Learning Hub · Testing & quality

HPLC purity vs net peptide content

Reviewed by Corex Vitality™ Research Team Last checked: 2 November 2026

Sources (4 peer-reviewed papers)

Wang X, Zhang F et al. Purity determination of synthetic glucagon using a mass balance approach. Sci Rep (2020). PMID 32157163 · DOI
Hoofnagle AN, Whiteaker JR et al. Recommendations for the Generation, Quantification, Storage, and Handling of Peptides Used for Mass Spectrometry-Based Assays. Clin Chem (2016). PMID 26719571 · DOI
D'Hondt M, Bracke N et al. Related impurities in peptide medicines. J Pharm Biomed Anal (2014). PMID 25044089 · DOI
De Spiegeleer B, Vergote V et al. Impurity profiling quality control testing of synthetic peptides using liquid chromatography-photodiode array-fluorescence and liquid chromatography-electrospray ionization-mass spectrometry: the obestatin case. Anal Biochem (2008). PMID 18342612 · DOI

Cited for chemistry and laboratory-method context only. Corex Vitality™ is not linked to the authors or journals.

For laboratory research use only. Not for human consumption.

Two numbers that often get mixed up. One tells you how clean the peptide is. The other tells you how much of the powder is peptide at all.

The short version

Two numbers, two questions

HPLC purity answers: of the peptide material in this sample, how much is the target peptide? Net peptide content (NPC) answers: of everything in the powder, how much is peptide? A batch can score 99% on the first and still be only around 80% on the second. That is normal and not a sign of a poor batch.

1. Purity

What HPLC purity measures

Peptides are usually tested by reversed-phase high-performance liquid chromatography (HPLC). The sample is pushed through a packed column, and its components come off at different times. A UV detector, normally set at 210–220 nm where the peptide bond absorbs light, draws each component as a peak.

Purity is the area of the main peak divided by the total area of all the peaks. If the main peak has an area of 9,850 and two small impurity peaks have areas of 95 and 55, purity is 9,850 ÷ 10,000 = 98.5%.

2. The blind spots

What HPLC cannot see

The detector only “sees” what absorbs UV light at that wavelength. So some things never show up as peaks:

WaterFreeze-dried peptides hold some moisture, and many pick up more from the air. Water is measured separately, for example by Karl Fischer titration.
Counter-ionsMost peptides are supplied as salts. Trifluoroacetate (TFA) or acetate binds to the N-terminus and to basic residues such as arginine, lysine and histidine. It adds weight, but it is not counted in the purity figure.
IdentityA clean single peak shows the sample is pure, but not that it is the right molecule. Identity is confirmed separately, usually by mass spectrometry.

This is why purity is reported as a percentage of the peaks, never as a percentage of the vial’s weight.

3. Content

What net peptide content measures

Net peptide content is the share of the powder’s weight that is peptide, as opposed to counter-ions and water. It is measured by amino acid analysis, where the peptide is broken into its amino acids and each is counted, or by nitrogen (elemental) analysis.

For research peptides it typically falls somewhere between 70% and 90%. Peptides with many basic residues carry more counter-ions, and water-loving peptides absorb more moisture, so both tend to sit lower. NPC also counts peptide-related impurities as “peptide”, which is one more reason it is not the same as purity.

Left: an HPLC chromatogram with one large main peak and two small impurity peaks, giving a purity of 98.5 percent. Right: a segmented bar showing that a weighed peptide powder also contains counter-ions and water, which HPLC does not see; net peptide content is peptide material divided by total weight. — HPLC purity (left) compares peaks with each other. Net peptide content (right) compares peptide with everything in the vial. Illustrative values only; bar not to scale.

4. Together

Putting the two together

To estimate how much target peptide a weighed amount of powder holds, laboratories multiply the gross weight by the net peptide content and then by the purity.

Illustrative example: 10 mg of powder × 80% net peptide content × 98.5% purity ≈ 7.9 mg of the target peptide. The remaining weight is mostly counter-ions and water, plus about 0.1 mg of peptide-related impurities.

Both figures can change from batch to batch, so they always belong to a specific batch and its certificate.

5. On a certificate

Reading both on a COA

Find the purity and its methodLook for the HPLC purity figure, plus the column and detection wavelength used to reach it.
Check the chromatogramOne dominant peak with small, well-separated impurity peaks is what a high purity figure should look like.
Look for identityA mass spectrometry result should match the molecule’s expected mass.
See whether content is reportedMany certificates report purity only. If there is no net peptide content line, the purity figure does not tell you how much of the powder is peptide.

You can check the certificate for any batch on our Verification page. For the basics of a COA, see our Peptide Science page, and for more articles, the Learning Hub.

Members Area

Go deeper with the member guide

Our free member guide, Reading a full COA and HPLC report, walks through the method block, the peak table and checking a mass spectrum, step by step.

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Sources

Bachem. Quality Control of Amino Acids & Peptides: A Guide.
Bachem. Care and Handling of Peptides.
AAPPTEC. Peptide Quality FAQ.
AmbioPharm. What is Net Peptide Content?

Research use onlyAll Corex Vitality™ products are supplied strictly for laboratory research use only. Not for human or veterinary use. This article is for general laboratory education.