selecting acids...

[matthew]

So, I received my triple quad last week (yeaaaah); immediately after it was unpacked, I grabbed a paper, and started injecting standards.

Only problem: one of the ions chosen by the authors to transition on has a mw very close to acetic acid. The authors specify integer transitions (API 3000), so I assume their quantitation is in error. I think that I received another free lesson on ionization enhancement (?): when just running solvent, the peak disappears in .05% HOAc-AcN and nearly does in .05% HOAc-Water, but the peak skyrockets as I mix the solvents.

Anyway, I'm writing to ask, what acids should I put in my cupboard? What are the most common acids to use, acetic acid, hmmm, formic acid, TFA? Are there others that I should have handy?

Thanks!

[MaryCarson]

Save the TFA for when you really need it, and buy the purest you can get in a small container. It doesn't keep all that well.

Formic and acidic acids for sure, but small bottles. Sometimes I see 1% acid in the mobile phase, but 0.1-0.5% is more common. You might want to stock some high purity NH4OAc as well. That is used a lot.

[matthew]

Thanks Mary!
Is there a particular vendor you use (fisher?). There are a few reagents that I'm considering that aren't labeled 'hplc', but they are > 99%, so I assume I won't destroy the machine if I use them (?).

[Stryder08]

We have quite a few LC/MS/MS instruments, as well as a couple of Ion Traps and TOFs. We tend to use Formic Acid in DI water/Formic Acid in Acetonitrile...but we have used Ammonium Hydroxide in Methanol previously...

[MaryCarson]

I have gotten away with using reagent grade glacial HOAc, and also formic acid. Formic azeotropes (is that a verb?) at about 88%, with the rest water. You can also buy 95% formic. We have used both--the trick then is whether "0.1%" is v/v from starting material, or final concentration. Usually, it won't make a difference....but best to know and repeat exactly.

It's possible to buy TFA in 1mL ampules. Expensive, but nice if you want to have some on hand for occasional use. They keep well. Otherwise, I'll look for a 50 mL or 100 mL bottle to buy. For us, discarding unused halogenated solvents can cost more than buying them in the first place, so we try not to have more on hand than we will use up before it expires.

I would recommend investing in LC grade NH4 OAc and formate--and I also believe in filtering mobile phases incorporating a solid buffer, even if it is only 5mM.

BTW, your new finding that water + ACN (+ acid modifier) gives the best ESI signal is not at all surprising. Some analytes prefer MeOH, but some organic really seems to create a better spray and nebulization than straight water, while straight organic, especially ACN, doesn't help ionization.

[matthew]

Hi Mary, thanks, I will also get NH4OAc. I received several ampoules of ammonium formate with my machine, and I agree that the approach is preferable (although single use containers are expensive, and a burden on the environment, if I go the ampoule route I won't be prone to buy a large bottle that will become cumbersome and possibly worse for the environment in a few years).

Thanks for the MeOH tidbit. I reran the sample in MeOH and it seems to behave much more predictably, and the peak is sharper. I also seemed to be (too easily) getting column overload with AcN, even with 10 volume washes. I'd like to explore that, because I believe that one question will help me really shake off all of the rust, but maybe this weekend.