Chem 3500
Mass Spectrometry
Y. Zhao
Lecture 2
• A Glimpse of Mass Spectrometry
Began from 1920s: Aston, Dempster, Thompson
MS is often an ideal starting point in organic structure analysis
H2N
O
C7H7NO
M.W. 121
EI-MS of benzamide
Chem 3500
Mass Spectrometry
Y. Zhao
What is Mass Spectrometry?
A technique used to determine the masses of atoms or molecules in
which electrical charges are placed on them and the resulting ions are
separated by their mass-to-charge ratios (m/z).
A Mass Spectrometer is an instrument that can separate
charged atoms or molecules according to their mass-to-charge
ratio.
Chem 3500
Mass Spectrometry
Y. Zhao
Presentation of Data
A Low-Resolution EI-MS of n-pentane. Each m/z value is the nominal
molecular mass of the fragment (z = +1)
Chem 3500
Mass Spectrometry
Y. Zhao
Species with a nominal mol. mass of 122 amu
Exact
mass
C9H14
C7H10N2
122.1096
122.0845
C8H10O
C 7 H 6 O2
122.0732 122.0368
C4H10O4
C4H10S
122.0579 122.0225
Exact mol. masses can be measured by using a High-Resolution
Mass Spectrometer (HR-MS)
The fate of a sample molecule in a mass spectrometer
• Ionization—Molecular Ion (M+·)
• Fragmentation and rearrangement—(fragment ions, radicals, neutral
species)
Chem 3500
Mass Spectrometry
Y. Zhao
• Principles and Instrumentation of MS
Major components of an MS instrument
Sample
introduction
Ionization
Analyzer
Detector
Data system
Chem 3500
Mass Spectrometry
Y. Zhao
Schematic of an electron-impact mass spectrometer with a single
magnetic sector analyzer
Chem 3500
Mass Spectrometry
Y. Zhao
• How does it work?
A single-focusing, 180º sector mass analyzer
Acceleration V
+
- Magnetic field B
305
185
173
100%
+
m/z = 185
+ + +
+
++ +
+
+
+
+
m
=
z
Base peak
185
173
305
Detector
m/z
A mass spectrum
Analyzer tube
B2r2
2V
Chem 3500
Mass Spectrometry
Resolution
C15H24NO2
250.1807
C16H26O2
250.1933
Y. Zhao
Two adjacent peaks with
almost equal intensity
height
valley
< 10%
m/z
R=
250.1933
= 20,000
250.1933-250.1807
High-resolution mass spectrum (HRMS) can
be used to measure exact masses
Chem 3500
Mass Spectrometry
Y. Zhao
Why HRMS?
Exact mass
H 1.0078522
N 14.0080744
C
O
12.0000000
15.99491502
CO
CH2=CH2
Nominal mass is 28
Nominal mass is 28
Exact mass
2  12.0000000 = 24.000000
4  1.00782522 = 4.03130
Exact mass
1  12.0000000 = 12.000000
1  15.99491502 = 15.99491502
28.03130
27.994915
Chem 3500
Mass Spectrometry
Y. Zhao
Electron Impact/Ionization (EI)
• EI is a “hard” ionization method
• One electron is removed to form
singly charged molecular ion, M+·
Anode
e
e
• M+· carries excess internal
energy and tends to fragment
• Give reproducible spectra but
limit to MW < 1,000 Da
+ ·e
M
• Pressure < 10-5 Torr
• ionization efficiency is high
• electron energies ~ 20-70 eV
Cathode
e
Chem 3500
Mass Spectrometry
Chemical ionization (CI)
Anode
e
+
R
Cathode
M
Y. Zhao
Chem 3500
Mass Spectrometry
Y. Zhao
Chemical ionization (CI)
• CI is especially useful (“soft” ionization) technique when
no molecular ion is observed in EI mass spectrum, and
also in the case of confirming the mass to charge ratio of
the molecular ion.
• Reagent gas is first ionized by EI.
• Sample molecules are ionized by interaction with reagent
gas ions.
• Reagent gas : sample molecules ~ 100 : 1
• Positive and negative ions are formed in the CI process.
However, depending on the instrument setup, only positive
ions or negative ions are recorded in the spectrum.
• Pressure = 0.1 – 2 Torr
Chem 3500
Mass Spectrometry
Y. Zhao
Chemical ionization (CI)
Positive ion mode:
G+· + M
G-H+ + M
TiCl2+ + M
M+· + G
charge transfer
M-H+ + G proton transfer
(M + TiCl2)+ adduct formation
Negative ion mode:
[G-H]- + M
M + e
[M-H]- + G proton transfer
M-·
electron capture
Chem 3500
Mass Spectrometry
Y. Zhao
Desorption ionization (DI)
• Fast atom bombardment (FAB)
++ +
Primary incident beam
Xe
Ion cluster
+++
+
Multiple
desolvation
steps
Sample solution
= sample molecule
= solvent molecule
Chem 3500
Mass Spectrometry
Y. Zhao
Desorption ionization (DI)
• Matrix assisted laser desorption/ionization (MALDI)
+
laser
substrate
Chem 3500
Mass Spectrometry
Y. Zhao
Electrospray ionization (ESI)
Drying gas
needle
+Sample
+ solution
+ + + +
+ + + + + +
+ +
+ + ++ + +
+
+
+
Voltage ~3 – 5 kV
Single ion in droplet theory: assume that increased charge density, due
to solvent evaporation, causes large droplets to divide into smaller
droplets eventually leading to single ions.
Ion evaporation theory: assumes the increased charge density that
results from solvent evaporation causes Coulombic repulsion to
overcome the liquid’s surface tension, resulting in a release of ions from
droplet surfaces.
Chem 3500
Mass Spectrometry
Y. Zhao
Mass analyzer
Magnetic sector
+
N
r
s
Time of flight (TOF)
V
d
detector
Field free region
m/z = 1.93 t2 V/d2
Acceleration region
Chem 3500
Mass Spectrometry
Quadrupole mass filter
-
+
+
+ -
Ion trap
Ion cyclotron resonance (ICR)
Y. Zhao