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Author: Subject: TFA Formation - How?
GreenD
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[*] posted on 1-9-2013 at 08:42
TFA Formation - How?


We have an LC/MS in our lab and its been plaguing us with this TFA signal for some time.

We know its TFA;
we get
(M-H)-
(2M-H)-
(M-H+Na-e)-

However, no where in our system do we add TFA! No where! Not in the solvent, the tuning mix or the reference mixes. And we certainly don't inject it.

My boss has conjured up the idea that TFA is being made by acetonitrile (our mobile phase) and the salt NH4F.

This doesn't burst out as a eureka to me - I don't think the system ionizes this stuff for a long enough period of time to be producing massive (it saturates our system) amounts of TFA.

Anyone have any input? I've been pulling my hair out for 2 weeks trying to get rid of this ion!




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sonogashira
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[*] posted on 1-9-2013 at 10:22


Have you checked to see if trifluoroacetic acid is eluting from the column by using a different detector? If it is not, then surely it is a well-known MS interaction - if your identification is correct. Sadly I know only the basics of MS, and I am not aware of how this ion may be formed.
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jwpa17
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[*] posted on 18-9-2013 at 17:19
TFA formation in MS


I'm certainly no expert, and in fact am not sure what you mean by
"We know its TFA;
we get
(M-H)-
(2M-H)-
(M-H+Na-e)-"
TFA - trifluoroacetic acid - i.e., the protonated form, right?
then (MH)- (2MH)- and (MH+Na-e)- are unclear to me. Maybe TFA to you means trifluoroacetate?
I tend to agree with you - if you aren't adding trifluoroacetic acid, saturating the detector with (TFAH)- seems unlikely. Do you see the signal if you use only ACN in the HPLC system? I.e., omit the ammonium fluoride and see if your signal disappears.
Roughly TFA is about 113 - do you have enough resolution to be certain it's TFA? Does the C12/C13 ratio indicate two carbon?
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AndersHoveland
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[*] posted on 18-9-2013 at 19:25


Are you sure your acetonitrile was not contaminated with traces of TFA ?


Another unlikely possibility, it is not completely inconceivable there could be some complex interaction and tautomerization. Remember, acetic acid can be readily chlorinated under very acidic and anhydrous conditions, and acetaldehyde is readily chlorinated to choral hydrate.


CH3-CN

CH3-CF=NH

CH2=CF-NH2

FCH2-CHF-NH2

FCH2-CH=NH

FCH2-CH=O

[Edited on 19-9-2013 by AndersHoveland]
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ziqquratu
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[*] posted on 19-9-2013 at 05:41


I assume you're using an ESI source, but I think my analysis should hold true for EI as well.

I can't imagine how you could possibly see an "(M-H+Na-e)-" ion (I assume you meant " [M - H + Na - e]-", where the symbols inside the brackets are add/subtract, whilst the one outside is the charge?). Such a beast would be impossible in ESI (and I don't think it would be possible for EI, either). First, I can't think of any example where I've seen ESI dislodge a single electron. Second, unless it becomes covalently bound to the molecule, there's no way Na+ (which is "Na - e") would be detected, since you're clearly working in negative mode. Finally, [2M - H]- is pretty rare for small molecules (I've seen it for peptides, but never for anything else - not to say it doesn't happen, just not very often!).

Also, looking at it again, ([M - H]-+Na+) must be an uncharged species and thus cannot be detected at all. That ion MUST be something else (or you've got a typo there, of course)!

In light of this, are you sure it's TFA? Could it be some other contaminant with M = 138 (for the weird sodium-containing ion you quoted), or even M = 228 (considering your "2M - H" ion), and which gives a fragment with M = 114? As jwpa17 asked, have you checked the isotopic ratios? Also, would it be possible to isolate enough for an NMR?

If it is definitely TFA, I seriously doubt that fluorination of acetonitrile is the culprit here. If nothing else, you should expect to see various intermediates (such as the mono- and di-fluorinated species) if that were the case.

In that case, you must be overlooking some source of TFA - and since you're running in negative mode, it's only going to take the tiniest trace of TFA to give a significant peak for [M - H]-
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