Introduction to Mass Spectrometry
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
1. Look for the molecular ion peak.
2. Try to calculate the molecular formula:
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,...).
3. Calculate the total number of rings
plus double bonds:
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
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,
Look for A+1 elements: C, N
"A" elements: H, F, P, I
4. Postulate the molecular structure
consistent with abundance and m/z of fragments.
For the molecular formula: CxHyNzOn
rings + double bonds = x - (1/2)y + (1/2)z
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More information on specific fragmentation
can be found in the quiz for each functional group.