Mass Spectrometry for Peptide Verification¶
Why Mass Spectrometry Matters¶
High-Performance Liquid Chromatography (HPLC) answers "how pure?" — but it does not answer "what is it?" A sample can display a single sharp peak at 99.5% purity and still be the wrong peptide. Mass spectrometry (MS) provides definitive molecular weight confirmation, ensuring the peptide's identity matches expectations.
At PeptideSourceHub, every production batch undergoes both HPLC and MS analysis before QC documentation is finalized. This dual verification is standard practice for research-grade peptide manufacturing.
ESI-MS: The Standard for Peptide Analysis¶
Electrospray Ionization Mass Spectrometry (ESI-MS) is the most commonly used MS technique for peptide analysis. It operates by:
- Ionization: The peptide solution is sprayed through a charged capillary, producing multiply-charged ions ([M+nH]ⁿ⁺)
- Mass Analysis: Ions are separated by mass-to-charge ratio (m/z) in a quadrupole or time-of-flight (TOF) analyzer
- Deconvolution: Software converts the charge-state envelope into a single zero-charge mass spectrum showing the molecular weight
Expected Output¶
| Peptide MW Range | Typical Charge States | Expected Accuracy |
|---|---|---|
| 300–1,000 Da | +1, +2 | ±0.5 Da |
| 1,000–5,000 Da | +2, +3, +4 | ±1.0 Da |
| 5,000–25,000 Da | +4 to +15 | ±1.5 Da |
Method Comparison: ESI vs MALDI-TOF¶
| Parameter | ESI-MS | MALDI-TOF |
|---|---|---|
| Ionization | Liquid-phase, electrospray | Solid-phase, laser desorption |
| Typical Charge | Multiply charged ions | Singly charged ions |
| Mass Range | Best for <30 kDa | Best for >10 kDa |
| Sensitivity | Excellent (fmol range) | Excellent (amol range) |
| Common Use | Routine peptide MW confirmation | High-throughput screening |
| At PSH | Every batch | On request for large peptides |
PeptideSourceHub uses ESI-MS as the standard release method for all catalog products. MALDI-TOF is available upon request for higher molecular weight products (e.g., HGH, ~22,125 Da).
Interpreting a Mass Spectrum¶
Key Features to Identify¶
- Parent Ion Peak: The deconvoluted [M+H]⁺ peak — should match theoretical monoisotopic MW ± acceptable tolerance
- Adduct Peaks: [M+Na]⁺ (+22 Da), [M+K]⁺ (+38 Da) — common in sodium/potassium-containing buffers
- Dimer Peaks: [2M+H]⁺ at 2× MW — indicates aggregation at higher concentrations
- Fragment Peaks: Low m/z signals — may indicate in-source fragmentation
Common Adducts in Peptide Mass Spectra¶
| Ion | Mass Shift | Origin |
|---|---|---|
| [M+H]⁺ | +1.0078 Da | Primary ion — this is what you confirm against theoretical MW |
| [M+Na]⁺ | +22.9898 Da | Sodium from glassware, buffers, or solvents |
| [M+K]⁺ | +38.9637 Da | Potassium from buffers |
| [M+NH₄]⁺ | +18.0344 Da | Ammonium from buffers |
| [M+2Na-H]⁺ | +44.9718 Da | High sodium environment |
A clean mass spectrum shows the [M+H]⁺ peak dominating, with [M+Na]⁺ at ≤10% relative abundance. Excessive sodium adduction suggests incomplete desalting during workup.
MS/MS: Sequence Verification¶
For high-value peptides or complex sequences, MS/MS (tandem mass spectrometry) provides sequence confirmation:
- Selection: The parent ion is isolated in the first mass analyzer
- Fragmentation: Collision-Induced Dissociation (CID) breaks the peptide at amide bonds
- Analysis: The fragment ions (b-ions and y-ions) are analyzed, producing a sequence ladder
MS/MS is particularly valuable for: - Verifying sequences over 20 amino acids - Confirming modified residues (acetylation, amidation, fatty acid conjugation) - Detecting deletion sequences at single-amino-acid resolution
Questions to Ask Suppliers About Their MS Method¶
| Question | Why It Matters |
|---|---|
| What MS instrument and ionization method do you use? | ESI vs MALDI — different mass ranges and accuracy profiles |
| What is your mass accuracy specification? | Research-grade should be ±1.0 Da or better |
| Do you run MS on every batch, or only periodically? | Batch-specific MS is a key quality indicator |
| Can you provide the actual mass spectrum, not just the MW value? | Enables independent verification of peak identity |
| Do you also monitor for common impurities by MS? | Shows whether the supplier actively screens for incorrect sequences |
| Is MS/MS available for sequence verification? | Essential for complex peptides, modified sequences, or high-value orders |