Mass Spectrometry

1
Mass Spectrometry
Hexane
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
• Due at end of lab period (so come prepared and
  ready to go to work).

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 - Introduction

• 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

6
Mass Spec - Introduction
7
Mass Spec - Instrumentation
Sample Introduction
Data Output
Data Systems
Inlet
Mass Analyzer
Ion Source
Ion Detector
High vacuum
8
Mass Spec - Instrumentation
Carey, Chapter 13
9
An Example Hexane (mw 86)
57
Base peak (always 100)
43
Molecular ion peak
Various cation fragments
29
71
M
Molecules with 13C
10
Hexane Fragmentation
CH3 - CH2 - CH2 - CH2 - CH2 - CH3
M 86
71
57
43
29
15
11
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

12
Common Fragment Ions
More in text, p. 169, Table 2P-2.
13
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

14
M 112
Chlorobenzene (MW 112)
77 (phenyl)
M2 114
15
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

16
Bromobenzene
Base peak at 77 (phenyl)
M 156
M2 158
17
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

18
1-iodobutane (MW 184)
57 (184-127)
M 184
19
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
20
M-18 56
1-butanol
M 74
21
77 base peak (phenyl)
Benzamide (MW 121)
M 121
105 (Loss of NH2)
22
M-17 43
M-15 45
M 60
Acetic Acid
23
M 106
Benzaldehyde (aldehydes often have a large
M-1 peak)
77
24
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

25
What is this compound?
M 122
26
What is this compound?
M 86