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The Basics of Mass Spectroscopy

! Purpose

Mass spectrometers use the difference in mass-to-charge ratio (m/z) of ionized atoms or molecules to separate them. Therefore, mass spectroscopy allows quantitation of atoms or molecules and provides structural information by the identification of distinctive fragmentation patterns.

The general operation of a mass spectrometer is:

  • 1.

    create gas-phase ions

  • 2.

    separate the ions in space or time based on their mass-to-charge ratio

  • 3.

    measure the quantity of ions of each mass-to-charge ratio

Ionization sources

SORTING

DETECTION

IONIZATION

OF IONS

OF IONS

! Instrumentation

gaseous

mass

ion

ion source

analyzer

transducer

Chemical Ionisation (CI) Atmospheric Pressure CI (APCI) Electron Impact (EI) Electrospray Ionization (ESI) Fast Atom Bombardment (FAB) Field Desorption/Field Ionisation (FD/FI) Matrix Assisted Laser Desorption Ionisation (MALDI) Thermospray Ionisation (TI)

Analyzers

vacuum pump 1 0 - 5 1 0 - 8 t o r r

signal processor

quadrupoles Time-of-Flight (TOF) magnetic sectors Fourier transform and quadrupole ion traps

inlet

Detectors

mass spectrum

electron multiplier Faraday cup

Ionization Sources: Classical Methods

! Electron Impact Ionization

A beam of electrons passes through a gas-phase sample and collides with neutral analyte molcules (M) to produce a positively charged ion or a fragment ion. Generally electrons with energies of 70 ev are used.

This method is applicable to all volatile compounds ( >103 Da) and gives reproducible mass spectra with fragmentation to provide structural information.

! Chemical Impact Ionization

Ionization bergins when a reagent gas (R) is ionized by electron impact and then subsequently reacts with analyte molecules (M) to produce analyte ions.

This method gives molecular weight information and reduced fragmentation in comparison to EI.

reagent gas (R)

2 C 3 4 H H 1 0 C NH H 3 O H NO

molecular ion

2 C H 4 H NH C H 3 O NO 10 3 H

reactive reagent ion

3 C + H 4 H NH4 C H 3 O NO H + 11 + +

2

+

:

M

Step 1

M

:

R

Ionization

R

Electron Ionization

Ionization

Chemical Ionization

Step 1

Step 2

Step 2

M , A + , B + , e t c .

RH+

EI spectrum

M

MH+ + R

Step 3

CI spectrum

2

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