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Chemistry LibreTexts

Interpreting a Mass Spectrum

Here are a list of steps to follow when interpreting a mass spectrum. This simplified list will help you to interpret many spectra, however there are other mechanisms of fragmentation which cannot be covered in this brief tutorial.

Steps to interpret a mass spectrum

1.  Look for the molecular ion peak.

  • This peak (if it appears) will be the highest mass peak in the spectrum, except for isotope peaks.
  • Nominal MW (meaning=rounded off) will be an even number for compounds containing only C, H, O, S, Si.
  • Nominal MW will be an odd number if the compound also contains an odd number of N (1,3,...).

2.  Try to calculate the molecular formula:

  • The isotope peaks can be very useful, and are best explained with an example.
    • Carbon 12 has an isotope, carbon 13.  Their abundances are 12C=100%, 13C=1.1%. This means that for every 100 (12)C atoms there are 1.1 (13)C atoms.
    • If a compound contains 6 carbons, then each atom has a 1.1% abundance of (13)C.
    • Therefore, if the molecular ion peak is 100%, then the isotope peak (1 mass unit higher) would be 6x1.1%=6.6%.
    • If the molecular ion peak is not 100% then you can calculate the relative abundance of the isotope peak to the ion peak.  For example, if the molecular ion peak were 34% and the isotope peak 2.3%: (2.3/34)x100 = 6.8%. 6.8% is the relative abundance of the isotope peak to the ion peak.  Next, divide the relative abundance by the isotope abundance:  6.8/1.1=6 carbons.
  • Follow this order when looking for information provided by isotopes:  (A simplified table of isotopes is provided in the introduction, more detailed tables can be found in chemistry texts.)
    • Look for A+2 elements:  O, Si, S, Cl, Br
    • Look for A+1 elements:  C, N
    • "A" elements:  H, F, P, I

3.  Calculate the total number of rings plus double bonds:

  • For the molecular formula:  CxHyNzOn
    • rings + double bonds = x - (1/2)y + (1/2)z + 1

4.  Postulate the molecular structure consistent with abundance and m/z of fragments.

  • More information on specific fragmentation can be found in the quiz for each functional group.

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