Mass Spectrometry Chem 3500 Mass Spectrometry Y. Zhao Lecture 2 A Glimpse of Mass Spectrometry Began...
Transcript of Mass Spectrometry Chem 3500 Mass Spectrometry Y. Zhao Lecture 2 A Glimpse of Mass Spectrometry Began...
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Lecture 2 Lecture 2 • A Glimpse of Mass Spectrometry
Began from 1920s: Aston, Dempster, Thompson
MS is often an ideal starting point in organic structure analysis
EI-MS of benzamideEI-MS of benzamide
H2N O
C7H7NOM.W. 121
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 SpectrometryMass Spectrometry Y. Zhao
Chem 3500 Mass SpectrometryMass 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)
C9H14 C7H10N2 C8H10O C7H6O2 C4H10O4 C4H10S
Exact mass 122.1096 122.0845 122.0732 122.0368 122.0579 122.0225
Exact mol. masses can be measured by using a High-Resolution Mass Spectrometer (HR-MS)
Species with a nominal mol. mass of 122 amu
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
The fate of a sample molecule in a mass spectrometer
• Ionization—Molecular Ion (M+·)
• Fragmentation and rearrangement—(fragment ions, radicals, neutral species)
Ionization Analyzer Detector
Data system
Sample introduction
Major components of an MS instrument
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
• Principles and Instrumentation of MS
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Schematic of an electron-impact mass spectrometer with a single magnetic sector analyzer
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
++
+ ++ + +
++
+
+
+ -Acceleration V
Magnetic field B
A single-focusing, 180º sector mass analyzer
+
m/z
185
173
185
m/z = 185
305
305173
Detector
A mass spectrum
Analyzer tube
100%
Base peakmz =
B2r2
2V
• How does it work?
Resolution
250.1933250.1807
m/z
Two adjacent peaks with almost equal intensity
height
valley < 10%
250.1933 R =
250.1933-250.1807= 20,000
High-resolution mass spectrum (HRMS) can be used to measure exact masses
C16H26O2C15H24NO2
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Why HRMS?
Exact massH 1.0078522 C 12.0000000N 14.0080744 O 15.99491502
CH2=CH2
Nominal mass is 28
CO
Nominal mass is 28
Exact mass2 12.0000000 = 24.0000004 1.00782522 = 4.03130
28.03130
Exact mass1 12.0000000 = 12.0000001 15.99491502 = 15.99491502
27.994915
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
e
Anode
Cathode
M
e
ee
+ ·
Electron Impact/Ionization (EI)
• EI is a “hard” ionization method
• One electron is removed to form singly charged molecular ion, M+·
• M+· carries excess internal energy and tends to fragment
• Give reproducible spectra but limit to MW < 1,000 Da
• Pressure < 10-5 Torr
• ionization efficiency is high
• electron energies ~ 20-70 eV
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
M
Chemical ionization (CI)
Anode
Cathode
e
R+
Chem 3500 Mass SpectrometryMass 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 SpectrometryMass Spectrometry Y. Zhao
Chemical ionization (CI)
Positive ion mode:
G+· + M M+· + G charge transferG-H+ + M M-H+ + G proton transferTiCl2
+ + M (M + TiCl2)+ adduct formation
Negative ion mode:
[G-H]- + M [M-H]- + G proton transferM + e M-· electron capture
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Desorption ionization (DI)
• Fast atom bombardment (FAB)
= sample molecule = solvent molecule
Xe
Sample solution
+++Ion cluster
Multiple desolvation steps
++ ++
Primary incident beam
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Desorption ionization (DI)
• Matrix assisted laser desorption/ionization (MALDI)
laser+
substrate
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Electrospray ionization (ESI)
needle
+ + +++ ++
++
++ ++ ++ +
+
+
+
+
+
++
Drying gas
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.
Sample solution
Voltage ~3 – 5 kV
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Mass analyzer
s
N+
r
Magnetic sector
Time of flight (TOF)
detector
V
Field free region
Acceleration regionm/z = 1.93 t2 V/d2
d
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao
Quadrupole mass filter
+
+
-
-
++
Ion trap
Ion cyclotron resonance (ICR)
Chem 3500 Mass SpectrometryMass Spectrometry Y. Zhao