When evaluating peptide Certificates of Analysis, two testing methods appear most frequently: HPLC (High-Performance Liquid Chromatography) and LC-MS (Liquid Chromatography-Mass Spectrometry). Understanding what each test tells you — and what it doesn't — is essential for informed peptide procurement.
HPLC: The Purity Workhorse
HPLC is the industry standard for measuring peptide purity. It separates the components of a sample based on their chemical properties and measures the relative abundance of each component. The result is a purity percentage — typically expressed as the area percentage of the target peptide peak relative to all detected peaks.
What HPLC Tells You
- The relative purity of the sample (e.g., 98.3% means 98.3% of detected material is the target peptide)
- The presence and relative abundance of impurities (deletion sequences, truncated forms, oxidized variants)
- Batch-to-batch consistency when comparing chromatographic profiles
What HPLC Doesn't Tell You
- Whether the major peak is actually the correct peptide (it could be a different peptide of similar hydrophobicity)
- The exact molecular weight or identity of impurities
- Non-UV-absorbing contaminants (salts, counterions, water content)
LC-MS: The Identity Confirmer
LC-MS combines the separation power of liquid chromatography with the identification capability of mass spectrometry. It tells you not just how pure the sample is, but confirms that the major component has the correct molecular weight — confirming identity.
What LC-MS Tells You
- Molecular weight of the target compound (confirms identity)
- Molecular weight of impurity peaks (identifies what the impurities are)
- Detection of modifications (oxidation, deamidation, incomplete deprotection)
What LC-MS Doesn't Tell You (Without Additional Methods)
- Sequence information (which amino acids are where) — requires MS/MS or peptide sequencing
- Biological activity or potency
- Endotoxin or microbial contamination levels
Which Test Do You Need?
| Scenario | HPLC | LC-MS | Both |
|---|---|---|---|
| Routine batch release QC | ✓ | ||
| New supplier qualification | ✓ | ||
| Complex peptides (>30 aa) | ✓ | ||
| Ongoing monitoring of established supplier | ✓ | ||
| Dispute resolution / failure investigation | ✓ | ||
| GLP-1 agonists (semaglutide, tirzepatide) | ✓ |
The short answer: HPLC alone is sufficient for routine quality monitoring from established, trusted suppliers. LC-MS should be required for initial supplier qualification, complex or high-value peptides, and any situation where identity confirmation is critical.
What to Look for on a COA
When reviewing HPLC results: look for the actual chromatogram image (not just a number), check that the method description is specific (C18 column, gradient conditions), and verify that the purity exceeds the supplier's stated specification.
When reviewing LC-MS results: confirm the observed molecular weight matches the theoretical molecular weight within 1 Da for peptides under 5,000 Da, check for major adduct peaks that might indicate salt contamination, and look for deamidation peaks (+1 Da) that could indicate degradation.
For a comprehensive guide to reading and verifying COAs, including annotated examples, see our COA Verification Guide.
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