Has anyone found anything better than formic for ionization supression in the elution of "stronger" aromatic acids from RP columns? Of course better also must consider the MS sensitivity. This is a trace analysis.

If I understand you correctly, you want to use the pH of the mobile phase to reduce the ionization of acids in the interface, and therefore their response in MS. While the idea is good, it does not work that simply. The MS response of compounds correlates only very mildly with their ionization in the mobile phase, probably due to differences in the evaporation of the different components of the mobile phase in the droplets in the interface, causing pH shifts.
If you don't mind cleaning the interface once in a while, you may want to see if phosphoric acid does the trick for you. We have run non-volatile mobile phase components once in a while. At a low concentration, it is not impossible, but more frequent cleaning is needed. If you have an interface that gives you easy excess, this is not a major problem.

Easy access - not excess - Sorry

Quite the opposite. I need an acidic mobile phase to effect good retention and peak shape on a reversed phase column. The acidic analytes are fairly stong aromatic acids. It is a trace analysis so I need as great a sensitivity as possible for the mass spec detection. A stronger acid than formic that doesn't degrade MS sensitivity is desired.

If the issue is not the MS sensitivity, but peak shape, I would recommend to look at a good reversed-phase packing that gives good peak shape for acids. Also, for retention there are other options than straight RP, for example RP with ion-exchange.

If the pKa's are that low, it may be worth trying 2 to 10 mM ammonium acetate, and chromatograph the acids well above their pKa in the ionized form. If they have any hydrophobic character, you can still get some retention with the appropriate column choice (as per A.Mouse), as well as improved sensitivity in negative mode.

More information, and a plea to get back to the original question which is, is there a better acid to use than formic that will be strong and not detract from MS sensitivity?:

The suggestions so far are, change the chromatography. Not a likely, nor certainly desirable, option. There are years of experience in the chromatography of these compounds and not many choices given the complex sample and other considerations. If phosphoric acid is used instead of formic, the chromatography is just fine in all regards with UV detection. The new wrinkle is using MS detection which precluded phosphoric acid in eluant and a switch to very dilute formic. If enough formic is used to get good chromotography, then MS sensitivity suffers. I have less experience with MS than chromatography, so with the collective experience of this group, is there something better than formic that someone has discovered?

None that I know of. TFA is a stronger acid and volatile, but will absolutely kill your sensitivity in negative mode. I have never tried phosphoric with negative mode. Perhaps it would be worth a try in very low concentrations, as was previously suggested. I'd be interested to hear how this turns out.

OK Bill, then let me go back to the previous suggestion: why not using phosphoric acid and cleaning the interface more frequently. Use it at a low concentration, 10 mM! In some of the modern MS instruments, you can see the interface and observe the build-up. You can then clean it as needed. A cleaning every two weeks or so is not a problem. We actually ran the interface for much longer, before we cleaned it.

Just because you get stuff building up at the interface does not mean that you can't use it at all...

Hey A.Mouse, for my own interest, what kind of ionization were you using with the phosphoric acid? Any idea what that stuff is that builds up? I would think that phosphoric acid alone would be volatile enough to boil off in a heated source, but something nonvolatile stays behind.

It actually was a buffer instead of straight phosphoric acid. You are right, phosphoric acid itself is reasonably volatile, so it was not phosphoric acid that showed up on the interface.

We used both APCi and eletrospray at the time, depending on the compounds that we were after.

Phosphoric acid can hardly be considered volatile. In fact, phosphoric acid and sulfuric acid are amongst the least volatile of the common inorganic acids. References cite a boiling point for phosphoric acid of 158C but the fact that you cannot buy 100% phosphoric acid stems from the tendency of this acid to undergo dehydration based condensation reactions to form higher molecular weight products such as pyrophosphoric acid at elevated temperature (i.e. under conditions necessary for distillation). And, of course, these condensation products are even less volatile so if your interface operates at elevated temperature (as is normally the case) you'll produce a film of the polyphosphoric acid on the surfaces of your interface.

Thanks Chris, I had always wondered about that.