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.