report back on that...

[Mary Carson]

Yes, I would be interested in hearing a report back on that application, as it is pretty much the same use we had. The main difference could be that almost all our LCs have online degassers, so even though we swapped out columns (we consider them cheap enough to be "dedicated") and cleaned source, we still saw a persistent effect, not in chromatography, but in negative ion suppression.

[mardexis]

Most of the online degassers use Teflon AF as the membrane. Very permeable towards gases (good) and very high affinity for fluorinated compounds. We use negative mode routinely for many compounds as I'm sure most people do. I was thinking of trying some ion pairing for a tricky separation on our triple quad. I will steer clear of the perfluoro compounds or certainly not let them into my degasser.

[Kostas Petritis]

Mary,

Do you see the [HFBA-H]- ion in negative ion mode after cleaning? If yes, then ion suppression can be from that, if not I do not believe that this is the reason.

[Mary Carson]

Kostas,

I don't remember if we actually checked, since we mostly used the HFBA on an ion trap, and we mostly did not use that instrument in negative mode. I think one of my colleagues did look and did see HFBA ions in negative mode, but I can not swear to that. One triple that we have used TFA on extensively has extremely poor negative ion response, but other comments I've seen on this forum suggest that may be an inherent characteristic of that particular model. I think that is also the only LC we have on a MS that doesn't have a membrane degasser. Mardexis' comment about the Teflon AF membrane makes a lot of sense to me. So maybe one only has to dedicate a degasser to perfluorinated acid mobile phases, and not an entire LC-MS system.

BTW, as an aside to the moderator, should this entire topic be moved over to "hyphenated techniques"?