Defined term · Analytical chemistry
Counter-ion
The ion paired with a charged peptide, most often trifluoroacetate or acetate from purification. Can constitute a substantial fraction of the mass of a basic peptide and is not detected by a purity…
§1Counter-ion
§1.1Also known as
Salt form
§1.2Category
Analytical chemistry — Terms describing the determination of identity, purity, content and quality.
§1.3Related terms
- Mass balance — The sum of peptide content, water content and counter-ion content, which should approach one hundred per cent. A certificate that does not permit a mass balance to be…
- Trifluoroacetate — The counter-ion most commonly introduced by reverse-phase purification with trifluoroacetic acid. Quantified by ion chromatography or fluorine nuclear magnetic…
Compound Evidence Institute · CEI-GT-138 · https://compoundevidence.com/glossary/counter-ion/ · retrieved 30 July 2026