Counter-ion content

Where do counter-ions come from?

Most peptides carry positive charges at basic residues and the free amino end, so they exist as salts with a negatively charged partner. A 2020 review of counter-ions in peptides explains that trifluoroacetic acid is used both to cleave peptides from the synthesis resin and as a mobile phase modifier in reversed-phase purification, so cationic peptides are obtained as TFA salts, and that most approved peptide medicines are acetate salts (Sikora and colleagues, Pharmaceuticals, 2020). Changing TFA to acetate or chloride is an extra step after purification.

Why this weighs more heavily on short peptides is covered in the lesson on net peptide content and counterions. This page is about how the counter-ion is measured.

Which methods measure counter-ions?

Sikora and colleagues list chromatographic separation (HPLC or ion chromatography, which they call the technique of first choice), electromigration techniques (capillary electrophoresis or isotachophoresis) and microtitration as the common approaches, with IR and NMR used for partial characterisation such as tracking TFA. A 2025 study set out to compare and validate methods for TFA specifically, noting that many published methods lacked validation to ICH standards (Erckes and colleagues, Pharmaceuticals, 2025).

How sensitive are these methods in practice?

Erckes and colleagues validated three TFA methods side by side and reported quantitation limits per milligram of peptide salt of 5.8 µg for HPLC-ELSD, 10.3 µg for 19F NMR and 178 µg for FT-IR. They concluded that 19F NMR and HPLC-ELSD both gave reliable quantification, while FT-IR is useful for confirming that TFA is present rather than for measuring how much.

In the same study, exchanging TFA for chloride by freeze-drying from hydrochloric acid worked best at 10 mM HCl, with residual TFA measured after each freeze-drying cycle. That repeated measurement is how an exchange is shown to be complete, and why a salt name on a label without a figure leaves the question open.

What do regulators expect for counter-ions?

The European Medicines Agency's draft guideline on synthetic peptides says the counter-ion should be indicated, along with whether it is present in stoichiometric or non-stoichiometric ratio. It lists acetic acid or counter-ion content, residual TFA content and mass balance among typical specification tests, says the amount of counter-ion should be controlled with both an upper and a lower limit, points to Ph. Eur. general chapter 2.5.34 for acetic acid in synthetic peptides, and names RP-HPLC and ion chromatography for identifying potential counter-ions (EMA draft guideline on synthetic peptides, 2023).

Counter-ion content, water by Karl Fischer, peptide content by amino acid analysis and the related impurities should together account for close to all of the powder. When a certificate gives each of them, check that they add up.

How does the counter-ion change the milligrams of peptide in a vial?

A vial is filled by weighing powder, and that powder is the peptide salt, not the bare peptide. Erckes and colleagues point out that TFA can introduce a weighing error because the salt forms have different molecular weights. Treat the whole weighed mass as peptide and the peptide amount is overstated by the counter-ion and any water, which is why the EMA draft guideline expresses assay limits on a counter-ion free, anhydrous basis. An HPLC amount read against a reference standard of known peptide content measures peptide directly; a figure taken from the fill weight does not.

Does Tested.ph offer counter-ion testing?

Planned, not yet offered. The standard package measures identity, purity and amount, performed by the partner laboratory when testing opens. The content calculator shows how a counter-ion figure from any certificate changes the peptide milligrams in a vial.