Chromatography Forum

Half a year ago, a group of me had to change the additive of our preparative HPLC from TFA to FA(formic acid) in mobile phase under the requirement of customer, they don't want to use fluoric acid in the final compound's purification. But FA can't give the same results as TFA do, even we increase the content of FA to 0.3%, the peak shape and resolution is bad, consequently, yield low purity and recovery; the MW of our compound is about 300-600, typically, it contains 3-4 N; we have many kinds of C18 columns from Waters, phenomenex,YMC, etc., but the results are almost the same. Is there any other acid for this application? or other mode? we use ammonia sometimes, but it isn't very good for a lot of compounds.

acetic acid!!

This might come down to a pKa issue.

TFA = 0.3
HOAc = 3.8
Formic = 4.8

If the compounds really need that low of a pH to become symmertical, then the closest acid to TFA would be phosphoric, with a pKa of 2.1. Hydrochloric acid might also work, but the pKa of HCl might also be too low ...

shaun78, your acetic and formic pKa are interchanged.

indeed the values are interchanged. Thanks for pointing that out!

Acetic acid is a weaker acid than FA, so it's not a choice. I notice oxalic acid's pka is 1.23, comparing to 3.75 of FA, it should be stronger, and acceptable by HPLC, but it's non-volatile, so it's not very good for preparative HPLC. If it's possible to use mixed volatile acid to get ideal pka?

Methanesulfonic acid might work? I guess they don't want a TFA salt for a drug.

Alternatively, maybe you should consider high-pH reverse phase or normal phase.

If volatility isn't an issue, how about phosphoric? pK1 around 2.1
I'd send the customer for retraining, or find out their issue with TFA, and help them work around it.

Please keep having fun,

Bruce Hamilton

Volatility is an issue, otherwise we need a process to get rid of this additive, it's too complicated to be a choice. We use high pH by using ammonia, but it doesn't work always. I guess if I add a little HCl into FA, I can get ideal pH(2~3) and ion strength?

Another department at work came up with this beauty aswell. I don't yet understand it and I guess we haven't yet been told the reason blackdrum's customer doesn't want TFA in the preparative run.

As far as I am concerned TFA is volatile and this is why it is accepted for use with electrospray mass spec. That's before you start applying vac or high-vac / lyophilisation to recover product from prep fractions.

Admittedly though, we have had projects where material prepped in TFA mobile phase was recovered as a multiple-TFA salt. We've also degraded product during an evaporative concentration.

I've also heard of people refusing to prep in a chromatograpically-advantageous TFA mobile phase, thus running 'neutral', when the product has to be converted to a stoichiometric HCl salt anyway.

Why are people averse to prepping with TFA?

And rather than mixing HCl / formic acid in the mobile phase, I've seen a few examples of mixing 0.1% FA with 0.01% TFA to try and reap the 'benefits' of both.

JA,

The residual TFA sometimes gives positive result when the final compound is performed bioacitvity test, which will mislead the research, so some companys avoid using it.

Does anybody here use mixed acid to meet such purpose? please give some advice.

What sort of volumes are you generating....is it not possible for the customer to allow the use of TFA to allow you to generate the best chromatography and then for them to SCX (or some other method) the final material and then generate the required salt for testing.

An alternative to reversed phase is normal phase (using NH4AcOH).

Below are peptides (calcitonin and angiotensins) analyzed
using Unison UK-Amino:

http://www.silvertonesciences.com/files/TI388E.pdf
http://www.silvertonesciences.com/files/TI374E.pdf

To echo JA's suggestion, we often use 0.09% FA with 0.01% TFA for LC/MS when we want the resolution power of TFA with the better volatility of FA.

You could try some mixed mode (SCX-RP) columns with formic acid for your application if the cost is not too high...