Mass spectrometry

Detection of ionic species to determine the molecular weight of and obtain structural

information on a molecule

Mass spectrometry

electron beam+ 1e-

parent ion

+

CH3

+

CH2CH3

etcFragments

The more stable a fragment, the more likely it will form

ionicdetected

not ionicnot detected

• ions only detected• individual molecules are detected• uses isotopic masses not MW

CH3CH2CH3

Exact Mass: 44.0626Mol. Wt.: 44.0956

CH4

Exact Mass: 16.0313Mol. Wt.: 16.0425

29

Base peakParent ion

Base Peak

Parent ion-15

-CH3

43

58

29 44

Acetone

H3C CH3

O

Exact Mass: 58.0419Mol. Wt.: 58.0791

H3C H

O

Exact Mass: 44.0262Mol. Wt.: 44.0526

Acetaldehyde

-15

-CH3

-15

-CH3

Isotope effects

Molecular weightBr = 79.9040

Br isotopic abundance79Br = 50.5 %81Br = 49.5 %

CH379Br

CH381Br

Isotope effects

Molecular weightBr = 79.9040

Br isotopic abundance79Br = 50.5 %81Br = 49.5 %

CH379Br

CH381Br

Isotope effects

Molecular weightCl = 35.453

Cl isotopic abundance35Cl = 75.4 %37Cl = 24.6 %35Cl : 37Cl Ratio = 3:1

CH335Cl

CH337Cl

MS Ionization Techniques

1. Electron Impact (EI)uses beam of electrons

2. Fast Atom Bombardment (FAB)uses beam of atoms

3. Chemical Ionization (CI)uses chemical reactions

4. Matrix Assisted Laser Desorption (MALDI)uses lasers to ionize molecules

5. Electrosprayuses compounds which are already ionic

High energyhard ionization

Low energysoft ionization