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 page, and read how to read an HPLC chromatogram. Compounds are listed in the catalogue.
Sources
1. Mant CT et al.. HPLC analysis and purification of peptides. *Methods Mol Biol* 2007;386:3-55. PMID 18604941. 2. Rutherfurd SM et al.. Amino acid analysis. *Curr Protoc Protein Sci* 2009;Chapter 11:11.9.1-11.9.37. PMID 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. 4. Fenn JB et al.. Electrospray ionization for mass spectrometry of large biomolecules. *Science* 1989;246:64-71. PMID 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
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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.