Counterion
A counterion is the charged species that pairs with a peptide to form a neutral salt. Common peptide counterions include trifluoroacetate (TFA) and acetate. The counterion contributes to total mass and can affect solubility and downstream suitability, so the salt form should be confirmed against the agreed specification.
Why does Counterion matter when sourcing?
The counterion is part of the material mass and therefore affects how much actual peptide a weighed quantity contains. It also belongs in technical comparisons because changing the salt form may change handling or downstream performance. Two offers with the same gross weight and HPLC purity are not necessarily equivalent on a net-peptide basis.
What should you verify about Counterion?
Confirm the salt form in the specification and batch documents, including the result basis and analytical method where the counterion is quantified. If an exchange step is requested, establish the target form, acceptance limit and any additional tests before quotation. Record these requirements alongside sequence, purity and net-content targets.
What can Counterion not tell you?
A salt-form name alone does not state the counterion amount, residual solvents, water, identity or purity. Nor does it establish suitability for every formulation or regulatory use. Those questions require separate evidence and project review; an HPLC area-percent result by itself is not a counterion assay.
Related terms
Category: chemistry · Part of our peptide glossary · Sourcing question? Ask our engineers for a qualified project review.