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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. C 7 H 7 NO M.W. 121. EI-MS of benzamide.

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Chem 3500 mass spectrometry y zhao

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

C7H7NO

M.W. 121

EI-MS of benzamide


Chem 3500 mass spectrometry y zhao

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

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

Chem 3500 Mass Spectrometry Y. Zhao

Species with a nominal mol. mass of 122 amu

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

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

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

+

+

+

+

+

+

+

m/z = 185

+

+

+

B2r2

m

185

=

100%

z

2V

173

305

A mass spectrum

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

+

+

Base peak

Analyzer tube

Detector

m/z


Chem 3500 mass spectrometry y zhao

Chem 3500 Mass Spectrometry Y. Zhao

Resolution

Two adjacent peaks with almost equal intensity

C15H24NO2

C16H26O2

250.1933

250.1807

height

< 10%

valley

m/z

250.1933

= 20,000

R =

250.1933-250.1807

High-resolution mass spectrum (HRMS) can be used to measure exact masses


Chem 3500 mass spectrometry y zhao

Chem 3500 Mass Spectrometry Y. Zhao

Why HRMS?

Exact mass

H 1.0078522 C 12.0000000

N 14.0080744 O 15.99491502

CO

CH2=CH2

Nominal mass is 28

Nominal mass is 28

Exact mass

1  12.0000000 = 12.000000

1  15.99491502 = 15.99491502

27.994915

Exact mass

2  12.0000000 = 24.000000

4  1.00782522 = 4.03130

28.03130


Chem 3500 mass spectrometry y zhao

e

e

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+·

  • 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

Anode

e

e

M

+ ·

Cathode


Chem 3500 mass spectrometry y zhao

Chem 3500 Mass Spectrometry Y. Zhao

Chemical ionization (CI)

Anode

e

M

+

R

Cathode


Chem 3500 mass spectrometry 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

Chem 3500 Mass Spectrometry Y. Zhao

Chemical ionization (CI)

Positive ion mode:

G+· + M M+· + G charge transfer

G-H+ + M M-H+ + G proton transfer

TiCl2+ + M (M + TiCl2)+ adduct formation

Negative ion mode:

[G-H]- + M [M-H]- + G proton transfer

M + e M-· electron capture


Chem 3500 mass spectrometry y zhao

+

+

+

+

Multiple desolvation steps

+

+

Ion cluster

+

Chem 3500 Mass Spectrometry Y. Zhao

Desorption ionization (DI)

  • Fast atom bombardment (FAB)

Primary incident beam

Xe

Sample solution

= sample molecule

= solvent molecule


Chem 3500 mass spectrometry y zhao

+

laser

Chem 3500 Mass Spectrometry Y. Zhao

Desorption ionization (DI)

  • Matrix assisted laser desorption/ionization (MALDI)

substrate


Chem 3500 mass spectrometry y zhao

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

N

+

r

s

Chem 3500 Mass Spectrometry Y. Zhao

Mass analyzer

Magnetic sector

Time of flight (TOF)

V

d

detector

Field free region

m/z = 1.93 t2 V/d2

Acceleration region


Chem 3500 mass spectrometry y zhao

Chem 3500 Mass Spectrometry Y. Zhao

Quadrupole mass filter

+

-

+

+

-

+

Ion trap

Ion cyclotron resonance (ICR)