Peptide Purity Testing: HPLC, Mass Spectrometry, and COA Analysis
A comprehensive guide to peptide purity testing methods including HPLC analysis, mass spectrometry confirmation, residual solvent testing, and how to read a Certificate of Analysis (COA).
Introduction
Peptide purity is the single most important quality metric for research compounds. This guide explains the analytical methods used to verify purity and how to interpret a Certificate of Analysis (COA).
Analytical Methods
HPLC (High-Performance Liquid Chromatography)
HPLC separates peptide molecules by their interaction with a stationary phase. The output is a chromatogram showing peaks at different retention times.
Purity calculation: The area of the main peak divided by the total area of all peaks, expressed as a percentage.
| Purity Grade | HPLC Result | Use Case |
|---|---|---|
| Research | >= 95% | General in vitro |
| High-purity | >= 98% | Quantitative assays |
| Ultra-purity | >= 99% | Clinical/standard |
Mass Spectrometry (MS)
MS confirms the molecular weight of the peptide. The observed mass should match the theoretical mass within 1 Da.
Residual Solvent Analysis
After lyophilization, trace solvents from synthesis remain. ICH Q3C sets limits:
- Acetonitrile: < 410 ppm (Class 2)
- TFA (trifluoroacetic acid): < 250 ppm
- DMF: < 880 ppm
How to Read a COA
A proper COA should include:
- Peptide name and sequence
- Lot/batch number
- HPLC purity percentage
- MS confirmation (observed vs theoretical mass)
- Residual solvent levels
- Appearance (white/off-white powder)
- Storage conditions
- Test date and expiry
Common Purity Issues
TFA Salt Contamination
Most peptides are synthesized as TFA salts. High TFA residual (>500ppm) can interfere with cell-based assays.
Truncated Sequences
Incomplete coupling during SPPS produces deletion peptides. These appear as secondary peaks on HPLC.
Racemization
Epimerization during synthesis can produce D-amino acid containing variants. These have identical mass but different biological activity.
Frequently Asked Questions
What does >= 98% purity mean?
At least 98% of the sample (by peak area on HPLC) is the target peptide. The remaining 2% may include truncated sequences, deletion peptides, or salts.
Why is mass spectrometry important?
HPLC alone cannot distinguish between peptides of similar hydrophobicity. MS confirms the molecular weight, proving the correct peptide was synthesised.
Conclusion
Understanding peptide purity testing ensures research reproducibility. Always request a full COA with HPLC and MS data from your supplier.