Measuring residual TFA in synthetic peptides: ion chromatography and 19F NMR

Why TFA is there

In reversed-phase HPLC, TFA pairs with positively charged groups and sharpens peaks. The same pairing means the peptide that is collected and freeze-dried carries trifluoroacetate on its basic residues and N-terminus. The choice of ion-pairing reagent changes separation selectivity, which is why it is studied in purification.

Source: The importance of ion-pairing in peptide purification by RPLC, J Chromatogr A 2017 (PubMed 28363419); Effect of anionic ion-pairing reagent hydrophobicity on peptide separations, J Chromatogr A 2005 (PubMed 16013616)

Ion chromatography

Ion chromatography separates trifluoroacetate from other anions (acetate, chloride, phosphate) and detects it by conductivity. It can measure residual TFA in both buffers and peptide preparations and is a common choice for counter-ion assays.

Source: Determination of residual trifluoroacetate by ion chromatography, J Chromatogr A 2004 (PubMed 15250411); Determination of counter-ions in synthetic peptides, J Pept Sci 2012 (PubMed 22252914)

19F NMR

Because TFA contains three equivalent fluorine atoms and peptides normally contain none, 19F NMR gives a clean signal that can be integrated against a fluorinated internal standard. A 2026 method correlates 1H and 19F spectra to quantify TFA alongside the peptide in one measurement.

Source: Quantifying TFA in peptides by 1H-19F NMR spectral correlation, J Org Chem 2026 (PubMed 42695718)

How the result is reported

• As % w/w TFA in the powder, or as moles of TFA per mole of peptide.
• Moles per mole can be compared with the number of basic sites in the sequence as a sanity check.
• Residual TFA, water and peptide content should add up to roughly 100% of the powder weight.

Practical notes for labs

If an assay is sensitive to TFA, ask for the counter-ion form before purchase, or plan a salt exchange. A consensus paper on TFA analysis and exchange (2025) is a useful starting point for method choice.

There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.

Source: Towards a consensus for the analysis and exchange of TFA, Pharmaceuticals 2025 (PubMed 40872554)

Frequently asked questions