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.

Purity is not quantity either#

There is a third question, and it gets confused with purity more often than identity does.

Purity asks what proportion of the detected material is your compound. Quantity asks how much compound is in the vial.

They can disagree, and a strong chromatographic result does not rule out a short fill. Material missing from a vial is not impurity, it is absence, and chromatography has nothing to say about it. Counterion, residual water and salt also make up real weight in a lyophilised vial without ever appearing as a separate peak, so a purity figure cannot tell you what the powder weighs out at. That is a separate determination against a reference, and it is reported as a quantity found rather than a percentage.

Which is why a certificate should say which determination it carries. Ours report quantity found alongside purity, and where a batch went to the laboratory for quantification rather than purity determination, the library states that instead of leaving a gap you would have to interpret. The practical consequence is on your bench: if your method depends on knowing the mass you dissolved, the purity figure is not the number you need — see how much is actually in the vial.

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 the vial supplied? 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.

Regulators treat this scale as material rather than cosmetic. In FDA’s guidance on synthetic peptide products, a peptide-related impurity is to be identified — structurally characterised — at 0.10% of the drug substance or above, and a newly specified one should be no more than 0.5%. That is a regime built around tenths of a per cent, in a context with far more at stake than ours; the gap between 98% and 99% is twenty times the figure at which someone is expected to find out what the impurity actually is.

Where we stand#

Our certificates of analysis are being re-checked, and we are not publishing them or making testing claims until that is finished. Ask us about the documentation for a specific batch before you order.

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

Sources#

• ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin — FDA guidance for industry, May 2021. The impurities described above, in regulatory language: sequences differing by insertion, deletion or modification of amino acids. Each is to be identified at 0.10% of the drug substance or above, and a new specified peptide-related impurity should be no more than 0.5%
• ICH Q2(R2), Validation of Analytical Procedures — EMA/CHMP/ICH/82072/2006, adopted 14 December 2023. Purity is not identity, as a regulator puts it: specificity may be verified against a second procedure that ideally applies a different measurement principle, and lack of specificity in one procedure should be compensated by others

Our certificates#

Our certificates of analysis are being re-checked, and until that is finished we are not publishing them or making testing claims. Ask us about the documentation for a specific batch before you order, and run the checks on this page against anything we send you. Products you can ask about include:

• BPC-157 — Body Protection Compound
• BPC-157 / TB-500 Blend — Pre-mixed blend (1:1)
• CJC-1295 (without DAC) + Ipamorelin — Pre-mixed GH research blend (5mg+5mg)
• CJC-1295 (without DAC) — GHRH analogue (no DAC)
• DSIP — Delta sleep-inducing peptide
• GHK-Cu — Copper tripeptide-1
• KLOW Blend — BPC-157 + GHK-Cu + TB-500 + KPV
• KPV — Tripeptide (Lys-Pro-Val)
• MOTS-c — Mitochondrial-derived peptide
• NAD+ — Nicotinamide adenine dinucleotide
• Selank — Heptapeptide (Thr-Lys-Pro-Arg-Pro-Gly-Pro)
• Semax — Heptapeptide, ACTH(4-7) analogue
• Sermorelin Acetate — GHRH (1-29) analogue
• SS-31 (Elamipretide) — Mitochondrial-targeted peptide
• GLOW Blend — BPC-157 + GHK-Cu + TB-500

Pioneer Research Peptides — UK supplier of research peptides for laboratory use. Ask about batch documentation before you order.

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