Isotopic Distributions in Peptide Mass Spectrometry
Elements occur as mixtures of isotopes. A peptide therefore generates a cluster of isotope peaks rather than one infinitely narrow mass.
Monoisotopic mass
Monoisotopic mass is calculated using the lightest common isotope of each element.
Average mass
Average mass uses natural isotopic abundance and is often more relevant for larger unresolved molecules.
Isotope envelope
The envelope shape depends on elemental composition and charge state. Higher charge compresses isotope spacing on the m/z axis.
Practical interpretation
Confusing monoisotopic and average mass can create an apparent mismatch even when the correct peptide is present.
Frequently asked questions
Why are isotope peaks separated by less than one m/z unit?
The ion may carry multiple charges.
Can isotope pattern support composition?
Yes, especially with high-resolution data.
Does one envelope represent one compound?
Usually, but overlapping species can complicate interpretation.
Why do larger peptides have broader envelopes?
They contain more atoms and possible isotope combinations.
Key takeaways
Isotopic distributions are fundamental to peptide MS interpretation. Reports should clearly state the mass convention used.
References
- Aebersold R, Mann M. Mass-spectrometric exploration of proteome structure and function. Nature. 2016.
- Gross JH. Mass Spectrometry: A Textbook. Springer.
TSMS Labs educational disclaimer: For laboratory research and educational purposes only. Not for human consumption. This content is not medical, clinical, or regulatory advice.