How to Read a Peptide COA

You’ve found a supplier, picked your peptide, and the listing says “99%+ purity, COA included.” Sounds good. But what does that actually mean? And more importantly, how do you know they’re not just slapping a recycled document on every product page?

If you’re spending money on research-grade peptides, understanding how to read a Certificate of Analysis is non-negotiable. It’s the single most important document separating legitimate suppliers from the ones cutting corners. Here’s what to look for, what the numbers mean, and the red flags that should make you close the tab immediately.

What Is a Certificate of Analysis?

A COA is a lab report that confirms what’s in your vial. At minimum, it should tell you two things: the peptide is what it claims to be (identity), and it’s as pure as advertised (purity). That sounds simple enough, but the detail matters. A proper COA isn’t a single number on a page. It’s a multi-method analytical report with supporting data.

Think of it this way. If someone sold you a gold ring, you’d want more than their word. You’d want hallmarks, an assay certificate, maybe an independent valuation. Peptide COAs work the same way. The methods are the hallmarks. The data is the proof.

HPLC Purity: The Number Everyone Quotes

High-Performance Liquid Chromatography is the standard method for measuring peptide purity. When a supplier says “99.2% purity,” this is what they’re referencing.

HPLC works by dissolving the peptide and passing it through a column that separates the components by their chemical properties. The result is a chromatogram, which looks like a graph with peaks. Your target peptide shows up as the dominant peak. Any impurities (deletion sequences, truncated fragments, oxidised variants) appear as smaller peaks alongside it.

What the purity percentage actually measures: the area of the target peak divided by the total area of all peaks, expressed as a percentage. So 99.2% means the target peptide accounts for 99.2% of all peptide-related material detected.

Here’s what most people miss. HPLC purity only measures peptide-related impurities. It doesn’t account for water content, residual salts, or counterions trapped in the lyophilised powder. A vial labelled 5mg at 99% HPLC purity doesn’t necessarily contain 4.95mg of active peptide. The actual peptide content could be 70-85% of the total weight, with the rest being water and salts. This is called “net peptide content,” and it’s a separate measurement.

What to look for on a COA:

• HPLC purity of 98% or above (99%+ for premium research grade)
• The actual chromatogram image, not just the number
• Method details (column type, mobile phase, detection wavelength)
• A single clean dominant peak with minimal secondary peaks

Red flag: If the COA lists a purity percentage but no chromatogram, that’s a problem. The chromatogram is the evidence. Without it, the number is just a claim.

Mass Spectrometry: Confirming You’ve Got the Right Molecule

HPLC tells you how pure something is. Mass spectrometry tells you what it is. These are two fundamentally different questions, and you need both answered.

Mass spec works by measuring the molecular weight of the compound. Every peptide has a known theoretical molecular weight based on its amino acid sequence. The observed mass on the COA should match the theoretical mass within a very tight tolerance, typically plus or minus 1 Dalton.

Why does this matter? Because a peptide could be 99.5% pure by HPLC and still be the wrong molecule entirely. Contamination with a similar-weight peptide, a synthesis error producing a related but incorrect sequence, or deliberate substitution with a cheaper compound would all look fine on HPLC alone. Mass spec catches these problems.

What to look for:

• Observed molecular weight matching the theoretical value
• The actual mass spectrum graph (peaks at the expected m/z values)
• Method specified (ESI-MS or MALDI-TOF are most common for peptides)

Red flag: A COA with HPLC data but no mass spectrometry. Identity confirmation isn’t optional in serious research.

Batch Numbers: The Detail That Catches Fakers

Every legitimate COA is tied to a specific production batch. The batch number on the COA should match the batch number on your vial label. Full stop.

This is where many suppliers fall apart. Generic COAs, ones that get reused across multiple batches or don’t reference a batch number at all, are essentially worthless. They might represent testing done months ago on a completely different production run. Your vial could contain anything.

What to check:

• Batch/lot number printed on the COA
• Same batch number on the physical vial label
• Date of analysis (should be recent and relevant to the production date)

Some suppliers now include QR codes or verification links that let you cross-reference a batch number against their testing database. It’s not foolproof, but it adds a layer of accountability.

Third-Party Testing vs. In-House Testing

There’s a meaningful difference between a supplier testing their own products and sending them to an independent laboratory. In-house testing isn’t inherently dishonest, but it does create an obvious conflict of interest. The company producing the peptide is also the one confirming its quality.

Third-party COAs from ISO 17025-accredited laboratories carry significantly more weight. These labs have no financial relationship with the supplier and follow standardised, audited testing procedures. When a COA comes from an independent facility, the data is considerably harder to fabricate.

Look for:

• The name and accreditation of the testing laboratory
• ISO 17025 or GLP certification referenced on the report
• Contact details for the lab (a real lab has a real address and phone number)

At Peptides UK, every batch undergoes independent HPLC and mass spectrometry analysis. We publish batch-specific COAs because generic reports don’t prove anything about the vial you’re holding.

Net Peptide Content: The Number Most People Ignore

This catches out even experienced researchers. A 10mg vial at 99% HPLC purity does not contain 9.9mg of usable peptide. It contains 10mg of powder, of which the peptide component might be 75-85% by weight. The rest is moisture, acetate or TFA counterions from the synthesis process, and residual salts.

Net peptide content is measured separately, usually by amino acid analysis or nitrogen determination. It tells you the actual mass of active peptide in the vial. Reputable suppliers either list this on the COA or account for it in their stated vial contents.

Does it matter for most research purposes? Sometimes. If your protocol requires precise dosing in an in vitro assay, the difference between 10mg gross weight and 7.8mg net peptide content is significant. If you’re running preliminary screening, it’s less critical. But knowing the distinction makes you a better-informed buyer.

Quick Checklist: What a Proper COA Should Include

• Product name and peptide sequence
• Batch/lot number matching your vial
• HPLC purity percentage with chromatogram
• Mass spectrometry data with observed vs. theoretical mass
• Testing laboratory name and accreditation
• Date of analysis
• Storage recommendations
• Net peptide content (ideal, not always provided)
• Appearance description (white/off-white lyophilised powder)

The Bottom Line

A COA is only as valuable as the data it contains. A single purity number on a branded PDF means nothing without the supporting chromatograms, mass spec data, and batch-specific identification. When you’re evaluating a peptide supplier, the COA is where the claims meet the evidence. Read it properly, and you’ll never waste money on substandard compounds again.

Every peptide we sell at Peptides UK ships with a full, batch-specific Certificate of Analysis. If you want to see what a proper COA looks like before you buy, check the product pages in our shop, where we publish reports for current stock.