What a Purity Figure Actually Means: HPLC Testing Explained

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

Almost every peptide supplier claims . Very few explain what the number measures, how it's produced, or why two vials both labelled "99%" can be materially different. Here's what's behind the figure.

Purity is a measurement, not a promise

A purity percentage is the output of an analytical test. Without the method stated, the number means nothing — it's a claim about a claim.

The standard method is HPLC: High-Performance Liquid Chromatography.

How HPLC works

A small sample is dissolved and pushed under high pressure through a column packed with a stationary material. Different molecules travel through at different speeds depending on how strongly they interact with that packing. A detector at the far end records what emerges and when.

The output is a chromatogram — a trace with peaks. Each peak is a distinct compound. The horizontal position tells you how long it took to travel (retention time), and the area under the peak is proportional to how much of it there is.

Purity is then simple arithmetic: the area of your target peak divided by the total area of all peaks, expressed as a percentage. A result of 99.2% means the target compound accounts for 99.2% of everything the detector saw.

What the other 0.8% is

Peptides are built one amino acid at a time by solid-phase synthesis. The chemistry is efficient but not perfect, so a batch inevitably contains small quantities of:

• Truncated sequences — chains where a coupling step failed and the peptide is missing a residue
• Deletion sequences — an amino acid skipped mid-chain
• Residual reagents and protecting groups from the synthesis
• Scavengers and salts left from cleavage and purification

Purification removes the bulk of this. How thoroughly it's removed is exactly what the purity figure quantifies.

This matters for research because impurities aren't inert. A truncated analogue of your target compound may still bind the same receptor, with different affinity. That's a variable you didn't intend and can't account for.

Purity is not identity

This is the distinction most people miss.

HPLC tells you how pure the sample is. It does not tell you what the sample is.

A vial could be 99.8% pure and contain entirely the wrong compound. Chromatography separates by physical behaviour, not by identity — a peak is just a peak.

Identity requires mass spectrometry. MS measures the mass-to-charge ratio of the molecule and compares it against the theoretical mass calculated from the sequence. If your peptide should have a molecular weight of 1419.55 and the instrument reports 1419.5, that's confirmation you have the right molecule.

A serious certificate of analysis carries both. Purity without identity is half a test.

Reading the result properly

When you receive a COA, check four things:

1. Is the method named? "99% purity" with no method stated is unverifiable.
2. Is there a chromatogram? The trace itself is the evidence. A number typed into a template is not.
3. Does the batch number match your vial? A generic certificate that isn't tied to your specific lot tells you about some other batch.
4. Is identity confirmed separately? Look for mass spectrometry alongside the HPLC result.

Why the difference between 98% and 99% matters more than it looks

It sounds marginal. In practice it's a doubling.

At 98% purity, 2% of the material is something else. At 99%, it's 1%. You have halved the quantity of unknown compounds in your experiment. If you're running comparative work across batches, that's the difference between a controlled variable and an uncontrolled one.

For reproducibility — the entire point of specifying reagents carefully — the tighter figure is meaningfully better, not marginally.

Where we stand

We publish the certificate for each batch in full, in the COA library, with the issuing laboratory's own verification key — so every figure on this site can be checked against the lab's records without going through us.

Results are published as measured. Purity figures across current batches run from 98.8% to 99.9%, and where a batch was submitted for quantification rather than purity determination the library says so rather than leaving you to assume. We don't round up to a marketing number, and batches that don't meet specification aren't offered for sale.

Related reading: how to read a COA · how to verify a COA · what are research peptides

Pioneer Research Peptides — UK supplier of research peptides for laboratory use. Independently tested by batch, certificates published with verification keys.

For laboratory research use only — not for human consumption. 18+.