Chromatography Forum

Hi, All, I am confronted with a question about an ion-pair reagent. Pentanesulfonic acid is used as an ion-pair reagent for methylene blue measurement. We are thinking about hooking HPLC to Mass Spec. But, not sure, whether, pentanesulfonic acid is bad for Mass Spec or not.

Thank you for your help!!!

I am curious about this as well. I don't think it is a good idea.

I have almost always used volatile mobile phase additives for LCMS (Ammonium formate, ammonium acetate, and their acids, ammonium hydroxide, tetrabutyl ammonium hydroxide, TFA, .....). I think everyone will tell you the same, stay away from non-volatile additives. As for volatiles...I did try very dil HCl once (millimolar conc) and that did not seem to do the mass spectormeter any good so you might want to stay away from that as well.

I remember the first (and last) time I used phosphoric acid (non-volatile) as an additive on a triple quad instrument, the signal kept dropping until I saw nothing. We had to clean out the front end from orifice through to the end of Q0 rods.

Alp

Replace your sulfonic acid with a long perfluorocarboxylic acid (length up to pentadecafluorooctanoic acid) and you should be fine... A word of caution, people reported problems with using of perfluorocarboxylic acid with on line degasseurs...

Quote: "Replace your sulfonic acid with a long perfluorocarboxylic acid (length up to pentadecafluorooctanoic acid) and you should be fine... "

provided you never ever want to work in negative mode again. These ion pair reagents can be incredibly hard to remove from your system, and ionise extremely strongly in their own right. Depending on applications, your system, and how you use it, you may find that full scan negative data contain no detectable signal except long perfluoro carboxylic acids for several months after you've used them.

That has not be my experience at least with triple quadrupole instruments. As mentioned, a lot of times the problem comes from the on-line degasser. I can not comment on the effect at ion-trap instruments.

Imh, has it be your experience that it is difficult to clean the instrument up after using the perfluorocarboxylic acid? Have you tried to clean the instrument and then infuse the mobile phase (so that you isolate the mass spectrometer from the LC)?

I agree the perfluorocarboxylic acids are a bad idea. Aside from the almost fatal impact on negative ions - ok you can flush out with e.g. aq ammonia for a long time and recover some [M-H]- your sensitivity will be severely compromised, you may well/will probably see a moderate to dramatic decrease in positive ion sensitivity. Years ago we looked at a number of acidic additives, compared to formic acid TFA and other perfluorocarboxylic acids gave as little as 8% response in some cases.

Contamination issues are probably going to depend very much on individual design of instruments, and therefore an additive that is a major problem on one instrument may be only a minor inconvenience on another. My experience was heptafluoropentanoic acid, I think, and was in a single quad and an ion trap. Obviously the ion-trap was the most badly affected because the majority of its trap capacity was filled by the heptafluoropentaonate ion, meaning it lost all sensitivity. The single quad was still OK for SIM mode (except that coincidentally one of my favourite analytes happened to have the same nominal mass as heptafluoropentanoate!), but in scan mode it was hopeless. In the end I had to run repeated washes through the LC systems, copious washings of spray chambers, but also I had to replace the electrospray needle in one of the instruments, and the spray chamber drain system in the other (I assume that the plastics in the drain system were particularly attractive to perfluorinated acids, and that eddies of drying gas were enough to pick them up, but I don't know for sure). It was one of the less happy incidents of my life, but after a month or two, the ion was gone.