“99% pure” is not 99% peptide

What the measurement shows

An HPLC purity of 99% compares peak areas: of the peptide-like material the detector sees at 214–220 nm, 99% is the intended sequence. It says nothing about material the detector does not see. A freeze-dried peptide is a salt, and it holds water. Counter-ions such as trifluoroacetate and the water taken up by the solid add weight but give almost no peak.

The share of the weighed mass that is peptide, the net peptide content, is measured separately, usually by amino acid analysis. For a typical trifluoroacetate salt it is often in the range of about 70 to 85%. A lot can be 99% pure and 78% peptide at the same time, and both numbers are correct.

Why it matters

A stock solution made up from the weighed mass as if it were all peptide is more dilute than intended, by the inverse of the peptide content: at 78%, a solution meant to be 1 mM is about 0.78 mM. Comparisons between lots, or with published concentrations, inherit the error.

How to check

• Look for a net peptide content, water content or counter-ion content on the certificate, as separate results from purity.
• Where it is not reported, measure the concentration of the solution itself: by absorbance at 280 nm for peptides containing tryptophan or tyrosine, or at 205–215 nm, or by amino acid analysis of an aliquot.
• Read Net peptide content and purity for how the two numbers are measured and combined.

Frequently asked questions

Is the purity figure wrong, then?

No. It answers a different question: how clean the peptide material is, not how much of the powder is peptide.

Does a higher purity mean a higher peptide content?

Not necessarily. Peptide content depends mostly on the salt form and the water in the solid, which purity does not measure.

Sources & Further Reading

1. Westwood S, Choteau T, Daireaux A, Josephs RD, Wielgosz RI. Mass balance method for the SI value assignment of the purity of organic compounds. Analytical Chemistry 2013;85(6):3118–3126. doi:10.1021/ac303329k
2. Sikora K, Jaśkiewicz M, Neubauer D, Migoń D, Kamysz W. The role of counter-ions in peptides—an overview. Pharmaceuticals 2020;13(12):442. doi:10.3390/ph13120442
3. Rutherfurd SM, Gilani GS. Amino acid analysis. Current Protocols in Protein Science 2009;58(1):Unit 11.9. doi:10.1002/0471140864.ps1109s58

Cosmic Peptides. (2026, October 25). “99% pure” is not 99% peptide. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/myths/purity-is-not-content

@misc{cosmicpeptides_research_myths_purity_is_not_content,
  author       = {{Cosmic Peptides}},
  title        = {“99\% pure” is not 99\% peptide},
  howpublished = {Cosmic Peptides Research Library},
  year         = {2026},
  month        = oct,
  url          = {https://cosmicpeptides.com/research/myths/purity-is-not-content},
  urldate      = {2026-10-26},
  note         = {Last reviewed 2026-10-25}
}

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