Peptide purity is critical for reliable research results. This guide explains how to evaluate peptide quality, read Certificates of Analysis (COAs), and identify trustworthy suppliers.
TL;DR: A trustworthy peptide COA states HPLC purity above 98%, confirms identity via mass spectrometry, and matches the lot number on your vial. Request the COA before ordering, cross-check it against third-party lab data when possible, and treat suspiciously perfect (99.9%) purity claims as a red flag rather than reassurance.
New to this area? Start with our Beginner’s Guide to Peptide Research for foundational context, then return here for the purity-verification details.
Why Purity Matters
Impure peptides can cause:
- Inconsistent results — Variable potency between batches
- False conclusions — Contaminants may produce effects
- Safety concerns — Unknown substances present
- Wasted resources — Failed experiments, repeated work
The Purity Standard
| Purity Level | Quality | Typical Use |
|---|---|---|
>99% | Pharmaceutical grade | Clinical research |
>98% | High research grade | Most research |
| 95-98% | Standard research | General use |
<95% | Low grade | Avoid for research |
Target: >98% purity for reliable research peptides.
For a broader look at how quality control fits into supplier selection, see our Peptide Quality & Safety Guide.
Understanding Certificates of Analysis (COAs)
A COA is a document from the manufacturer verifying peptide quality. Here’s how to read one:
Key COA Components
1. Product Identification
| Field | What to Check |
|---|---|
| Peptide name | Matches your order |
| Sequence | Correct amino acids |
| Molecular weight | Matches expected value |
| Lot/Batch number | Unique identifier |
2. Purity Results
| Test | Method | Target |
|---|---|---|
| Purity | HPLC | >98% |
| Identity | Mass Spec (MS) | Confirmed |
| Salt content | Various | As specified |
| Water content | Karl Fischer | <10% typical |
3. Physical Description
| Property | Typical Result |
|---|---|
| Appearance | White/off-white powder |
| Form | Lyophilized |
| Solubility | Soluble in water/BAC |
Solubility and mixing ratios are covered step-by-step in our Peptide Reconstitution Guide.
Sample COA Interpretation
CERTIFICATE OF ANALYSIS
Product: Semaglutide
Lot Number: SEM-2025-1234
Molecular Formula: C187H291N45O59
Molecular Weight: 4113.64 g/mol
TEST RESULTS:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Purity (HPLC): 98.7% ✓ Good
Mass Spec (MS): 4113.5 ✓ Confirmed
Appearance: White powder ✓ Normal
Endotoxin: `<`0.5 EU/mg ✓ Safe
Water Content: 5.2% ✓ Acceptable
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━This particular example references semaglutide; if you’re new to that compound, our semaglutide primer and the 5mg product listing provide useful sourcing and dosing context.
Testing Methods Explained
HPLC (High-Performance Liquid Chromatography)
Purpose: Determines purity percentage
How it works:
- Sample dissolved in solvent
- Passed through chromatography column
- Components separate by properties
- Detector measures each component
- Main peak = target peptide
What to look for:
- Purity percentage (
>98%) - Single dominant peak
- Retention time matches standard
Mass Spectrometry (MS)
Purpose: Confirms peptide identity
How it works:
- Peptide ionized
- Ions separated by mass-to-charge ratio
- Detector creates mass spectrum
- Compare to expected molecular weight
What to look for:
- Observed mass matches expected (±1 Da)
- Clean spectrum without major impurity peaks
Other Tests
| Test | Purpose | Typical Spec |
|---|---|---|
| Amino Acid Analysis | Verify sequence | Matches expected |
| Endotoxin (LAL) | Bacterial contamination | <1 EU/mg |
| Sterility | Microbial contamination | Sterile |
| Water Content | Moisture level | <10% |
Red Flags: Signs of Low Quality
COA Red Flags
- ❌ No lot number
- ❌ Missing purity percentage
- ❌ No testing date
- ❌ Generic/templated appearance
- ❌ No company contact info
- ❌ Purity claimed exactly 99.9% (suspiciously perfect)
Supplier Red Flags
- ❌ No COA available
- ❌ Won’t provide COA before purchase
- ❌ Prices far below market rate
- ❌ No third-party testing
- ❌ Poor/no customer reviews
- ❌ No physical address
- ❌ Payment only via untraceable methods
Third-Party Testing
Why It Matters
Manufacturer COAs have inherent bias. Third-party testing provides:
- Independent verification — No conflict of interest
- Unbiased results — Testing lab has no stake
- Quality assurance — Catches manufacturer errors
How to Verify
- Request lab name — Ask supplier which lab tests
- Check accreditation — Lab should have ISO/GLP certification
- Independent testing — Consider sending samples yourself
DIY Third-Party Testing
You can send samples to independent labs:
| Service | Tests Available | Cost Range |
|---|---|---|
| Janoshik | HPLC, MS | $50-150 |
| Other peptide labs | Various | Varies |
Note: Some researchers test new suppliers before committing to larger orders.
Evaluating Suppliers
What to Look For
| Factor | Good Sign | Bad Sign |
|---|---|---|
| COA availability | Provided before purchase | Not available |
| Testing | Third-party verified | Self-tested only |
| Purity claims | 98-99% (realistic) | 99.9% (suspicious) |
| Pricing | Market rate | Far below average |
| Reviews | Multiple verified | None or fake-looking |
| Communication | Responsive, professional | Difficult to reach |
| Shipping | Cold chain available | No temp control |
Questions to Ask Suppliers
- “Can I see the COA before ordering?”
- “What lab performs your testing?”
- “Do you use third-party testing?”
- “What is your typical purity range?”
- “How do you handle quality issues?”
Purity vs. Price
The Quality-Cost Balance
| Price Point | Typical Quality | Risk Level |
|---|---|---|
| Very cheap | Low/unknown purity | High |
| Below average | Variable quality | Moderate |
| Market rate | Good quality expected | Low |
| Premium | Highest quality | Lowest |
Remember: Cheap peptides often mean:
- Lower purity
- Poor testing
- Contamination risk
- Inconsistent results
- Wasted time/money
Cost Per Quality Unit
Better to calculate cost per mg of pure peptide:
Effective cost = Price ÷ (Amount × Purity)Example:
- Supplier A: 5mg at $50, 95% purity = $50 ÷ 4.75mg = $10.53/mg pure
- Supplier B: 5mg at $60, 99% purity = $60 ÷ 4.95mg = $12.12/mg pure
The $10 savings costs you 0.2mg of actual peptide plus quality risks.
Price divided by (amount x purity). The cheaper vial is not always the better value.
Storage Impact on Purity
Even pure peptides degrade with improper storage:
| Storage Error | Purity Impact |
|---|---|
| Room temperature (reconstituted) | Rapid degradation |
| Freeze-thaw cycles | Significant loss |
| Light exposure | Photodegradation |
| Moisture (powder) | Hydrolysis |
| Old reconstituted solution | Bacterial/chemical degradation |
Protect your investment: Proper storage maintains the purity you paid for. For the full protocol, see our dedicated Peptide Storage Guide.
Quality Assurance Checklist
Before Ordering:
- Supplier provides COA
- Purity
>98% documented - Third-party testing mentioned
- Reviews are positive
- Pricing is reasonable
Upon Receipt:
- COA included with shipment
- Lot number matches COA
- Peptide appearance normal
- Proper shipping temperature
During Use:
- Solution clear after reconstitution
- No particles or cloudiness
- Expected research results
- Stored properly
Summary
| Priority | Action |
|---|---|
| Essential | Request and read COA |
| Important | Verify >98% purity |
| Recommended | Choose third-party tested |
| Optional | Consider independent testing |
Quality peptides cost more but provide:
- Reliable results
- Consistent performance
- Better value long-term
- Peace of mind
FAQ
What purity percentage should I look for on a peptide COA? Aim for HPLC purity above 98% for general research use, and above 99% if you need pharmaceutical-grade consistency. Purity between 95-98% is acceptable for less sensitive applications, but anything below 95% introduces meaningful batch-to-batch variability and should generally be avoided.
Can a Certificate of Analysis be faked? Yes. Some suppliers issue generic or altered COAs that don’t correspond to the actual batch shipped. Cross-check the lot number against your product, look for consistent formatting from a named lab, and consider independent third-party testing if a supplier’s documentation seems templated or vague.
What’s the difference between HPLC and mass spectrometry testing? HPLC (High-Performance Liquid Chromatography) measures purity as a percentage by separating and quantifying components. Mass spectrometry (MS) confirms identity by measuring molecular weight. A complete COA should report both: HPLC tells you how pure the sample is, MS tells you it’s the right peptide.
Is third-party testing worth the extra cost? For any supplier you plan to order from repeatedly, independent lab verification (from a service like Janoshik) is a reasonable investment. It removes the inherent bias of manufacturer self-testing and can catch discrepancies before you commit to larger orders.
Does improper storage invalidate a COA’s purity claim? Yes — a COA reflects purity at the time of testing, not after months of poor storage. Freeze-thaw cycles, light exposure, moisture, and room-temperature storage of reconstituted solution all degrade purity over time, regardless of what the original certificate stated.
Last updated: December 17, 2025
Disclaimer: This information is for educational and research purposes only. Peptides are research chemicals not intended for human consumption.