The peptide is not 100% of the vial
When a peptide is purified on a preparative RP-HPLC with trifluoroacetic acid (TFA) in the mobile phase, the product comes off the column paired with one or more TFA molecules. TFA is toxic at even trace levels in many assay systems, so we exchange it for a less problematic counter-ion — usually acetate — by an additional chromatography step.
The result is a peptide that is between 5% and 15% acetate by mass. This is normal. It is not contamination. But it affects your dosing math.
How a dosing error compounds
Say you want to dose a 5 mg/mL working solution of a peptide. If the vial is labeled 5 mg, and the lot is 15% acetate, the vial actually contains ~4.25 mg of peptide and ~0.75 mg of acetate. Reconstitute to 1 mL and you have 4.25 mg/mL — not 5. A 15% underdose can be the difference between a significant and non-significant result.
This is why our COA publishes both the purity percentage and the peptide content percentage. The first tells you how clean the peak was. The second tells you how much of the vial’s mass was actually peptide — which is the number you multiply the fill weight by when computing doses.
When acetate drifts high
Acetate above 15% usually means one of two things: incomplete counter-ion exchange (more TFA still present, plus acetate), or multiple acetate salts formed on basic residues. Both are fixable with another chromatography pass. Our internal spec is ≤ 15%. If a lot exceeds it, we re-process, not relabel.
The short rule
Always dose on peptide content, not fill weight. For that, the supplier must tell you both.