HPLC Purity % Explained: What UK Labs Should Check

What is HPLC?

High-Performance Liquid Chromatography (HPLC) is a laboratory technique that separates, identifies, and quantifies components within a mixture. For peptide quality control, HPLC is the gold standard for purity determination.

How HPLC Works

HPLC forces the sample through a column packed with a stationary phase material under high pressure. Different compounds travel through the column at different rates based on their chemical properties, producing peaks on a chromatogram. The area under each peak corresponds to the percentage of that component in the sample.

What Purity Percentage Means

A peptide with 99%+ HPLC purity means:

• •99% or more of the detected material is the target peptide
• •Remaining ≤1% consists of impurities (synthesis by-products, degradation products, residual reagents)

This is the standard required for research-grade peptides. Lower purity peptides contain higher levels of impurities that may interfere with research results or introduce confounding variables.

Reading a Certificate of Analysis (COA)

A complete COA for a research peptide should include:

Why Third-Party Testing Matters

Some suppliers perform in-house testing, which creates a conflict of interest. At Peptide Central, every batch is tested by an independent, accredited third-party laboratory. This ensures:

• •No bias in reporting
• •Results are reproducible and traceable
• •Data meets research-grade standards

Our Testing Process

1. Batch synthesis by GMP-certified manufacturer

2. Sample sent to an independent third-party laboratory

3. HPLC purity analysis and chromatogram generation

4. Mass spectrometry identity confirmation

5. COA issued and published in our COA Library

Only batches passing all criteria are dispatched.

Disclaimer: For educational purposes related to in-vitro research quality standards. Not medical advice.

Purity is not net peptide content

This is the single most consequential misunderstanding about the figure, and it is rarely explained on a product page.

HPLC purity measures the proportion of detected material that is the target peptide. It says nothing about how much of the vial's total mass is peptide at all. A lyophilised powder also contains counter-ions from synthesis, typically acetate or trifluoroacetate, plus residual water. Those are not impurities in the chromatographic sense and do not appear as peaks competing with the target.

The result is that two vials can both honestly report 99% HPLC purity and still contain measurably different quantities of peptide, because one carries a larger salt and water fraction than the other. Net peptide content, determined separately, is what quantifies that.

For most research purposes the purity figure is the more relevant number. But when comparing suppliers on price per milligram, it is worth knowing that the milligram figure on the label is total mass rather than peptide mass unless the certificate says otherwise.

Why a blend cannot have a purity percentage

A purity percentage is a proportion. It requires a single target molecule for the rest of the sample to be measured against. A vial containing two or more actives has no single target, so the figure is undefined rather than merely difficult.

A blend certificate therefore quantifies each active by mass instead, which is the more useful number anyway: it states what is actually in the vial. Any supplier advertising a blend at a combined purity percentage is quoting a figure that does not describe the product, and that is worth treating as a signal about the rest of their documentation.

Reading the chromatogram itself

The chromatogram is the part most often cropped out of a published certificate, and it is the part that carries the most information.

• •The target peak should be dominant and well resolved, clearly separated from its neighbours rather than merging into them.
• •Peak shape matters. A sharp, symmetrical peak indicates a clean separation; significant tailing can indicate interaction with the column or a co-eluting species.
• •The baseline should be flat and quiet between peaks. A noisy or drifting baseline makes small impurity peaks difficult to distinguish from noise.
• •Impurity peaks are normal and expected. What matters is that they are small, resolved, and consistent with the synthesis route rather than unexplained.

A summary figure with no chromatogram asks you to accept all of the above on trust.

What HPLC does not tell you

Purity testing answers one question well and several others not at all.

• •Identity is confirmed by mass spectrometry, not by HPLC. A chromatogram shows that a sample is largely one thing; mass spec shows what that thing is. A complete certificate carries both.
• •Sterility is a separate screen entirely.
• •Endotoxin content is another, and is not implied by either purity or sterility.

This is why a certificate that reports purity alone is weaker than one reporting purity, identity confirmation and sterility together, even where the purity figure itself is higher.

What to do with the number

Treat a quoted purity figure as a claim rather than a fact until you have seen the document behind it, and check the document against the vial rather than against the product page. Our guide to reading a peptide COA works through each section of a report, and every batch we hold a certificate for is published in the COA library against its batch number.