Introduction
A Certificate of Analysis (CoA) is your primary documentation of peptide quality. Understanding how to read and interpret this document ensures you're working with appropriate quality materials.
Components of a CoA
Header Information
Peptide Identification:
- Product name or code
- Sequence (one-letter or three-letter code)
- Molecular formula
- Theoretical molecular weight
Batch Information:
- Lot/batch number
- Manufacturing date
- Expiry date (if applicable)
- Quantity
Specification Table
The core of the CoA lists tests performed:
| Test | Method | Specification | Result |
|---|---|---|---|
| Appearance | Visual | White to off-white powder | Complies |
| HPLC Purity | RP-HPLC | ≥95% | 97.3% |
| MS Identity | ESI-MS | 1234.5 ± 0.1% | 1234.4 |
| Water Content | Karl Fischer | ≤10% | 5.2% |
Supporting Data
Quality CoAs include raw data:
- HPLC chromatograms
- Mass spectra
- Any additional analytical data
Interpreting HPLC Data
Reading the Chromatogram
X-Axis (Time):
- Retention time in minutes
- Main peak position indicates compound identity
Y-Axis (Response):
- Absorbance units (AU)
- Peak height/area proportional to concentration
Understanding Purity Values
Area Percent:
``
Purity = (Main Peak Area / Total Area) × 100%
What 95% purity means:
- 95% of detected material is target peptide
- 5% is other UV-absorbing compounds
- Note: Some impurities may not be detected
Evaluating Peak Shape
Good peaks:
- Symmetrical
- Sharp
- Well-resolved from neighbors
Problem indicators:
- Tailing: Poor column condition or overload
- Fronting: Sample solubility issues
- Shoulders: Unresolved impurities
Integration Considerations
- Baseline should be flat
- Integration limits should be appropriate
- Minor peaks should be properly detected
Interpreting Mass Spectrometry Data
Reading Mass Spectra
X-Axis (m/z):
- Mass-to-charge ratio
- Position indicates molecular weight
Y-Axis (Intensity):
- Relative abundance
- Usually normalized to tallest peak
Molecular Weight Confirmation
ESI-MS (multiple charges):
For a peptide with MW = 2000:
- [M+H]+ at m/z 2001
- [M+2H]2+ at m/z 1001
- [M+3H]3+ at m/z 668
Calculating MW from multiply charged ions:
`
MW = (m/z × z) - z × 1.008
Common Observations
Expected peaks:
- Molecular ion ([M+H]+)
- Sodium adduct ([M+Na]+) at MW + 22
- Multiply charged ions for larger peptides
Concerning peaks:
- Mass 16 higher: Oxidation
- Mass 18 lower: Dehydration
- Other major peaks: Impurities
Other Common Tests
Amino Acid Analysis
Shows composition as ratios:
`
Expected: Gly(2), Ala(3), Lys(1)
Result: Gly(1.98), Ala(3.02), Lys(1.00)
Slight variations (±10%) are normal due to method precision.
Water Content (Karl Fischer)
- Expressed as % w/w
- Typical range: 3-10%
- Higher values mean less peptide per mg
Net Peptide Content
Accounts for:
- Water content
- Counterion content
- Residual solvents
`
Net Peptide = 100% - Water - Counterion - Other
Endotoxin Testing
- Expressed as EU/mg (endotoxin units per milligram)
- Typical specification: <0.25 EU/mg for injectables
- LAL method is standard
Solubility
Reported as approximate solubility in common solvents:
- Water
- DMSO
- Acetic acid
- Buffer solutions
