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Organic Chemistry, 7th Edition
L. G. Wade, Jr.
Chapter 12
Mass Spectrometry (質譜)
藥學系 藥物分析科
許秀蘊 教授
[email protected]
1
學習目標
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1. 光譜學簡介
2. 質譜儀之原理
3. 質譜儀之應用
2
Introduction
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Spectroscopy is a technique used to
determine the structure of a compound.
Most techniques are nondestructive (it
destroys little or no sample).
Absorption spectroscopy measures the
amount of light absorbed by the sample
as a function of wavelength.
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Types of Spectroscopy
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Infrared (IR) spectroscopy measures the bond
vibration frequencies in a molecule and is used
to determine the functional group.
Mass spectrometry (MS) fragments the molecule
and measures the masses.
Nuclear magnetic resonance (NMR)
spectroscopy detects signals from hydrogen
atoms and can be used to distinguish isomers.
Ultraviolet (UV) spectroscopy uses electron
transitions to determine bonding patterns.
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Electromagnetic
Spectrum
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Examples: X rays, microwaves, radio waves,
visible light, IR, and UV.
Frequency and wavelength are inversely
proportional.
c = ln, where c is the speed of light (3 x 1010
cm/sec).
Energy per photon = hn,
where h is Planck’s constant, 6.62 x 10-37
kJ•sec.
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The Spectrum and
Molecular Effects
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Mass Spectrum
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Mass Spectrometry
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Molecular weight can be obtained from a very
small sample.
It does not involve the absorption or emission
of light.
A beam of high-energy electrons breaks the
molecule apart.
Destructive technique, the sample cannot be
recovered.
The masses of the fragments and their relative
abundance reveal information about the
structure of the molecule.
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Mass Spectrometry
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1. Ionizes molecules in a high vacuum
2. Sorts the ions according to their
mass
3. Records the abundance of ions of
each mass
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Mass spectrometry -1
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Molecules (or atoms) are bombarded by
electrons, and become ionized
Mass spectrometer separates individual
particles (ionized atoms or molecules)
M + e[M].+ + 2e -
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Mass spectrometry -2
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The ions are then accelerated in an
electric field at a voltage V.
Energy of particle = e V (e：charge).
Kinetic energy = ½ mv 2 (m: mass of
particle)
e V = ½ mv 2
Velocity of a particle depends on its
mass.
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Mass spectrometry -3
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Charged particles enter at right angles
to a homogeneous magnetic field B.
Particles move in a circle
The radius of the circular path depends
on the accelerating voltage V, the
magnetic field, ratio m/e.
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Mass Spectrometer
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1. Inlet system
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All samples first be converted to the
gaseous state
Inlet system heated up to 400oC
Unstable samples should not be
exposed excessively high temp
The rate which the sample enters the
ionization chamber remains constant
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2. Ionization
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Electron Impact
Electron Spray Ionization (ESI)
(large molecules)
Fast Atom Bombardment (FAB)
(molecular ion; sample dissolved in a highboiling solvent/glycerol)
Field Ionization (high electrostatic field) : M+
Chemical Ionization (mix sample with a low
MW gas / methane)
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Radical Cation Formation
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M + e
-
[M].+ (molecular ion) + 2e
-
When a molecule loses one electron, it then
has a positive charge and one unpaired
electron. This ion is therefore called a
radical cation.
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Electron Impact Ionization
A high-energy electron can dislodge an electron from a
bond, creating a radical cation (a positive ion with an
unpaired e-).
H H
H C C H
H H
H H
H H
e- +
H C C+
H C C H
Other fragments can be formed H
when C—C or C—H bonds are
broken during ionization. Only the
positive fragments can be
detected in MS.
H
H H
H
H
H C+
H
H
C H
H
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3.Accelerating Chamber
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Ionized particles enter the accelerating
cell, under the influence of the
accelerating voltage V
Under fixed voltage, only one mass
would reach the detector
In order to scan the entire mass range
of fragments, the voltage is varied at a
constant rate
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4. Drift Chamber
(Magnetic Field Path)
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Accelerated fragmented ions enter the
drift chamber and come under the
influence of the magnetic field
The drift chamber is under a high
vacuum to prevent fragment from
colliding with each other
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5. Detector
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Most common detector: electron multiplier
Fragments from the drift chamber arrive at the
electron –emissive surface of a dynode amplifier
中間極放大器
A collision releases several electrons
Electrons are then accelerated to a second such
surface
Process is continued until a cascade of electrons
arrives at the collector
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Analytical Sequence
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1. gas sample is bled very slow at a constant rate into
ionization chamber (ionized and fragmented)
2. A beam of electrons causes molecules to ionize and
fragment.
3. Accelerating voltage (V) ： The mixture of ions is
accelerated
4. Magnetic field: Ions passes through a magnetic field,
where the paths of lighter ions are bent more than those
of heavier atoms. By varying the magnetic field, the
spectrometer plots the abundance of ions of each mass.
5. The exact radius of curvature of an ion's path depends
on its mass-to-charge ratio, symbolized by m/z. In this
expression, m is the mass of the ion (in amu) and z is its
charge.
6. Collector
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Separation of Ions
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A beam of electrons causes molecules to ionize and
fragment.
The mixture of ions is accelerated and passes
through a magnetic field, where the paths of lighter
ions are bent more than those of heavier atoms.
Only the cations are deflected by the magnetic field.
Amount of deflection depends on m/z.
The detector signal is proportional to the number of
ions hitting it.
By varying the magnetic field, ions of all masses are
collected and counted.
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Analytical Uses of Mass
Spectrometry
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1. Elemental Analysis
2. Molecular Analysis
3. Protein Sequencing Analysis
4. Special Equipment for GC-MS Combinations
5. GC-FTIR-MS Systems (separation;
functional group; MW)
6. Thermal Analysis-MS interface (physical
measurements with temperature change)
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Interpretation of Mass Spectra
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1. Identification of the Parent Mass
2. Counting Oxygens
3. Compounds with Hetero Elements
4. Clusters of Halogens
5. Using Electron Impact
6. Rings Plus Double Bonds
7. Important Neutral Species
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The Mass Spectrum
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In the spectrum, the tallest peak is called the base
peak and it is assigned an abundance of 100%. The
% abundance of all other peaks are given relative to
the base peak.
The molecular ion (M+) corresponds to the mass of
the original molecule.
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Mass Spectrum
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Isotopic Abundance
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Recognizable elements in the
Mass Spectrum
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Br M+2 as large as M+
Cl M+2 a third as large as M+
I
I+ at 127; large gap
N
odd M+, some even fragments
S
M+2 large than usual (4% of M+)
Isotopes: present in their usual abundance.
Hydrocarbons contain 1.1% C-13, so there
will be a small M+1 peak.
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Mass Spectrum
with Sulfur
Sulfur has three isotopes: 32S (95%), 33S (0.8%), and 34S
(4.2%).
The M+ peak of ethyl methyl sulfide has an M+2 peak that
is larger than usual (about 4% of M+).
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Mass Spectrum
with Chlorine
• Chlorine is a mixture of 75.5% 35Cl and 24.5% 37Cl.
• The molecular ion peak M+ is 3 times higher than the M+2 peak.
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Mass Spectrum
with Bromine
•Bromine is a mixture of 50.5% 79Br and 49.5% 81Br.
•The molecular ion peak M+ has
that has 81Br.
79Br
be as tall as the M+2 peak
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Fragmentation Patterns in MS
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Ionization
R：R’ + e-
[R.R’].+ + 2e-
radical cation
(molecular ion)
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Fragmentation
[R.R’].+
R+ + .R’
cation fragment
(observed)
radical fragment
(not observed)
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Mass Spectra of Alkanes
Fragmentation of the Hexane Radical Cation
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Mass Spectrum of n-Hexane
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Groups of ions correspond to loss of one-,
two-, three-, and four-carbon fragments.
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Fragmentation of Hexane
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[CH3CH2CH2--CH2CH2CH3]+.
Hexane radical cation
M+86
CH3CH2CH2+ + .CH2CH2CH3
propyl cation propyl radical(43)
detected at m/z 43
not detected
[CH3CH2CH2CH2CH2--CH3]+.
Hexane radical cation
M+86
CH3CH2CH2CH2CH2+ + .CH3
pentyl cation methyl radical(15)
weak at m/z 71
not detected
[CH3CH2CH2CH2CH2--CH3]+. X
Hexane radical cation
M+86
CH3CH2CH2CH2CH2. + +CH3
pentyl radical methyl cation(15)
not detected
(too unstable)
The stability of the cation is more important than the radical.
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Mass Spectra
of Alkanes
More stable carbocations will be more
abundant.
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Fragmentation of Branched
Alkanes
• The most stable carbocation fragments form in greater
amounts.
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Fragmentation Giving
Resonance-Stabilized Cations
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[R－CH＝CH－CH2－R’]+.
[R－CH＝CH－+CH2
allylic cation
R－+CH－CH ＝ CH2]
+ .R’
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Mass Spectra
of Alkenes
Resonance-stabilized cations favored.
Chapter 12
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Fragmentation Giving
Resonance-Stabilized Cations
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Benzylic cations
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Fragmentation Giving
Resonance-Stabilized Cations
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Ketones & aldehydes：loss of alkyl groups to
give acylium ions
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Mass spectra of Alcohols
Fragmentation Splitting Out a Small
Molecule
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Mass Spectra of Alcohols
Alcohols usually lose a water molecule.
M+ may not be visible.
44
MW=144
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The GC-MS
A mixture of compounds is separated
by gas chromatography, then identified
by mass spectrometry.
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High Resolution MS
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Masses measured to 1 part in 20,000.
A molecule with mass of 44 could be C3H8, C2H4O,
CO2, or CN2H4.
Using a mass with more significant figures would
help identify the correct formula.
For example, let’s say the compound we are
looking for has mass of 44.029, pick the correct
structure from the table:
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Analytical Uses of Mass
Spectrometry






1. Elemental Analysis
2. Molecular Analysis
3. Protein Sequencing Analysis
4. Special Equipment for GC-MS Combinations
5. GC-FTIR-MS Systems (separation;
functional group; MW)
6. Thermal Analysis-MS interface (physical
measurements with temperature change)
49
Common Fragmentation
Patterns-1
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Common Fragmentation
Patterns-2
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Common Fragmentation
Patterns-3
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Common Fragmentation
Patterns-4
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學習目標



1. 光譜學簡介
2. 質譜儀之原理
3. 質譜儀之應用
藥學系
許秀蘊
藥物分析科
教授
[email protected]
54