Peptide purity on a COA: what the number means, what it does not tell you and how to test for it

What peptide purity means on a COA

Purity on a peptide COA is a share. It is the target peptide’s share of the total UV peak area in an HPLC chromatogram, measured at one stated wavelength. The formula is simple: the main peak’s area divided by the total area of all integrated peaks, times 100.

The wavelength is usually 214 or 220 nm, where the peptide bond absorbs UV light. At these wavelengths most peptide-related impurities show up as peaks next to the main one. Typical impurities are shorter or longer sequences from synthesis, oxidised forms and other side products.

So a purity of 98% means: of everything the detector saw at that wavelength, 98% of the peak area was the main peak. Bachem notes that research peptides are typically above 95% pure by HPLC.

What purity does not tell you

Purity answers one narrow question. Most of the questions buyers care about need a different test.

Question	Does purity answer it?	Test that does
Is it the right peptide?	No. A wrong peptide can be 99% pure	Identity by LC-MS
How many mg are in the vial?	No	Content per vial
How much water is in the powder?	No. Water gives no UV peak	Water content (Karl Fischer)
How much salt or counter-ion?	No	Counter-ion test (TFA, acetate)
Is there endotoxin?	No	Endotoxin (LAL)
Can microbes grow from it?	No	Sterility (USP <71>, 14 days)
Are solvents left over?	No	Residual solvents
Are same-mass isomers present?	Only if the method separates them	A method shown to separate them

The first row matters most. Purity confirms that one main peak dominates. It does not say what that peak is. A vial of the wrong peptide can show high purity and still be the wrong product. That is why purity and identity are run together.

The gap between purity and amount is also large. Bachem also notes that the peptide content of the solid may range from 70 to 90%, because the powder also holds counter-ions and water. Our guide on net peptide content explains the difference.

Why the same vial can give different purity numbers

ICH Q6B, the guideline for biotechnological products such as proteins, calls purity “method-dependent”. Q6B excludes synthetic peptides, but the point holds for them too: one powder can give different numbers on different methods, and the method is set by the lab. These are the main levers:

• Wavelength. Impurities absorb differently at different wavelengths, so a peak can grow or shrink as the wavelength changes.
• Separation. A short run or a steep gradient can leave impurities hidden under the main peak. Some impurities, such as D-amino-acid isomers, have the same mass as the peptide and can sit very close to the main peak. Only chromatography that separates them can count them.
• Reporting threshold. Peaks below the threshold are not reported. The EMA synthetic peptide guideline, in force from 1 June 2026, applies the Ph. Eur. thresholds for peptide-related impurities: report above 0.1%, identify above 0.5%, qualify above 1.0%. ICH Q2(R2) says a method’s quantitation limit should not be above its reporting threshold.
• Integration settings. Baseline placement, minimum peak area and peak width settings decide which small peaks count. Changing them can move the result.
• Exclusions. Labs leave out peaks that also appear in a blank injection, and often the counter-ion peak near the start. A written rule should say which peaks were left out and why.
• Detector and trace. UV at 214 or 220 nm sees most peptide impurities. A trace that follows only the target’s mass sees almost nothing else, so it can report very high purity by design. An evaporative light-scattering detector (ELSD) responds differently again.
• System suitability. The harmonised chromatography chapter (USP <621>, Ph. Eur. 2.2.46) sets checks such as resolution, peak symmetry and repeatability. Those checks show whether the run could separate what it claims to separate.

This is why a purity number without its chromatogram is hard to judge. In HK Analytical’s September 2026 review of public COAs from 25 labs and one rating site’s certificate wrapper, only 4 printed a proper peak table, and none printed its integration settings. Common problems were charts cropped to the main peak, single-mass traces that cannot show impurities, and values printed as “above 99%”.

How purity numbers are printed

The way a number is printed tells you how much the lab is claiming.

How purity is printed	What it tells you
“Above 99%”	A censored value. The real number is unknown, and impurities are not listed
Three decimals, no uncertainty	False precision. No stated uncertainty backs the extra digits
A value with its stated uncertainty	A number you can judge against its precision
One purity for a whole blend	Not meaningful. A blend is a mix by design, so each component needs its own identity and mg

Very high results printed to several decimals deserve extra care. Our guide on why a 99.9% purity result can be a warning sign covers this in depth. In Peptigrity’s lab directory in September 2026, the average purity of tested samples ran from about 98.8% to 99.8% across the labs we recorded. Peptigrity notes that this figure describes the products tested, not the lab.

How to read a purity result

Use this checklist on any COA:

1. Method and wavelength. “HPLC-UV, 214 nm” or similar. No wavelength, no context.
2. Full chromatogram. The whole run, from injection to the end, with the baseline visible. A chart cut down to the main peak hides the impurities.
3. Peak table. Retention time and area for each integrated peak, so you can recompute the purity.
4. Reporting threshold. The smallest peak that counts. A threshold of 0.1% or lower matches the EMA reporting level.
5. Impurities listed. The largest impurities by retention time, with their area.
6. Uncertainty. An honest result carries an uncertainty, or says why none is given.
7. Identity alongside. An LC-MS mass match for the main peak.

Our guide to HPLC purity testing explains the method in more detail, and HPLC vs mass spectrometry shows which test answers which question. For the whole report, see how to read a COA.

How to test peptides for purity

Purity needs a lab with an HPLC system. You cannot get an HPLC purity number at home. Reagent kits and colour tests react with broad groups of chemicals. They cannot separate a peptide from its related impurities, so they cannot give a peak-area share. Our guide on peptide test kits covers what they can show.

When you choose a lab, ask four questions. Does it show the chromatogram and peak table? Does it state the wavelength? Does it run identity by mass? Does it report mg per vial against a reference standard? A lab that answers yes to all four gives you a purity number you can read for yourself. Most labs accept samples by post, so “near me” matters less than the evidence on the report. Our guide on where to test peptides lists mail-in labs by country, including the USA. For what drives the price, see peptide purity testing cost.

How HK Analytical reports purity

HK Analytical reports Purity (HPLC) as the main peak’s share of all detected peaks, in % area at the stated wavelength. Every number comes with its raw data. On our example report, the purity section shows the full chromatogram, the peak table and every integration setting.

Purity can be ordered with identity (LC-MS), content per vial, counter-ion (TFA, acetate), water content (Karl Fischer) and the safety tests in one order, so each question gets its own answer. Results within days of receipt. Sterility (USP <71>, 14 days) is the one test that takes 14 days, because the method incubates for 14 days.