Guide · 6 min read

What is a good HPLC purity for peptides?

By the PeptideCOACheck team · Last updated November 2, 2026 · How we check

HPLC purity is the headline number on most peptide COAs, and the question people ask most is simply "is this number good?" This page gives practical benchmarks and explains what moves the number. For how HPLC works and how to read a chromatogram, see HPLC purity explained.

The benchmarks

These are the bands our COA checker uses. They reflect common practice in the research-peptide market rather than any official standard.

The checker also warns when a product's label claims a purity more than 0.5 percentage points above what the lab measured. A label saying "99%+" over a COA showing 98.1% doesn't mean the material is bad, but it does mean the marketing is ahead of the data.

What the percentage actually means

HPLC purity is normally calculated as the area of the main peak divided by the total area of all integrated peaks, at one detection wavelength. So "98.5%" means that, among everything the detector saw at that wavelength, 98.5% of the signal came from the main peak. Three consequences follow:

• It is relative, not absolute. Anything the detector can't see, such as water, salts or many non-peptide contaminants, isn't in the calculation.
• It depends on the method. A different column, gradient or wavelength can separate impurities differently and give a different number for the same sample.
• It says nothing about identity. A pure sample of the wrong peptide can score 99%. Always pair purity with a mass spectrometry identity result.

Why 98% became the benchmark

Synthesizing a peptide one amino acid at a time leaves related by-products: sequences missing a residue (deletions), incompletely deprotected chains, oxidized or otherwise modified forms. Purification removes most of them, and reaching about 98% is routinely achievable for common research peptides. Going from 98% to 99%+ takes more purification effort and yield loss, which is part of why higher-purity material can cost more.

For perspective, pharmaceutical peptide manufacturing works at a far finer level. In U.S. FDA presentations on generic peptide drugs, individual peptide-related impurities at 0.10% and above are expected to be identified, and new impurities above 0.5% face extra scrutiny. Research-grade COAs rarely break impurities down at all, so a single purity number is a coarse summary by comparison.

What makes the number go down

• Synthesis by-products that weren't removed during purification.
• Degradation over time: oxidation (often +16 Da on the mass spec), deamidation or aggregation, which is why test dates matter.
• Handling: heat, moisture and repeated temperature changes can degrade some compounds.
• Harder sequences: long peptides and those with difficult residues are harder to make and purify.

98% vs 99%: does it matter?

Going from 98% to 99% halves the impurity share, from about 2% to about 1%. Whether that difference matters depends on what the impurities are and why the material is being studied. More useful than chasing a decimal point:

1. Confirm identity by mass spec first. Purity on the wrong compound is meaningless.
2. Check the result is recent and for your batch. Our checker warns past 12 months and fails past 24.
3. Look at the chromatogram: one dominant peak with small, well-separated minor peaks is what a credible 98–99% looks like.
4. Prefer a verifiable third-party report over a slightly higher in-house number. See third-party testing explained.

Purity vs content

A COA may also report content, or net peptide content: how many milligrams of peptide are actually in the vial. Lyophilized peptide powder also contains water and counter-ions such as acetate or TFA, so content is usually lower than the gross powder weight even when HPLC purity is high. The two numbers answer different questions. Our checker compares measured content with the label: under 80% of label is an issue, under 90% is a warning, and over 120% is a warning.

Purity numbers that deserve questions

• Every product from one seller at exactly the same purity, such as 99.9%.
• A purity with no chromatogram, peak table or method details.
• A summary purity that doesn't match the main peak's area % in the peak table.
• A "certificate of purity" with no identity test. See COA vs certificate of purity.

Educational content only. Not medical advice. Purity benchmarks describe analytical results, not suitability for any use. Research peptides are sold for laboratory research purposes only.