By mwingerd on Thursday, October 25, 2001 - 04:35 pm:
We have recently been having a discussion of TFA vs. formic acid as an additive or modifier in Reversed Phase HPLC.
I also believe that I have seen a recent article that discusses the difference in performance of TFA vs. formic acid, relative to both UV detectors and LC-MS detection.
Does anyone know of this Article?
Does anyone care to comment?
Thanks
Mark Wingerd
By H W Mueller on Friday, October 26, 2001 - 06:37 am:
The articles on TFA and MS which I have seen are all stating that TFA usually reduces the MS signal, apparently, all alternatives are usually better. Unfortunatly, only the following refs. were among my files: http://www.abrf.org/ABRF/ABRFMeetings/ABRF98prog/p67.html;
D Temesi, B Law, GC LC Int., March 1999, 175.
Regarding TFA and HPLC I can, and have related own experience. In short: The only analysis which worked with TFA/water was: oxalic acid on Hypersil´s Hypercarb column (graphitized carbon). Changing the TFA concentration only slightly resulted in peak shapes that I had never seen before or since (except maybe for some matrix elements).
In the analysis of a peptide (ANF) a useful chromatogram could not be obtained in spite of closely emulating literature methods. Using phosphate buffer, instead, solved the problem immediatly.
Using 0.1% aqu. TFA, and even more so heptafluoropropionic acid, badly deteriorated separation of an antibody from its Fab fragment on a Toso TSK-Gel Super SW 3000 size exclusion column. As a matter of fact the resolution is still not recovered after many long sessions with favorable mobile phases.
The result is that TFA is last on the list of substances to try in this lab. Also, the Ka of TFA is near 0.6, that is, highly dissociated. It will buffer practically only against more acidity.
On HPLC-UV: Absorbance of TFA = 0.01 at 250 nm and 0.1% (~0.009M)
Absorbance of Na formate = 0.02 at 254 nm and 0.1M.
It would be interesting to hear of some research which showed that TFA was undisposable in a particular separation.
By Anonymous on Tuesday, October 30, 2001 - 05:48 am:
The use of trifluoroacetic acid in API-Electrospray may lead to the formation of ion pair with analytes and the consequence is ion suppression. However the use of micromolar concentration of TFA gave very good results (see article "Development of a Multiresidue Method for Analyzing Pesticides Trace in Water Based on SPE and Electrospray Liquid Chromatogrphy Mass Spectrometry" Environ. Sci. Technol. 1997, 31, 479-488 wich report a useful discussion about use of TFA in LC-MS).
We currently use, with good results, TFA at 800 µL/L in both acetonitrile and water for RP separation and analysis of sorbic and benzoic acids on C18 ABZ column and UV-DAD detection.
Formic acid have a good volatility and is capable of lowering pH at value 2-3: therefore it is good choice in API-ES for compounds with a protonable group.
For other informations see article "The Effect of LC Eluent Composition on MS Responses using Electrospray Ionization" LC-GC International, March 1999 already cited by H.W. Mueller