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