---
title: "What is the difference between area %, weight % and assay on a peptide CoA? | Sable Peptides"
description: "A glossary deep dive into three numbers that are often confused: HPLC area percent (purity), weight percent (content), and assay against a reference standard. What each measures, how they relate, and which to use for concentration. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/area-percent-vs-weight-percent-vs-assay
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# 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)](https://pubmed.ncbi.nlm.nih.gov/11043783/); [Quantitation of proteins using HPLC detector response, J Pharm Biomed Anal 1995 (PubMed 8562598)](https://pubmed.ncbi.nlm.nih.gov/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](/guides/peptide-purity-vs-net-peptide-content)
-   [Amino acid analysis](/guides/amino-acid-analysis-for-peptide-content)
-   [Glossary of CoA fields](/learn/coa-fields)
-   [Measurement uncertainty](/guides/measurement-uncertainty-in-peptide-purity-results)
-   [How are HPLC peaks integrated, and how can integration change purity?](/guides/chromatographic-peak-integration-rules)

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