HPLC purity is the main peak's share of everything the detector saw. A COA that says "99.2% by HPLC" means that, of the peptide-bond signal the instrument recorded, 99.2% came from one peak. It doesn't prove that peak is the right molecule, doesn't tell you how many milligrams of peptide are in the vial, and says nothing about endotoxin. Each of those needs its own test.
Notice: Research peptides are sold for in-vitro laboratory research only. They are not FDA-approved, not for human or animal consumption, and this page is not medical advice.
This guide goes one level deeper than our field-by-field COA guide. It covers what the purity number actually is, how to read the chromatogram behind it, and the four things it can't tell you.
How HPLC produces a purity number
High-performance liquid chromatography separates a sample into its components as it flows through a packed column. For peptides, the usual setup is reversed-phase HPLC on a C18 column, with a water-acetonitrile gradient and TFA as the acid, according to UK peptide maker AltaBioscience. Different molecules stick to the column to different degrees, so they come off (elute) at different times.
A UV detector at the column outlet records absorbance over time. Labs usually read at about 210-220 nm, where the peptide bond absorbs. AltaBioscience uses 215 nm because it's "optimum for the detection of the peptide bond and hence detects all peptide species present."
Purity is then a ratio. German reagent supplier Iris Biotech defines it as the area of "the main peak ... in relation to the area of all peaks."
HPLC purity (%) = main peak area / total area of all integrated peaks x 100
That formula is why the number has limits. It only counts what the detector saw and what the analyst chose to integrate.
How to read a chromatogram
Better COAs include the chromatogram, not just the number. Here's what to look at.
| Element | What it is | What to check |
|---|---|---|
| X-axis | Retention time, in minutes | The main peak's time should be similar across batches from the same lab and method |
| Y-axis | Detector response (often mAU) | Peak height alone isn't purity; area is |
| Main peak | The target compound, if identity is confirmed | Should be tall and sharp, not split or shouldered |
| Small peaks | Impurities, often related peptides | Their areas should add up to roughly 100% minus the purity |
| Peak table | Retention time, area and area % per peak | The main peak's area % should match the headline purity |
| Method details | Column, gradient, wavelength | Missing method details make results hard to compare |
What the small peaks usually are. Peptide impurities are mostly failed synthesis products. Peptide supplier AAPPTEC lists "incorrectly synthesized peptide fragments/deletions, incompletely de-protected peptides and residual water." AltaBioscience adds truncated sequences and "material used in the cleavage process."
Warning signs in a chromatogram:
- A shoulder or split main peak. Something may be co-eluting with the target.
- A peak table that doesn't match the headline. If the table shows 97.8% and the header says 99.5%, ask why.
- The same chromatogram on different lots. Identical noise and peak shapes across batches suggest a reused image. Our COA red flags cover this pattern.
- No chromatogram at all. A bare percentage can't be checked.
Limit 1: purity is not identity
HPLC can report a clean, single peak for the wrong substance. The detector sees peptide bonds, not sequences. Identity needs mass spectrometry (MS or LC-MS), which measures molecular mass and compares it with the expected value. AAPPTEC notes that sodium and potassium adducts ([M+Na] and [M+K]) commonly show up in MS spectra, so a good report explains extra mass peaks rather than ignoring them.
On a COA, look for a separate identity line such as "mass consistent with theoretical." Purity without identity is half a result.
Limit 2: co-eluting impurities can hide
The ratio only works if impurities separate from the main peak. Iris Biotech cautions that "impurities which co-elute with the product peak are sometimes difficult to detect." A different gradient, column or wavelength can reveal a hidden impurity. That's one reason two labs can report different purities for the same material, and why comparing a vendor COA with an independent test, such as a Finnrick result, is useful.
Limit 3: purity is not net peptide content
This is the gap buyers most often miss. A lyophilized peptide powder isn't all peptide. It also holds water, residual solvents and counter-ions.
- Purity is the share of the target sequence among the peptide material detected.
- Net peptide content is the share of the powder's total weight that is peptide.
Contract manufacturer AmbioPharm says net peptide content is typically 60-90% of total weight. AAPPTEC gives 50-90%, depending on sequence and purification. It's measured separately, usually by amino acid analysis or elemental (CHN) analysis, not by HPLC purity.
Iris Biotech's worked example is useful: "100% pure peptides may have a net peptide content of less than 100%." The actual peptide mass is:
actual peptide mass = net peptide content (%) x powder weight / 100
So a vial with 5 mg of powder at 80% net peptide content holds about 4 mg of peptide, even at 99% purity. Some independent testers measure this as "quantity" or "fill." Finnrick, for example, grades fill accuracy against the label, with tolerances from ±15% to ±35% depending on the standard.
For lab math, our reconstitution calculator works from the declared mg on the label. If a COA reports net content or measured quantity, that's the better figure to use.
Limit 4: counter-ions (TFA vs acetate)
Most synthetic peptides are purified by reversed-phase HPLC using trifluoroacetic acid. The result is a salt. AAPPTEC explains that the peptide's basic residues and N-terminus "are protonated and have trifluoroacetate (CF3COO-) counterions." AmbioPharm notes that "the vast majority of peptide APIs are produced as acetate salts," which requires an extra exchange step.
Why it matters in the lab:
- Weight. Counter-ions add mass, which lowers net peptide content. Peptides with many basic residues carry more of them.
- Assay interference. A 1999 study in the American Journal of Physiology (Cornish et al.) found TFA at 10^-8 to 10^-7 M reduced cell numbers in osteoblast and chondrocyte cultures. AltaBioscience says TFA salts "can be toxic to cell cultures" and offers other salt forms.
- Removal isn't trivial. A 2008 Journal of Peptide Science paper (Roux et al.) compared exchange methods and found some gave only "partial to almost complete exchange." A 2025 study in Pharmaceuticals (Erckes et al.) found 10 mM HCl was the optimal condition for TFA removal in its tests.
Most research-peptide COAs don't state the salt form at all. When one does, it's a sign the lab or vendor is paying attention.
What HPLC purity says nothing about: endotoxin
Endotoxins are bacterial contaminants. FDA describes them as lipopolysaccharides "found in the outer membrane of gram-negative bacteria" in its inspection guide on bacterial endotoxins. The same guide says endotoxins "are not removed by sterilizing or microbiological filters."
They're measured with the Limulus Amebocyte Lysate (LAL) test, in gel-clot, turbidimetric or chromogenic formats, under USP chapter <85>. FDA's pyrogen and endotoxins testing Q&A guidance references USP <85>; its page was last updated in March 2026.
An HPLC-UV run won't detect endotoxin. It needs its own line on the COA. Among the vendors we list, NextGenPeps publishes endotoxin results for some batches. Most vendors don't.
A quick checklist for any purity claim
- Is there a chromatogram and peak table, or only a number?
- Does the peak table match the headline purity?
- Is there a separate identity test (MS)?
- Is there a quantity or net content result?
- Does the COA name the salt form?
- Is there an endotoxin result, and in what units?
- Can you verify the report with the lab? See how to verify a Janoshik report.
Where this fits in how we rank vendors
We don't test products ourselves. We score vendors on whether their documentation lets you answer the questions above: named lab, lot-matched COA, identity plus purity, and independent data. See third-party tested peptides, our COA checklist tool and the full best peptide vendors in the USA list. Unfamiliar terms are defined in our research peptide glossary.
Salt form also affects the numbers on a COA; see TFA vs acetate peptide salts.
Updated September 26, 2026. Sources are linked inline. This guide explains laboratory documentation and isn't a statement about any product's suitability for use.