Summary

Purity percentages are the headline number in research peptide supply, and they are almost universally understood to mean something they do not mean.

An HPLC purity figure is an area-percent: the target peak expressed as a proportion of the total detected peak area. It is a ratio against other things the detector could see. It is not a fraction of the mass in the vial, and it does not account for counterions, residual water or solvent.

What HPLC Purity Measures

High-performance liquid chromatography separates the components of a sample as they pass through a column. Each component emerges at a characteristic time and produces a peak. A detector — typically ultraviolet absorbance at around 214nm, where the peptide bond absorbs — measures how much light each component absorbs as it passes.

The purity figure on a certificate is normally an area-percent: the area of the target peak expressed as a percentage of the total area of all detected peaks.

Read that definition carefully. It is a ratio between the target compound and other things the detector could see. It is not a fraction of the total mass in the vial.

What Area-Percent Leaves Out

Several things in a lyophilised peptide vial are effectively invisible to that measurement, either because they do not absorb meaningfully at the detection wavelength or because they pass through unretained.

Trifluoroacetic acid deserves particular mention. TFA is standard in reverse-phase peptide purification and remains bound to the peptide as a counterion. Depending on the sequence and the number of basic residues it can account for a meaningful proportion of the powder’s mass. It does not appear in an area-percent purity figure at all.

Net Peptide Content Is a Different Measurement

The figure that does describe mass is net peptide content, sometimes called peptide content. It is determined separately — commonly by amino acid analysis or elemental nitrogen determination — and reports what proportion of the powder’s mass is actually peptide.

For a typical lyophilised TFA-salt peptide, net peptide content is usually well below the HPLC purity figure. The three common measurements answer three different questions.

Most research peptide certificates report the first two and not the third. That is normal industry practice rather than concealment, but it means comparing vials purely on purity percentage compares them on the narrowest of the three measures.

Why Mass Spectrometry Matters More Than People Assume

Purity answers a question about proportion. Mass spectrometry answers a prior and more fundamental question: is the compound the one named on the label?

A sample can return an excellent area-percent figure while being the wrong peptide entirely, or a close analogue. The chromatogram shows one clean dominant peak either way. Only the observed mass, checked against the theoretical molecular weight for the stated sequence, confirms identity.

When reading a certificate, check that a mass spectrometry result is present and that the observed mass is close to theoretical. A certificate reporting purity alone has skipped the identity question.

Reading Our Own Figures

Every batch we hold is tested by Janoshik Analytical and the certificate published in full on the lab reports page. These are HPLC area-percent figures, with mass spectrometry confirming identity, and they should be read with everything above in mind.

The spread is worth noting. These run from 98.0% to 99.9%, and we publish the lower figures alongside the higher ones because a supplier who shows only favourable results has not told you anything.

A 98% area-percent result on a difficult short sequence is a good result. A uniform 99.9% across an entire catalogue, with no certificates published, is a marketing decision rather than a measurement.

Each compound in that table has its own page carrying the current batch reference and a link to the certificate for that batch: Selank, Semax, MOTS-c, KPV, GHK-Cu, Kisspeptin-10, PT-141, Ipamorelin, Melanotan I and Retatrutide.

Practical Implications

Common Questions

Is a 98% peptide worse than a 99.5% one?

Not necessarily, and not in a way that generalises across compounds. Some sequences are harder to synthesise and purify than others. A 98% result on a difficult peptide can represent better work than 99.5% on an easy one.

Why do some certificates show exactly 100%?

A genuine chromatogram rarely returns exactly 100.00%. Real analyses produce figures with decimal places that are not round. A flat 100% is usually a sign of a self-issued document rather than a laboratory report.

Does purity change over time?

A certificate records a moment. Peptides can degrade through hydrolysis, oxidation and aggregation, and the rate depends heavily on sequence and storage conditions. This is why cold chain storage and a recent test date both matter.

What is a realistic net peptide content?

It varies substantially with sequence, counterion and residual water, so a single figure would be misleading. The point is not to assume it equals the HPLC purity figure, and to ask the supplier directly where the distinction matters for the work.

Does a higher purity figure justify a higher price?

Not on its own. Price reflects synthesis difficulty, order volume, testing cost and storage. A small difference in area-percent is a weak basis for a price comparison; the presence or absence of verifiable testing is a much stronger one.

Is area-percent the same as weight-percent?

No. Weight-percent would require knowing the mass of every component. Area-percent assumes comparable detector response across peaks, which is an approximation rather than a measurement of mass.

References

1. Janoshik Analytical. Analytical testing services and report verification.
2. Pureline Biolabs. Published batch certificates.
3. Pureline Biolabs. How and Why to Reconstitute Peptides — UK Guide 2026.

Last reviewed September 2026. Questions or factual corrections: contact Pureline Biolabs.