will TFA change selectivity of neutral analytes

[jiang295]

I know that it will decrease retention of analytes because of adsorption of tfa anion, but i do not know whether it will change selectivity of neutral analytes or not?
and i guess TFA will decrease loadibility of neutral analytes because of decreased C18 adsorption surface , can anyone confirm with this?
thanks a lot if you can help.

[adam]

My guess is that it would have very little effect on most neutral analytes.

If it is not protonating the analyte or ion pairing with it, then it wouldn't be expected to do much.

I guess - for certain analytes - protonating the silica stationary phase could result in less tailing and maybe even a somewhat reduced retention.

[jiang295]

I think so too. thanks.

My guess is that it would have very little effect on most neutral analytes.

If it is not protonating the analyte or ion pairing with it, then it wouldn't be expected to do much.

I guess - for certain analytes - protonating the silica stationary phase could result in less tailing and maybe even a somewhat reduced retention.

[Jack Silver]

Although it may not interact with the analyte, the TFA will affect the polarity of the solvent system and could cause the analytes to elute a little later on reverse phase.

[HW Mueller]

It seems one can make a very rough estimate on the polarity change. If one has, as an example, 60 units H2O + 40 units MeOH and adds 0,1 units TFA you probably get less of a polarity change than adding another 0.1 units of MeOH.
I have no idea what the effect will be if an analyte interacts with silanols via hydrogen bonding. But hydrogen bonding is a partial dissociation so one should, maybe, not include such compounds in "neutral" analytes.

[Alera]

It sounds like you would like to change the selectivity of your separation by using TFA in the mobile phase. I would try other venues before turning to TFA (column, temperature, organic component), as it is not a very nice compound to work with in terms of chromatography. It is very volatile, so mobile phase is not stable, it absorbs a lot below 240nm and it degraded very quickly generating ghost peaks.

[HW Mueller]

Alera, we just had a discussion in which some people claimed that TFA is quite stable. Now I know it is not a particular problem at the usual 0.1% in water. The admonition to use high grade 1mL ampoules might come from stability problems due to impurities. Do you have any references to unequivocal evidence that it "degraded very quickly"?
Of the problems which I had using TFA (mentioned several times some years ago) volatility of aqueous solutions was definatly not one of them.

[Alera]

HW, I completely agree with you about stability of TFA, if you start with very pure material. It is definitely volatile though, so if your separation is really sensitive to TFA concentration (which I have seen also), your peaks WILL move around. It is all relative though (compared to non-volatile mobile phases), so if you are using high purity stuff (ampules) and making fresh mobile phase every week, you should be fine. I also displike it since I worked with LC-MS a lot and that stuff sticks around the source forever.

[HW Mueller]

That I would like to see in well documented experimental results: RT changing due to evaporation of TFA in a mobile phase of the type, 0.1% TFA + x% H2O + y%Org. At such low pH one has to change the acid considerably before getting a pH change which is significant in a HPLC setting. How much org. modifyer might evaporate before TFA does significantly?
It is interesting that the H2O azeotrope of TFA is a high boiling one (above the bp of H2O and thus also of TFA).

[danko]

0.1% TFA is under all circumstances (typically anyway) in a considerable redundancy and one can convince him/her self that it’s the case by measuring pH in the aqueous eluent. Last I did that the pH was 1.9 which typically isn’t that good for many columns, but that’s another story. The point is that if TFA evaporation causes the problem here it should be possible to measure a pH fluctuation - upwards.
Finally, if the above is the case, after all, then the retention time should only fluctuate in one direction – typically forward.

Best Regards

[Alera]

HW, in my case, the RT shift was not due to the pH of the mobile phase change per se, but rather TFA's role as a modifier, which as you know could have a profound effect on separation of ionic compounds.

[danko]

Trifluor acetate does not evaporate alone – it takes the hydrogen ion along. That would result in higher pH. OK? So, reworded it goes like this: If TFA evaporates you should see pH increase.

Best Regards

[HW Mueller]

Alera, the concern here is that 0.1% TFA is usually more than enough to do the job, that such aqueous solutions of TFA are not particularily prone to evaporation problems (unless you provoke it), and that evaporation of organic modifyer might be a more serious problem.
Besides, why are we discussing that? It seems to me that jiang just asked a theoretical/academic question, I doubt that he will try to use TFA as a modifyer in "neutral" compounds, and if he does his least concern would be the evaporation of TFA.

[Alera]

I agree, I had the same thought