---
title: "Purity vs Peptide Content: Why a 99% Pure Vial Is Not 99% Peptide by Weight"
description: "HPLC purity and net peptide content measure different things. What each means, why counter-ions and water matter, and what to look for on a certificate of analysis. Research use only."
canonical: https://www.sablepeptides.co.uk/learn/purity-vs-peptide-content
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# Purity vs Peptide Content: Why a 99% Pure Vial Is Not 99% Peptide by Weight

Purity vs Peptide Content: Why a 99% Pure Vial Is Not 99% Peptide by Weight is supplied for laboratory research only. Key facts are below; stored as lyophilised powder.

Last verified 16 Oct 2026.

A CoA for every lot; each vial carries a printed lot number that matches it. Request yours and we email it within 2 hours.

The short answer

\*\*HPLC purity\*\* answers: of the material the detector sees, how much is the target peptide? \*\*Peptide content\*\* (net peptide content) answers: of the powder's total weight, how much is peptide? They are different numbers, and a vial can be high on one and lower on the other \[1\]\[2\].

> Research use only. Sable Peptides supplies laboratory reagents for in vitro and laboratory research by qualified people. Not a medicine. Not for human or veterinary use.

## The short answer

**HPLC purity** answers: of the material the detector sees, how much is the target peptide? **Peptide content** (net peptide content) answers: of the powder's total weight, how much is peptide? They are different numbers, and a vial can be high on one and lower on the other \[1\]\[2\].

## What HPLC purity measures

Reverse-phase HPLC separates components and a UV detector records peaks. Purity is the main peak area as a percentage of all detected peak area \[1\]. It ranks the target against related impurities such as deletion or truncated sequences. It does **not** weigh anything, and it does not see components that do not absorb UV at the detection wavelength.

## What is in the powder besides peptide

\- **Counter-ions.** Peptides purified with trifluoroacetic acid usually carry trifluoroacetate as a salt; acetate is another common form \[3\]. These ions add weight but are not the peptide. - **Residual water.** Lyophilised powder holds some bound moisture. - **Bulking agents**, if any are used, also add weight.

## How peptide content is measured

Net peptide content is usually determined by quantitative **amino acid analysis** (hydrolyse the peptide and quantify the amino acids) or by elemental nitrogen analysis \[2\]. It is a separate test from HPLC purity and from mass-spectrometry identity \[4\].

## Reading a certificate with this in mind

\- A purity figure with a chromatogram tells you about related impurities. - An observed vs theoretical mass tells you the molecule has the expected mass. - A peptide-content figure, if listed, tells you how much of the weight is peptide. If it is not listed, the certificate does not make a content claim.

See how Sable lots are tested on the [testing methodology](/testing-methodology) page, and read [how to read an HPLC chromatogram](/learn/how-to-read-an-hplc-chromatogram). Compounds are listed in the [catalogue](/catalogue).

## Sources

1\. Mant CT et al.. HPLC analysis and purification of peptides. \*Methods Mol Biol\* 2007;386:3-55. PMID [18604941](https://pubmed.ncbi.nlm.nih.gov/18604941/). 2. Rutherfurd SM et al.. Amino acid analysis. \*Curr Protoc Protein Sci\* 2009;Chapter 11:11.9.1-11.9.37. PMID [19937719](https://pubmed.ncbi.nlm.nih.gov/19937719/). 3. Cornish J et al.. Trifluoroacetate, a contaminant in purified proteins, inhibits proliferation of osteoblasts and chondrocytes. \*Am J Physiol\* 1999;277:E779-83. PMID [10567002](https://pubmed.ncbi.nlm.nih.gov/10567002/). 4. Fenn JB et al.. Electrospray ionization for mass spectrometry of large biomolecules. \*Science\* 1989;246:64-71. PMID [2675315](https://pubmed.ncbi.nlm.nih.gov/2675315/).

\*Last reviewed: 2026-10-16. Original Sable Peptides content; general reference, not a preparation method. Sources checked against PubMed and live links on 2026-10-16.\*

Last reviewed: 2026-10-16

## How much does Purity vs Peptide Content: Why a 99% Pure Vial Is Not 99% Peptide by Weight cost in the UK?

## Frequently asked questions

### Does 99% HPLC purity mean 99% of the vial weight is peptide?

No. HPLC purity is a ratio of detected peak areas. Counter-ions and water add weight without appearing as impurities.

### What test gives peptide content?

Quantitative amino acid analysis or nitrogen analysis, reported separately from HPLC purity.

Reviewed by Sable Peptides editorial team · 14 October 2026 · [Editorial policy](/editorial-policy)