Net peptide content and counterions

Lesson 4 showed an amount figure swinging by twelve points depending on whether the reference standard was counted as salt or as peptide. This lesson shows where that gap comes from, how far apart purity and peptide content can be in published data, how to estimate the gap with a calculator, and which tests measure it.

Why does a peptide powder contain counter-ions at all?

Peptides carry positively charged groups: the free amine at the start of the chain and the side chains of basic amino acids such as lysine and arginine. In the acidic conditions used to purify and freeze-dry a peptide, each of those groups picks up a proton and pairs with a negatively charged partner from the acid used. If that acid was trifluoroacetic acid (TFA), the powder is a TFA salt. If the manufacturer exchanged it, it may be an acetate or chloride salt.

The European Medicines Agency's guideline on synthetic peptides notes that "usually acetate is used as counter ion, however, other counter-ions are also possible (e.g. trifluoroacetate or chloride)", and expects the counter-ion type to be defined and its amount controlled with a justified upper limit (EMA, synthetic peptides guideline, 2025).

How do you estimate the counter-ion share? A worked example

A 2025 study on TFA in synthetic peptides calculated counter-ion weight percentages "assuming each positively charged position is stoichiometrically paired by a counterion" (Pharmaceuticals, 2025). That gives a simple formula:

The masses you need come from PubChem. Trifluoroacetic acid has a molecular weight of 114.02 g/mol (PubChem, CID 6422) and acetic acid 60.05 g/mol (PubChem, CID 176). Start with an illustrative peptide of 1,400 g/mol carrying three positive groups. These are invented teaching numbers, not a real compound.

Before any impurity or water is counted, a fifth of the TFA salt's weight is not peptide. Acetate recovers about eight points because its partner is lighter.

Why do short peptides lose more weight to salt?

Because the counter-ion mass depends on the number of charged groups, not on the length of the chain. Apply the formula to two real compounds, using PubChem masses and assumed counts of positive groups. The counts are teaching assumptions; a real batch's counter-ion content has to be measured.

BPC-157 is listed in PubChem at 1,419.5 g/mol, and its sequence contains one lysine plus the free amine at the start of the chain (PubChem, CID 9941957). Assume two TFA partners. Tirzepatide is listed at 4,813 g/mol (PubChem). Assume, for comparison only, three TFA partners.

Even with one more counter-ion, the long peptide gives up a smaller share of its weight. The same 2025 study also found more TFA than the stoichiometric amount in peptides straight after purification, unbound TFA that further freeze-drying cycles removed, so a fresh TFA salt can carry more counter-ion than this estimate suggests (Pharmaceuticals, 2025). The same study tabulated this calculation for seven real peptides of about 770 to 2,850 g/mol: as TFA salts, the counter-ion came to between 21.9% and 35.2% of the weight, and as chloride salts between 8.0% and 17.1%.

What about water and other non-peptide material?

Peptide powders are often very hygroscopic, and the EMA guideline expects water content to be part of the stability protocols for hygroscopic powders (EMA, synthetic peptides guideline, 2025). Water is subtracted on top of the counter-ion. Continue the illustrative 1,400 g/mol TFA example: suppose the powder also holds 6% water (illustrative). The peptide share becomes 80.4% x 0.94, about 75.6%. A vial filled with 10 mg of that powder holds about 7.6 mg of peptide.

A full mass balance subtracts everything that is not the target peptide: water, counter-ion, residual solvents, non-peptide residues and peptide-related impurities. The synthetic glucagon study is a published example of this approach, using Karl Fischer titration for water and ion chromatography for TFA, then subtracting each from the total (Wang et al., Scientific Reports, 2020).

How far apart can purity and peptide content be?

Far enough to matter, and the published glucagon study shows it with three figures for one material. The authors measured 103.03 mg of TFA in every gram of the powder, which no UV purity figure sees.

The gap between the first and last rows is 87.36 mg in every gram, as the authors calculated, and they conclude that a UV-only result is not an accurate content figure "because some impurities do not absorb ultraviolet radiation" (Wang et al., Scientific Reports, 2020). A reference-standard paper gives a cleaner teaching case: a theoretical bulk peptide with HPLC impurities of 0.92% of detected area and 5.58% acetic acid by weight works out at 0.93 mg of peptide per mg of material on an anhydrous basis, and 0.91 to 0.92 mg once 1.5% to 2% water is counted (McCarthy et al., Pharmaceutical Research, 2023). An implied HPLC purity above 99% sat alongside a peptide content of about 91% to 92%.

Which tests actually measure each part?

The 2025 TFA study adapted and validated three methods for quantifying TFA: FT-IR, 19F NMR and mixed-mode HPLC with an evaporative light scattering detector. The EMA guideline lists amino acid analysis, nitrogen analysis by Kjeldahl, elemental analysis and qNMR as alternatives for assay. The method pages for counter-ion content and Karl Fischer water describe each test.

What should you ask a seller about salt form?

A seller who knows their product can answer all of these; one who cannot is telling you something too.

• Which salt is it? TFA, acetate, chloride or free base. The answer changes the expected peptide share by several points, as the tables above show.
• Is the label in peptide milligrams or powder milligrams? This decides whether a measured amount below the label is a shortfall or the expected salt and water gap.
• Was a correction applied when filling? If the filler weighed out extra powder to reach a peptide target, the label and the measured peptide should agree closely.
• Is there a counter-ion or water figure for this batch? A batch figure is evidence; a general statement about the product line is not.

Salt forms are normal, and manufacturers choose them for good reasons. But without these answers, a gap between label and amount cannot be read, because expected salt weight cannot be separated from missing peptide. For medicines, the EMA guideline defines strength by the mass of peptide base and expects any correction factors applied during filling, for example for assay, moisture or salt content, to be described. The content calculator applies this lesson's formula to any peptide mass, counter-ion and water figure. Next: Reading a chromatogram.

Check your understanding

1. An illustrative 2,000 g/mol peptide has four positive groups and is a TFA salt. Roughly what share of the salt is peptide? 2,000 / (2,000 + 4 x 114.02) = 2,000 / 2,456.08, about 81.4%.
2. Why does the same number of counter-ions matter more for a short peptide? The counter-ion mass is fixed by the number of charged groups, so it is a larger fraction of a lighter molecule.
3. Which test would you ask for to measure water in a peptide powder? Karl Fischer titration. HPLC-UV purity does not see water.