Mass Spectrometry - Examples
Following are examples of compounds listed by functional group, which demonstrate patterns which can be seen in mass spectra of compounds ionized by electron impact ionization. These examples do not provide information about the fragmentation mechanisms that cause these patterns. Additional information can be found in mass spectrometry reference books.
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Alcohol:
An alcohol's molecular ion is small or non-existent. Cleavage of the C-C bond next to the oxygen usually occurs. A loss of H2O may occur as in the spectra below. An alcohol's molecular ion is small or non-existent. Cleavage of the C-C bond next to the oxygen usually occurs. A loss of H2O may occur as in the spectra below.
3-Pentanol
C5H12O
MW = 88.15
Aldehyde:
Cleavage of bonds next to the carboxyl group results in the loss of hydrogen (molecular ion less 1) or the loss of CHO (molecular ion less 29).
3-Phenyl-2-propenal
C9H8O
MW = 132.16
Alkane:
Molecular ion peaks are present, possibly with low intensity. The fragmentation pattern contains clusters of peaks 14 mass units apart (which represent loss of (CH2)nCH3).
Hexane
C6H14
MW = 86.18
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Amide:
Primary amides show a base peak due to the McLafferty rearrangement.
3-Methylbutyramide
C5H11NO
MW = 101.15
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Amine:
Molecular ion peak is an odd number. Alpha-cleavage dominates aliphatic amines.
n-Butylamine
C4H11N
MW = 73.13
Another example is a secondary amine shown below. Again, the molecular ion peak is an odd number. The base peak is from the C-C cleavage adjacent to the C-N bond.
n-Methylbenzylamine
C8H11N
MW = 121.18
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Aromatic:
Molecular ion peaks are strong due to the stable structure.
Naphthalene
C10H8
MW = 128.17
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Carboxylic Acid:
In short chain acids, peaks due to the loss of OH (molecular ion less 17) and COOH (molecular ion less 45) are prominent due to cleavage of bonds next to C=O.
2-Butenoic acid
C4H6O2
MW = 86.09
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Ester:
Fragments appear due to bond cleavage next to C=O (alkoxy group loss, -OR) and hydrogen rearrangements.
Ethyl acetate
C4H8O2
MW = 88.11
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Ether:
Fragmentation tends to occur alpha to the oxygen atom (C-C bond next to the oxygen).
Ethyl methyl ether
C3H8O
MW = 60.10
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Halide:
The presence of chlorine or bromine atoms is usually recognizable from isotopic peaks.
1-Bromopropane
C3H7Br
MW = 123.00
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Ketone:
Major fragmentation peaks result from cleavage of the C-C bonds adjacent to the carbonyl.
4-Heptanone
C7H14O
MW = 114.19
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