Peptide Quality, Purity & COA Guide
How to evaluate peptide quality, read Certificates of Analysis, understand purity standards, and identify trustworthy suppliers — so you know exactly what you're getting.
In This Guide
Why Peptide Quality Matters
Peptides are not all created equal. The same peptide name on a vial can represent wildly different products depending on how it was synthesized, purified, and tested. Quality directly affects safety, effectiveness, and the reliability of your research.
Low-quality peptides may contain truncated sequences (incomplete peptide chains), deletion peptides (missing one or more amino acids), chemical contaminants from the synthesis process, or even entirely different compounds than what's labeled. These impurities can produce inconsistent results, unexpected side effects, or no effect at all.
The peptide market is largely unregulated for research purposes. There is no FDA oversight of research-grade peptide quality, which means the burden of verification falls entirely on the buyer. Understanding how to evaluate quality — and knowing what questions to ask — is essential for anyone working with these compounds.
What Is a Certificate of Analysis (COA)?
A Certificate of Analysis (COA) is a document issued by the manufacturer or an independent testing laboratory that reports the results of quality testing performed on a specific batch of peptide. It's the primary tool for verifying that the product you received matches what was ordered — in identity, purity, and safety.
A legitimate COA should be batch-specific, meaning it corresponds to the exact lot of peptide in your vial. Each synthesis run produces a unique batch with its own purity profile and impurity characteristics. A COA from a different batch, no matter how similar the product, does not apply to your vial.
What a Good COA Should Include
- Product name and catalog number — clearly identifies the specific peptide
- Batch/lot number — unique identifier matching the label on your vial
- Date of manufacture and testing — when the batch was produced and analyzed
- HPLC purity result with chromatogram — percentage purity plus the actual graph showing the separation
- Mass spectrometry (MS) result — observed molecular weight vs expected weight, confirming identity
- Appearance and solubility — physical description of the lyophilized powder
- Net peptide content — actual peptide weight (vs total weight including salts/counter-ions)
- Endotoxin test (for injectable peptides) — LAL test confirming bacterial endotoxin levels are below safe limits
How to Read a COA Step by Step
COAs can look intimidating if you're not familiar with analytical chemistry. Here's how to interpret each section and what to look for.
Check the Batch/Lot Number
Verify the lot number on the COA matches the label on your vial. If they don't match, the COA doesn't apply to your product. This is the most basic and important check.
Look at HPLC Purity
The HPLC purity percentage tells you what fraction of the sample is the target peptide. Look for ≥98% for standard research use. The chromatogram (a graph with peaks) should show one dominant peak (your peptide) with minimal smaller peaks (impurities).
Verify Molecular Weight via Mass Spectrometry
The MS result should show an observed molecular weight within ±1 Da (Dalton) of the theoretical molecular weight for your peptide. This confirms identity — the product is actually the peptide claimed on the label, not a different compound.
Check Net Peptide Content
A vial labeled '5mg' doesn't always contain 5mg of pure peptide. The net peptide content accounts for counter-ions (like TFA or acetate salts) and moisture. A 5mg gross weight might contain 3.5-4.5mg of actual peptide. Knowing the net content is essential for accurate dosing.
Review Endotoxin Results (If Available)
For injectable peptides, the LAL (Limulus Amebocyte Lysate) test measures bacterial endotoxin levels. Results should be below 0.25 EU/mg (Endotoxin Units per milligram). Elevated endotoxin levels can cause fever, inflammation, and serious immune reactions.
Note the Testing Laboratory
Was the testing done in-house by the manufacturer, or by an independent third-party lab? Third-party testing from a recognized laboratory (Eurofins, SGS, Janssen) provides stronger assurance because it eliminates conflicts of interest.
Purity Standards: What Percentage Is Acceptable?
Purity is measured by HPLC and expressed as a percentage — the fraction of the total sample that is the intended peptide. The remaining percentage is impurities (related substances, salts, solvents, etc.). Here's how to interpret different purity levels.
| Purity Level | Grade | Suitable For | Notes |
|---|---|---|---|
| ≥99% | Pharmaceutical / Premium Research | In vivo research, sensitive assays, cell culture | Highest quality; may cost 2-3x more |
| 98-99% | Standard Research | Most research applications, general use | The sweet spot — high quality at reasonable cost |
| 95-98% | Economy Research | Screening, preliminary studies | Acceptable for non-critical applications |
| <95% | Low Grade | Limited applications | Significant impurity load; generally avoid |
Why 98% Is the Standard Minimum
At 98% purity, 2% of the sample is impurities. For a 5mg vial, that's 0.1mg of non-target compounds — generally a negligible amount for most research applications. Below 95%, the impurity load becomes meaningful (0.25mg+ in a 5mg vial), potentially affecting results or introducing safety concerns.
Net Peptide Content vs Gross Weight
An important distinction: purity percentage doesn't equal net peptide content. A vial labeled "5mg, 98% purity" doesn't contain 4.9mg of peptide. The 5mg is the gross weight of the lyophilized material, which includes counter-ions (TFA, acetate) and residual moisture. The actual peptide content might be 60-80% of the gross weight, meaning 3-4mg of active peptide. This is normal and expected — the COA's net peptide content field tells you the actual amount.
Testing Methods Explained
Understanding the major testing methods helps you interpret COAs and recognize when critical tests are missing.
HPLC (High-Performance Liquid Chromatography)
HPLC is the gold standard for measuring peptide purity. It works by passing the sample through a column that separates compounds based on their chemical properties. The target peptide elutes (emerges) as a single dominant peak on the chromatogram, while impurities appear as smaller peaks at different retention times. The area under the main peak relative to the total area of all peaks gives the purity percentage.
What to look for: A single sharp, dominant peak with minimal shoulders or secondary peaks. The retention time should be consistent with the expected value for that peptide. Multiple large peaks or a broad, irregular main peak suggest purity issues.
Mass Spectrometry (MS / ESI-MS / MALDI)
Mass spectrometry measures the molecular weight of the sample with high precision. For peptides, this confirms identity — the observed mass should match the theoretical molecular weight of the target peptide within ±1 Dalton. This is critical because HPLC alone can show high purity without confirming that the pure compound is actually the correct peptide.
What to look for: The observed molecular weight (often labeled "found" or "observed") should match the "calculated" or "expected" molecular weight. Common variants include [M+H]+ (molecular weight plus one hydrogen), which is normal for ESI-MS ionization.
LAL Endotoxin Test (Limulus Amebocyte Lysate)
The LAL test detects bacterial endotoxins — fragments of gram-negative bacterial cell walls that can cause severe inflammatory reactions when injected. The test uses a reagent derived from horseshoe crab blood that clots in the presence of endotoxins. Results are reported in EU/mg (Endotoxin Units per milligram).
What to look for: Results should be <0.25 EU/mg for injectable peptides. Values above 5 EU/kg of body weight are considered dangerous. Not all COAs include endotoxin testing — for injectable peptides, this is a critical omission to note.
Amino Acid Analysis (AAA)
AAA breaks the peptide into its individual amino acids and quantifies each one. This confirms the amino acid composition matches the expected sequence and determines the net peptide content (actual peptide weight vs total weight). While not always included on standard COAs, it's the most accurate way to determine how much active peptide is in the vial.
Testing Methods Summary
| Test | What It Measures | Essential? |
|---|---|---|
| HPLC | Purity (% target peptide vs impurities) | Yes — minimum requirement |
| Mass Spectrometry | Identity (molecular weight confirmation) | Yes — confirms it's the correct peptide |
| LAL Endotoxin | Bacterial endotoxin levels | Critical for injectable peptides |
| Amino Acid Analysis | Composition and net peptide content | Recommended for accurate dosing |
| Residual Solvent | Leftover synthesis chemicals (TFA, DMF) | Recommended for safety |
Red Flags: Signs of a Low-Quality Supplier
Not all suppliers are trustworthy. These warning signs should make you think twice before purchasing.
Legitimate suppliers proactively provide COAs — having to ask is a yellow flag; refusing is a red flag
Each synthesis batch varies — a COA without a specific lot number matching your vial is meaningless
Raw analytical data (HPLC chromatograms, mass spectra) are easy to include and impossible to fake convincingly
Achieving >99.5% purity for peptides over 10 amino acids is extremely difficult — claims of 99.9%+ are likely fabricated
Quality synthesis, purification, and testing are expensive — rock-bottom prices usually mean corners were cut
Legitimate businesses are transparent about who they are — anonymity is common with fly-by-night operations
Research chemical suppliers should not make therapeutic claims — doing so suggests they prioritize marketing over compliance
If the COA lot number and date never change regardless of when you order, they're likely not testing each batch
How to Evaluate Peptide Suppliers
Finding a reliable supplier is one of the most important decisions you'll make. Here's a systematic approach to evaluation.
Green Flags to Look For
Every order comes with a unique COA matching your specific vial's lot number
Some or all testing done by external labs (Janssen, Eurofins, SGS, etc.) for unbiased results
Willingly shares information about where and how peptides are synthesized
Repeat orders show consistently high-quality COAs with similar purity levels
Staff can answer specific questions about synthesis, purification, and analytical methods
Goes beyond standard HPLC/MS to test for bacterial endotoxins — a critical safety measure
Evaluation Checklist for New Suppliers
Before placing your first order with a new supplier, verify these items:
- Request a sample COA — ask for a real COA from a recent batch (not a template) and evaluate it using the criteria above
- Search for reviews and community feedback — check peptide forums, Reddit communities, and review sites for experiences with the supplier
- Verify business legitimacy — check for a real physical address, phone number, and business registration
- Ask about testing practices — do they test every batch? In-house or third-party? What tests are performed?
- Start with a small order — test quality with a single product before committing to larger purchases
- Compare COAs across suppliers — ordering the same peptide from two suppliers and comparing COAs reveals quality differences
Once you've found a supplier you trust, browse our peptide directory to research specific compounds with detailed profiles, mechanisms, and dosing information before purchasing.
Common Impurities and What They Mean
Not all impurities are created equal. Understanding the types of impurities found in peptide preparations helps you assess the significance of COA results.
| Impurity Type | Source | Risk Level | Detected By |
|---|---|---|---|
| Truncated peptides | Incomplete synthesis — chain terminated early | Low — generally inactive | HPLC + MS |
| Deletion peptides | Missing one or more amino acids in the sequence | Low-Moderate — may have partial activity | MS |
| Oxidized forms | Methionine or tryptophan oxidation during synthesis/storage | Moderate — reduced or altered activity | HPLC + MS |
| TFA (trifluoroacetic acid) | Counter-ion from HPLC purification | Low at normal levels — contributes to gross weight | Ion chromatography |
| DMF (dimethylformamide) | Residual solvent from synthesis | Moderate — toxic at high levels | GC (gas chromatography) |
| Bacterial endotoxins | Contamination during manufacturing or handling | High — can cause severe immune reactions | LAL test |
| Heavy metals | Contamination from reagents or equipment | High — toxic even at low levels | ICP-MS |
Synthesis-Related vs Process-Related Impurities
Synthesis-related impurities (truncated, deletion, and oxidized peptides) are inherent to the manufacturing process and are present in all peptide preparations to some degree. They're generally less concerning because they're chemically similar to the target peptide and mostly inactive.
Process-related impurities (residual solvents, endotoxins, heavy metals) are contaminants introduced during manufacturing, handling, or storage. These are more concerning because they're chemically unrelated to the peptide and can have their own toxicological effects. Proper storage practices can also prevent post-purchase contamination and degradation.
Frequently Asked Questions
Next Steps
Now that you understand how to evaluate peptide quality, explore these resources to continue your research.
Storage Guide
Learn how to properly store peptides to maximize shelf life and maintain potency.
Read GuideDosage Calculator
Calculate exact reconstitution volumes and injection doses for any peptide.
Open CalculatorPeptide Directory
Browse all peptides with full profiles, mechanisms, and cited research.
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