Short answer: identity testing answers “is this the right compound?” and purity testing answers “how much of the detected material is that compound?”. They are different questions, and a reliable COA reports both.
Identity testing
The most common identity method for peptides is mass spectrometry, often LC-MS. The instrument measures the molecular mass of the sample and compares it with the theoretical mass of the expected peptide, usually within a stated tolerance (for example ±1 Da). For blends, each component’s mass should be confirmed.
Some labs use UV spectral matching (comparing absorbance maxima with a reference standard). Mass spectrometry is generally more specific, because many peptides absorb UV light at similar wavelengths.
Purity testing
Purity is usually measured by HPLC with UV detection and reported as an area percentage of the main peak. See how purity testing works.
Why you need both
| Result | What it shows | What it misses |
|---|---|---|
| Identity only | The compound is present | How much impurity is present |
| Purity only | One main component dominates | Whether that component is the right compound |
| Identity + purity | The right compound, at a known purity | Quantity per vial and sterility (separate tests) |
Content testing
Net peptide content is a third measurement: how many milligrams are in the vial. It is useful for checking that the label amount is accurate.
Related: what a peptide COA should contain.
Related guides
- What Should a Peptide COA Contain?
- How to Read a Peptide COA
- Why Batch-Specific Testing Matters
- How to Verify Third-Party Lab Testing
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