What Does 99% Purity Mean? Reading the HPLC Chromatogram on a Peptide COA

For laboratory research use only. This article explains how researchers read the purity section of a Certificate of Analysis. It describes analytical documentation only and contains no dosing, human-use, veterinary, or clinical guidance.

A "99% purity" figure on a peptide COA means that 99% of the UV-absorbing material a reverse-phase HPLC run detected eluted as one main peak. It does not mean the vial is 99% peptide by weight, it does not confirm which peptide that peak is, and it says nothing about biological activity in an assay. This guide shows how to read the chromatogram behind the number, and the three things the number cannot tell you.

What HPLC actually measures

High-performance liquid chromatography pushes a dissolved sample through a packed column. Molecules travel at different speeds according to how strongly they interact with the column, so they leave ("elute") at different times. A UV detector at the outlet, usually set near 214 nm where peptide bonds absorb, records a signal over time. The result is the chromatogram: time on the x-axis, detector response on the y-axis. The purity figure is the area of the main peak divided by the total area of all detected peaks.

Reading the trace, line by line

1. The main peak

The tallest peak is the target compound. On a good result it is sharp and symmetrical rather than a broad hump. Its retention time, for example 6.8 minutes, is a fingerprint of that compound on that method: the same peptide run the same way should elute at close to the same time each batch.

2. The baseline

Between peaks the trace should settle to a flat, quiet line near zero. A noisy or drifting baseline can hide small impurity peaks.

3. The impurity peaks

Every peak that is not the main one is an impurity or a related substance: a deletion sequence missing one residue, an oxidised form, a truncated chain. Their combined area relative to the main peak is what the purity figure summarises. A second peak that is a large fraction of the main one contradicts any high purity claim; trust the trace over the headline.

4. The integration table

A full report lists every detected peak with its retention time, area and area percentage. The main peak's area percentage should equal the stated purity. A purity claim with no table, or a table that does not add up to the claim, is unsupported.

What a purity figure does not confirm

Identity. A clean dominant peak shows the sample is chemically homogeneous, not what the homogeneous substance is. A related but incorrect sequence can elute close to the expected time and produce an equally tidy peak. Identity is established separately, most directly by mass spectrometry, which matches the observed molecular mass to the theoretical mass of the sequence; for BPC-157 that is about 1,419 Da. A report that gives purity without an identity result is answering one of the two questions.

Weight content. Purity is a ratio of peak areas, not a mass measurement. Water, counter-ions such as trifluoroacetate or acetate, and inorganic salts do not show up as peaks, so a lyophilised powder at 99% HPLC purity can still be well below 100% peptide by weight. That is why the better reports give a separate measured amount of peptide in the vial.

Activity. Purity describes chemical composition. Biological activity in an assay also depends on whether the chain folds correctly, whether it has aggregated, and whether it was degraded by heat, light or repeated freeze-thaw after testing. The international guideline for biological product specifications, ICH Q6B, treats purity and potency as separate attributes for exactly this reason.

Purity and net content are two different numbers

Purity answers "of the peptide-related material detected, how much is the target?" Net content answers "how many milligrams of target peptide are in this vial?" They move independently. A vial can be 99.9% pure and underfilled, or overfilled at a lower purity. The certificate published for Selank 10mg on this site shows both side by side: 11.37 mg of Selank measured in a vial labelled 10 mg, at 99.882% purity — both figures on the Selank certificate page. Read both before drawing a conclusion about a batch.

Why 99% versus 98% versus 95% matters

Small percentage gaps are large ratios. At 99% purity, 1% of the detected material is non-target; at 95%, 5% is. That is five times as much impurity by peak area, and in experiments that compare batches or replicates, the extra material is an uncontrolled variable. For quantitative or comparative work, the difference between 99% and 95% is rarely cosmetic.

How a purity result is produced, and what makes it trustworthy

A reliable purity figure comes from a method that has been shown to separate the target from its likely impurities, typically a reverse-phase C18 column with a water-acetonitrile gradient containing an ion-pairing acid. Validating that a method measures what it claims to measure is the subject of ICH Q2(R2), validation of analytical procedures, one of the ICH quality guidelines. On a certificate, the practical signs are a named laboratory, a stated method, figures reported to decimals, and a way to confirm the report with the lab.

Green flags and red flags

Green: one sharp dominant peak, a flat baseline, an integration table that matches the claim, a named method and run date, and a batch identifier.
Red: a purity number with no trace or table; a blurry or axis-less image; a large secondary peak beside a high claim; a round ">99%" with no decimals; no batch identifier.

Putting it into practice with the certificates on this site

The certificates published in the Optimized Aminos COA archive are Janoshik Analytical test reports. They report HPLC purity and the measured amount of peptide in the tested vial, and each carries a verification key you can check on the laboratory's site. They print the results table rather than the chromatogram image, and they do not include a mass-spectrometry identity panel, so read them as purity and content results. Each compound's certificate also has its own page, such as Tesamorelin at 99.834% or Semax at 99.019%. Our guide to reading a peptide COA and spotting a fake one walks through verifying one with its key.

Frequently Asked Questions

Why do real purity figures have so many decimals?

Integration software calculates peak areas to fine precision and reports what it measures, which produces values such as 99.882% or 97.649%. A value that is always exactly 99.0% or 99.5% across batches is more likely a specification written onto a document than a measurement.

Is a counter-ion peak an impurity?

No. Trifluoroacetate and acetate are counter-ions left from purification. They elute near the start of the run, are often labelled, and are normally excluded from the purity calculation because they are a known component rather than a synthesis impurity.

Can a purity figure change after testing?

Yes. A purity result describes the sample on the day it was analysed. Heat, light, moisture and repeated freeze-thaw cycles can degrade a peptide afterwards, which is why storage conditions matter as much as the original figure.