What to Look for in a Certificate of Analysis Before Buying Peptides
August 7, 2026
A Certificate of Analysis (CoA) is the difference between a labeled vial and a characterized compound. It's the document that tells you what a peptide actually is, how pure it is, and how it was tested. For research use, that matters. Impurities, water content, and misidentified sequences all corrupt experimental data before you run a single assay.
Most buyers skim the purity number and move on. That's a mistake. A CoA carries several data points, and each one answers a specific question about the material. This guide walks through what a complete CoA should contain, how to read the analytical methods behind it, and the red flags that should make you pause.
Everything below refers to peptides sold for research use only — not for human consumption.
What a Certificate of Analysis Actually Documents
A CoA is a batch-specific analytical report. It records the identity, purity, and physical properties of one production lot. A generic CoA that isn't tied to a lot number is close to useless — it tells you about the product in theory, not the vial in your hand.
A complete CoA should include:
Product name and sequence — the full amino acid sequence, not just a trade name
Batch or lot number — matching the label on the vial
Molecular formula and molecular weight — theoretical vs. observed
Purity result with the method used to determine it
Analytical method data — HPLC and mass spectrometry at minimum
Appearance — typically a white to off-white lyophilized powder
Test and release dates
Water content if measured by Karl Fischer titration
If the sequence isn't printed, you can't confirm the vendor is even selling what you ordered. That's the first thing to check.
Purity is usually reported as a percentage from HPLC peak area — commonly 95%, 98%, or 99%. The number reflects the proportion of the main peak relative to detected impurities. Higher is better, but the method behind it matters as much as the figure.
A few points worth understanding:
95% is a common research-grade floor. Below that, the impurity fraction gets large enough to interfere with sensitive assays.
98–99% is preferable for work where deletion sequences or truncated peptides could confound results.
Purity is method-dependent. A purity claim with no HPLC chromatogram attached is a claim, not a measurement.
Deletion sequences — where an amino acid is missing during synthesis — are the most common impurity in solid-phase peptide synthesis. They can co-elute or show up as close side peaks on the chromatogram. A CoA that shows the actual trace lets you judge how clean the separation is.
The QC Methods You Should See
The strength of a CoA is in the analytical methods, not the summary numbers. Two methods are non-negotiable for peptides.
HPLC (High-Performance Liquid Chromatography)
HPLC separates the peptide from its impurities and quantifies purity by peak area. A reversed-phase HPLC trace should show a dominant main peak with minimal shoulders or satellite peaks. Look for the gradient conditions and column details when they're provided — they tell you the separation was set up properly.
Mass Spectrometry
Mass spec (often ESI-MS or MALDI-TOF) confirms identity by measuring the peptide's molecular weight. The observed mass should match the theoretical mass for the sequence within a small tolerance. This is how you confirm the vial contains the compound you ordered and not a similar sequence.
Karl Fischer Titration and Water Content
Lyophilized peptides absorb water. Karl Fischer titration measures residual moisture, which affects the net peptide content per vial. High water content means less actual peptide than the label weight suggests — relevant when you're standardizing concentrations across experiments.
Endotoxin and Sterility Testing
For peptides used in cell culture or animal research, endotoxin testing (LAL assay) matters because bacterial endotoxins skew results in biological systems. Not every research application needs it, but if yours does, confirm it's on the CoA or ask for it.
How to Read a Peptide Certificate of Analysis, Step by Step
Here's a practical order to work through a CoA when it lands in your inbox.
Match the lot number. Confirm the CoA lot matches the vial you received. A mismatch means you're reading someone else's data.
Check the sequence. Verify it against the compound you ordered, character by character for short peptides.
Compare observed vs. theoretical mass. They should agree within a tight margin on the mass spec report.
Read the HPLC trace, not just the number. One clean dominant peak is what you want. Multiple significant peaks undercut a high purity claim.
Note the water content. Factor it into your net peptide calculations.
Check dates. A recent test and release date is a good sign; a CoA years older than the batch is not.
Once you've confirmed the material, researchers often use a Peptide Calculator to standardize concentrations across experiments so results stay comparable between lots.
Common Red Flags
Some patterns should slow you down:
No lot number or a CoA that appears reused across products
A purity figure with no supporting chromatogram
Missing mass spec data — you can't confirm identity without it
No sequence printed on the document
Blurry or clearly edited images of analytical traces
Refusal to provide batch-specific documentation when asked
A reputable Canadian supplier should provide batch-specific CoAs without friction. Peptide Depot publishes analytical documentation for its research compounds, and you can review the full product catalog to see what's available. When a compound like BPC-157 or semaglutide is described only by class — for example, a GLP-1 receptor agonist studied in metabolic research — the CoA is where you confirm the specific material behind that description.
Why Batch-Specific Testing Matters
Every synthesis run is slightly different. Purity, water content, and impurity profiles vary batch to batch, even from the same manufacturer using the same protocol. A CoA tied to one lot captures those differences; a generic spec sheet averages them away.
For reproducible research, this is the whole point. If you're comparing results across time, you want to know whether the material changed between runs. Batch-specific CoAs let you trace variability in your data back to the material — or rule it out. Keep the CoA for each lot on file alongside your experimental records.
Frequently Asked Questions
What is a Certificate of Analysis for peptides?
A Certificate of Analysis (CoA) is a batch-specific document that reports a peptide's identity, purity, and physical properties. It typically includes the sequence, molecular weight, purity percentage, and the analytical methods used, such as HPLC and mass spectrometry.
What purity should I look for in a research peptide?
Research-grade peptides are commonly offered at 95%, 98%, or 99% purity, measured by HPLC peak area. Higher purity reduces impurities like deletion sequences that can interfere with sensitive assays. Always look for a chromatogram supporting the stated purity, not just the number.
Which QC methods should appear on a peptide CoA?
HPLC and mass spectrometry are the minimum. HPLC quantifies purity, and mass spectrometry confirms identity by matching observed to theoretical molecular weight. Karl Fischer titration for water content and endotoxin testing may also appear, depending on the intended research application.
Why does the batch or lot number matter on a CoA?
The lot number ties the analytical data to a specific production run. Purity and impurity profiles vary between batches, so a CoA is only meaningful if its lot number matches the vial you received. A generic CoA with no lot number doesn't describe your actual material.
Are these peptides safe for human use?
No. Peptides sold by Peptide Depot are for laboratory research use only and are not for human consumption. A CoA documents research-grade quality and identity; it is not a certification for any human or medical use.
A CoA is a research tool. Read it before you buy, keep it on file, and use it to confirm that the material in the vial matches your experimental assumptions. For more on sourcing and documentation, see the FAQ.
All products referenced are for research use only and not for human consumption.
Reading a Peptide Certificate of Analysis Before You Buy | Peptide Depot
