Mass Spectrometry

1
Mass Spectrometry
Hexane
Chem 338
2
Prelab and Datasheet

• Reading
• Technique P in lab text, p. 160-171
• Carey, Chapter 13 (4 pages at end of chapter)
• Notebook nothing
• Datasheet
• Handed out in class

3
Mass Spec - Introduction

• Very different from IR and NMR
• Absorption of electromagnetic energy
• Sample can be recovered and reused
• Mass spectrometry
• Records what happens when an organic molecule is
  hit by a beam of high-energy electrons
• Sample is completely destroyed

4
Mass Spec - Introduction

• What does a mass spectrum tell us?
• Molecular weight
• Molecular formula
• Either directly or in conjunction with other
  kinds of spectra such as IR or NMR
• Fragmentation pattern
• Key pieces of what the molecule looks like (such
  as methyl, ethyl, phenyl, or benzyl groups

5
Mass Spec - Instrumentation
Sample Introduction
Data Output
Data Systems
Mass Analyzer
Ion Source
Ion Detector
High vacuum (10-5 torr)
6
MS Ionization Methods
Silverstein, et. al., Spectrometric
Identification of Organic Compounds, 7th Ed, p 8.
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Electron Impact Ionization

• We hit an organic molecule with a beam of
  electrons (usually 70-75 eV)
• That removes an electron from the molecule
  resulting in the molecular ion (a radical cation)
• The molecular ion then fragments in smaller
  radicals and cations
• The cations are detected by the MS
  instrumentation
• Bond breaking is extensive and reproducible
• Unique compounds produce unique spectra
• Disadvantage M peak sometimes missing

8
Chemical Ionization

• A soft ionization technique
• Reagent gases are ionized
• methanol, methane, ammonia, others
• Sample molecules collide with the ionized reagent
  gas
• usually results in a proton transfer from the
  reagent gas to the sample compound
• so M1 ions are common
• Very little energy is transferred to the
  compound
• Very little fragmentation is observed
• Thus not much structural information is obtained
• Main advantage an abundance of the M1 ion

9
Mass Spec - Instrumentation
Sample Introduction
Data Output
Data Systems
Inlet
Mass Analyzer
Ion Source
Ion Detector
High vacuum
10
Mass Analyzers
Silverstein, et. al., Spectrometric
Identification of Organic Compounds, 7th Ed, p 13.
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Magnetic Sector Mass Spectrometer
Carey, Chapter 13
12
EI Mass Spec - Introduction
13
An Example Hexane (mw 86)
57
Base peak (always 100)
43
Molecular ion peak
Various cation fragments
29
71
M
Molecules with 13C
14
Hexane Fragmentation
CH3 - CH2 - CH2 - CH2 - CH2 - CH3
M 86
71
57
43
29
15
15
Fragmentation Guidelines

• Cleavage generally favors the formation of the
  most stable cations and radicals
• tertiary gt secondary gt primary gt methyl
• resonance stabilized cations often formed
• small, neutral molecules often eliminated (water,
  carbon monoxide, ammonia, ketene)
• Also depends upon the relative bond strengths of
  the bonds being broken

16
Common Fragment Ions
More in text, p. 169, Table 2P-2.
17
Isotopic Clusters

• Especially useful for identifying chlorine and
  bromine
• Chlorine has two isotopes 35Cl and 37Cl in about
  a 75/25 ratio (avg MW 35.45)
• So compounds that have a chlorine will have 35Cl
  (75) and 37Cl (25)
• The molecular ion peak will show as two peaks in
  a 75/25 ratio, 2 mass units apart

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M 112
Chlorobenzene (MW 112)
77 (phenyl)
M2 114
19
Isotopic Clusters

• Bromine also has two isotopes
• 79Br and 81Br (avg MW 79.9)
• Molecules that contain bromine have a 50/50
  chance of getting one or the other
• Molecular ions of compounds containing a single
  bromine will have two peaks, 2 mass units apart,
  of about the same relative intensity

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Bromobenzene
Base peak at 77 (phenyl)
M 156
M2 158
21
Mass Spec Rearrangements

• Intramolecular rearrangements often take place
  during fragmentation.
• Migration of hydrogen atoms in molecules
  containing heteroatoms are quite common.

McLafferty Rearrangement
Where Y H, R, OH, OR, NH2
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More Things to Look For

• Iodine does not have an isotope cluster, but
  often shows a major peak at M-127 due to loss of
  I
• Alcohols will often have a peak at M-18 (loss of
  water)
• Nitrogen rule if a compound has an odd number
  of nitrogens, the MW is odd (and M)
• If the number of nitrogens is even (or zero), the
  MW will be even
• Many compounds do not show a good M peak
• Alcohols, highly branched compounds

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1-iodobutane (MW 184)
57 (184-127)
M 184
24
45 (Loss of methyl from M)
2-propanol (MW 60) Alcohols often do not
show an M peak. M-1 is common, but may also be
missing
M-1 59
25
M-18 56
1-butanol
M 74
26
77 base peak (phenyl)
Benzamide (MW 121)
M 121
105 (Loss of NH2)
27
M-17 43
M-15 45
M 60
Acetic Acid
28
M 106
Benzaldehyde (aldehydes often have a large
M-1 peak)
77
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Interpreting Spectra

• If an IR is present, examine for functional
  groups
• If an NMR is present
• Validate IR functional groups via chemical shift
  data
• Look for characteristic shifts and splitting
  patterns
• Examine the mass spec
• Look for the presence of Cl, Br, I, and N
• Determine the MW of the compound from M
• Look for common fragments and patterns
• Subtract known atoms from M to determine C and H
• Postulate a molecular formula
• Postulate a structure consistent with the known
  data

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What is this compound?
M 122
31
What is this compound?
M 86