introduction to mass spectrometry and gc-ms 344 le… · introduction to mass spectrometry and...
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344 Organic Chemistry Laboratory
Introduction to Mass Spectrometry and GC-MS
Main topics
- Overview of gas chromatography (GC)
- Generation of molecular ions
- Fragmentation patterns
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Chromatography – separation of a mixture into individual components
Stationary phase = packed column Mobile phase = He
Gas Chromatography (GC) coupled to EI-MS
Organic sample needs to be sufficiently volatile to vaporize
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GC trace – mixture of aromatic hydrocarbons (BTEX)
Retention Time (min)
Detector
output (mV)
2.7 min
2.4 min
2.3 min
1.2 min
0.7 min
GC-MS - a mass spectrum is obtained for each compound as it elutes from the GC
www.npl.co.uk
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Molecular Fragments
Molecule
[M].+ gives the molecular mass in amu (m) of M
Information on connectivity of the molecule, isotopes
Molecular Ion
Uses high energy electron beam (70 eV), sample in gas phase
Ionization energy for most organic molecules ~8-15 eV
Electron Impact-Mass Spectrometry (EI-MS)
bond breaking bond forming
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31
EI-Mass Spectrum of Methanol CH3OH
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From where on the molecule is the electron most likely to be removed?
Alkenes – p bond
Heteroatom compounds (O, N, S, etc.) – non-bonding lone pairs
Alkanes – s bond
Generation of [M].+
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Molecular Ion
Radical cation
Cation Radical
CationRadical ✔
✔
✔
✗
✗
[A-B-C]
[A] [B-C]+
[A] [B-C]+
Generation of fragments from [M].+
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Molecular Ion
Radical cation
Only CATIONS and RADICAL CATIONS are detected by EI-MS.
[A-B][C] +Neutral Radical cation✗ ✔
Radicals and other neutral molecules (CO, H2O, alkenes) NOT detected by EI-MS.
[A-B-C]
✔
Generation of fragments from [M].+
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43
71
85
5729
114
EI-Mass Spectrum of Octane C8H18
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Both fragmentations involve formation of a Me radical or a Me cation
Octane
MW = 114 g/mol
3o > 2o > 1o > Me
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✔✔
Stability of cation and radical is important
Octane
MW = 114 g/mol
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✔✔
Octane
MW = 114 g/mol
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✔
Stability of cation and radical is important.
Fragmentations involving formation of a Me species are disfavored.
Octane
MW = 114 g/mol
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43
71
85
5729
114
EI-Mass Spectrum of Octane C8H18
MW = 114 g/mol
Base peak
15
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43
71
8629
EI-Mass Spectrum of 2-methylpentane
MW = 86 g/mol
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Branched alkanes fragment either side of the branch point(s).
m/z = 43 forms readily and is persistent in the chamber.
m/z = 71 may form readily, but fragments more rapidly.2-Methylpentane
MW = 86 g/mol
m/z = 71m/z = 86
m/z = 86 m/z = 43mass = 43
mass = 15
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43
71
86
29
EI-Mass Spectrum of 2-methylpentane
MW = 86 g/mol
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Atoms exist as isotopes (different # neutrons, same # protons)
12C is most abundant isotope of carbon
~1.08 % of C-atoms in any sample are 13C isotope (NMR active, useful)
~0.016% of H-atoms in any sample are 2H isotope (D)
~0.38% of N-atoms in any sample are 15N isotope
Atomic mass Cl = 35.48 amu
35Cl 75.8 % 37Cl 24.2 % ~3:1 ratio of 35Cl:37Cl isotopes
Atomic mass Br = 79.90 amu
79Br 50.7 % 81Br 49.3 % ~1:1 ratio of 79Br:81Br isotopes
Isotope patterns
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MW = 137 g/mol
57
EI-Mass Spectrum of 1-Bromobutane
29
Br = 79.90 g/mol
79Br 50.7 %81Br 49.3 %
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MW = 126.5 g/mol
EI-Mass Spectrum of Benzyl chloride
Cl = 35.48 g/mol
35Cl 75.8 % 37Cl 24.2 %
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EI-Mass Spectrum of Methanol CH3OH
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15
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43
58
128
EI-Mass Spectrum of 2-octanone
71
MW = 128 g/mol
2915
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43
58
128
EI-Mass Spectrum of 2-octanone
71
MW = 128 g/mol
2915