A purification salt at 10 nanomolar skewed cell experiments

That counterion adds mass to the powder, and a purity percentage, which compares peptide peaks with each other, does not count it. Researchers at the University of Auckland asked whether it also changed what cells did.

Trifluoroacetate at 10 to 100 nanomolar reduced cell numbers in fetal rat bone-cell cultures within 24 hours, and did the same in cartilage-cell cultures and mouse skull tissue. Peptides tested as trifluoroacetate salts consistently gave lower growth readings than the same peptides as hydrochloride salts.

Depending on the peptide, the salt hid a real growth signal or created the look of an inhibitory one. The authors judged the problem relevant to any peptide studied above about 1 nanomolar and advised converting peptides to hydrochloride or similar salts before testing.

The counterion was part of the experiment.