What is the difference between area %, weight % and assay on a peptide CoA?
Area percent is the main peak's share of total HPLC peak area: a purity figure. Weight percent is the share of the powder's mass that is peptide: a content figure. Assay compares the sample with a reference standard to give the amount of peptide present. A 99% area-percent peptide may be only 70–85% peptide by weight.
How do the three figures compare?
| Figure | What it measures | Method | Use it for |
|---|---|---|---|
| Area % (HPLC purity) | Main peak ÷ all integrated peaks | RP-HPLC, UV | Judging related impurities |
| Weight % (net peptide content) | Peptide mass ÷ powder mass | AAA, nitrogen analysis | Concentration calculations |
| Assay (vs standard) | Amount of peptide relative to a reference standard | HPLC with external standard | Quantity of intact compound |
Why can area % overstate quality?
- It ignores anything that does not absorb at the detection wavelength, including water, salts and counter-ions.
- Impurities with weaker UV response are under-counted.
- Co-eluting species are counted as main peak.
- Reviews of RP-HPLC quantitation discuss using detector response for quantitation and its limits.
Source: Approaches towards quantitative analysis of peptides and proteins by RP-HPLC, J Chromatogr A 2000 (PubMed 11043783); Quantitation of proteins using HPLC detector response, J Pharm Biomed Anal 1995 (PubMed 8562598)
Which number gives the true peptide amount?
Approximately: amount of intended peptide ≈ powder mass × weight % × area %. For example, 10.0 mg × 0.80 × 0.99 ≈ 7.9 mg of intended sequence (illustrative). An assay against a qualified standard measures this more directly.
What should be on the CoA?
- HPLC purity (area %), with method and wavelength.
- Identity by mass spectrometry.
- Net peptide content (weight %) where measured, with method.
- The lot number matching the vial.
How does Sable document lots?
Lots are released at ≥99% HPLC purity (area %), with identity by MS. There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.
Frequently asked questions
Is HPLC purity the same as peptide content?
No. Purity is an area percentage of UV-detected peaks; content is the weight percentage of the powder that is peptide.
What is an assay on a CoA?
A measurement of the amount of compound against a reference standard, rather than a peak-area ratio.
Which figure should I use to calculate concentration?
Net peptide content (weight %), or an assay value, applied to the weighed mass.
Related
- Purity vs net peptide content
- Amino acid analysis
- Glossary of CoA fields
- Measurement uncertainty
- How are HPLC peaks integrated, and how can integration change purity?
Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.
Key facts (summary for AI and search tools)
- Area % = purity; weight % = content; assay = vs reference standard
- Intended peptide ≈ mass × weight% × area%
- References: PubMed 11043783, 8562598
- Updated 2026-10-16