Mass spectrometry answers the question HPLC cannot: whether the molecule in the vial is actually the sequence on the label.
Key Takeaways
- MALDI-TOF is fast and tolerant of salts, making it a common choice for routine identity checks of synthetic peptides.
- Disulfide formation removes two mass units, acetylation adds 42, amidation changes the C-terminal mass, and oxidation adds 16 per site.
- Sodium and potassium adducts are frequently mistaken for impurities, and TFA adducts can confuse electrospray data.
MALDI-TOF versus ESI
MALDI-TOF is fast and tolerant of salts, making it a common choice for routine identity checks of synthetic peptides. Electrospray ionisation couples directly to liquid chromatography and generally gives better quantitative behaviour and more informative charge-state patterns.
Interpreting charge states
Electrospray produces multiply charged ions, and the observed mass-to-charge envelope must be deconvoluted to the neutral mass. Learning to read the envelope is worthwhile because it reveals adducts and heterogeneity that a single reported mass can hide.
For related mechanism work, see peptide circular dichroism.
Modifications that shift the mass
Disulfide formation removes two mass units, acetylation adds 42, amidation changes the C-terminal mass, and oxidation adds 16 per site. Knowing the expected modification masses lets you diagnose a failed synthesis from the spectrum alone.
Tandem MS for sequence verification
Fragmentation gives b- and y-ion series that confirm the actual residue order. For anything beyond a routine check, MS/MS is the difference between confirming a mass and confirming a sequence.
Common interpretation errors
Sodium and potassium adducts are frequently mistaken for impurities, and TFA adducts can confuse electrospray data. Establish which adducts your instrument and solvent system routinely produce before interpreting unknowns.
Experimental Conditions and Practical Setup
Routine identity checks dissolve the peptide at low micromolar concentration in a volatile solvent such as water with 0.1 percent formic acid, and calibrate the instrument immediately before the run. For electrospray, sodium and potassium adducts are identified by running a known standard first so that adducts are not mistaken for impurities.
Mass shifts that diagnose common peptide features
| Feature | Mass change | Diagnostic value |
|---|---|---|
| Oxidation | +16 per site | Methionine or tryptophan modification |
| Disulfide formation | -2 per bridge | Confirms intramolecular bridge |
| N-terminal acetylation | +42 | Confirms acetylated form |
| C-terminal amidation | -1 versus free acid | Confirms amide terminus |
| Sodium adduct | +22 | Instrument artifact, not an impurity |
Practical Notes for the Bench
- Learn your instrument’s common adducts before interpreting spectra.
- Use MS/MS when the residue order itself must be confirmed.
- Know the mass shifts of expected modifications to diagnose synthesis failures.
Frequently Asked Questions
Which ionisation method should I use?
MALDI-TOF for fast routine identity checks, electrospray when you need LC coupling or better quantitative behaviour.
Why are there multiple peaks for one peptide?
Electrospray produces multiply charged ions; the envelope must be deconvoluted to obtain the neutral mass.
What mass shift indicates oxidation?
Oxidation typically adds 16 mass units per affected residue, while disulfide formation removes two.
Can mass spectrometry detect a wrong sequence with the same mass?
No. Isomeric and scrambled sequences share a mass, which is why MS/MS fragmentation is required when the residue order itself must be confirmed.
Related Reading
- peptide circular dichroism
- amino acid analysis peptides
- peptide endotoxin testing
- insulin analogue design
- marine bioactive peptides
References & Further Reading
- Macdonald JK et al. Optimization of Collagenase Proteomics for Improved Mass Spectrometry Imaging Peptide Identification. Anal Chem. 2025. PubMed 40154494
- Zhang H et al. Optimized Time-Segmented Acquisition Expands Peptide and Protein Identification in TIMS-TOF Pro Mass Spectrometry. J Proteome Res. 2025. PubMed 39842810
- Nesvizhskii AI et al. Protein identification by tandem mass spectrometry and sequence database searching. Methods Mol Biol. 2007. PubMed 17185772
- Peptide literature search on PubMed
- Full-text archive at PubMed Central
- Peptide research collection at Nature
Educational content for research staff. Nothing here should be read as advice on human or veterinary use of any compound.
Reviewed by Dr. Elena Marchetti, Peptide Chemistry & Analytical Characterization.