Peptide purity vs peptide content: what each percentage measures

Two percentages often get mixed up when people read a peptide certificate: purity and peptide content. They sound alike, and both can appear near the top of a document, but they answer different questions. A material can have a high purity figure and a noticeably lower peptide content at the same time, and neither figure is wrong.

This guide explains what each percentage measures, how each one is found, and how to read them together. It's a neutral reference for reading documents. It doesn't cover handling, preparation or use of any material, and we don't provide instructions of that kind.

The short version

What purity measures

On most peptide certificates, purity comes from high-performance liquid chromatography (HPLC). The sample is separated into its components, a UV detector records each one as a peak, and purity is reported as the main peak's share of the total detected peak area.

So a purity figure is a ratio between peaks. It tells you how much of the detected peptide material is the target peptide, compared with related substances such as shorter or modified sequences. Our guide to HPLC vs LC-MS covers how the test works in more detail.

What it doesn't tell you is how much of the powder's weight is peptide. Water and counter-ions such as acetate or trifluoroacetate give little or no signal at the wavelengths usually used for peptides, so they don't appear as peaks and aren't counted.

What peptide content measures

Net peptide content is the share of the powder's total weight that is peptide. It's a weight-for-weight figure, so everything in the powder counts towards the total, including the parts HPLC can't see.

A freeze-dried peptide powder is rarely 100% peptide by weight. The rest is usually made up of:

• Counter-ions. Synthetic peptides are normally supplied as salts, so acetate or trifluoroacetate ions sit alongside the peptide. Our guide to peptide salt forms explains why.
• Water. Freeze-dried powders keep some bound water, and certificates often report it as a separate row, sometimes measured by Karl Fischer titration.
• Small amounts of other material, such as residual solvents, which usually have their own rows and limits.

Peptide content is measured in a few ways:

• Amino acid analysis. The peptide is broken down into its amino acids, which are then measured. Because the sequence is known, the total amount of peptide can be worked out from the results.
• Nitrogen (elemental) analysis. The nitrogen in the sample is measured and compared with the nitrogen expected from the peptide's formula.
• By difference. Some documents estimate it by subtracting the measured water, counter-ion and solvent content from 100%.

Peptide content counts all peptide material, so it doesn't distinguish the target peptide from related peptide impurities. That's the job of the purity figure.

A worked example

These numbers are illustrative only. They don't come from any certificate or product.

Imagine a 100 mg sample of freeze-dried peptide powder made up of:

• 6 mg of water
• 12 mg of counter-ions
• 82 mg of peptide material

The net peptide content is 82 mg out of 100 mg, or 82%.

Now suppose HPLC gives a purity of 98%. That means 98% of the peptide material is the target peptide and 2% is related substances.

To estimate the share of the whole powder that is the target peptide, you combine the two: 82% × 98% ≈ 80.4%.

Both figures in this example are accurate. The purity of 98% and the peptide content of 82% simply measure different things, which is why one is much higher than the other.

Common misreadings

• "98% purity means the powder is 98% peptide." It doesn't. Purity is a ratio between HPLC peaks, not a share of the total weight.
• "A lower peptide content means a less pure material." Not necessarily. Peptide content falls as water and counter-ion content rise, even if the peptide itself is highly pure.
• "If no peptide content is shown, the purity figure covers it." It doesn't. Many certificates report purity and identity only. Where peptide content isn't stated, the purity figure shouldn't be read as one.
• "Two certificates with the same purity describe the same material." They may differ in salt form, water content or peptide content, so it's worth comparing every row, not just the headline figure.

Where to find each figure on a certificate

• Purity usually appears as "Purity (HPLC)" with a specification such as ">98%" and a result. It's often supported by a chromatogram.
• Peptide content, where it's reported, appears as "Peptide content" or "Net peptide content", sometimes with the method named, such as amino acid analysis or nitrogen analysis.
• Water and counter-ion content may appear as separate rows. Together with peptide content, they help explain why the figures add up the way they do.

For a full walkthrough of a certificate, see how to read a peptide certificate of analysis. For the difference between a manufacturer's certificate and testing by a separate laboratory, see manufacturer COA vs independent testing.

A quick checklist

1. Is the percentage a purity figure (HPLC area %) or a peptide content figure (weight for weight)?
2. Which method is named for each result?
3. Are water and counter-ion content reported, and are they consistent with the peptide content?
4. If peptide content isn't reported, are you reading the purity figure only as purity?
5. When comparing two documents, are you comparing the same kind of figure?

We publish Manufacturer COAs (SENOBIO), listed by product, where available, on our COAs page. Our vial labels don't currently show a lot number, so a certificate may not be for the lot you receive. See also our FAQ, or browse Research Peptides UK, including BPC-157 and GHK-Cu.

For laboratory research use only. Not for human or veterinary use. 18+