Mass Spectrometry

To here!From here…

Detection of ionic species to determine the molecular weight of

and obtain structural information on a molecule

Molecular Weight: 70 amu

Molecular Formula:

C5H10

Mass spectrometry

electron beam+ 1e-

Molecular ion (M+)

+

CH3

+

CH2CH3

etcFragments

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

ionicdetected

not ionicnot detected

Fig. 12-1, p. 410

Gas Chromatography –Mass Spectromety (GC-MS)

Liquid Chromatography –Mass Spectromety (LC-MS)

CH3CH2CH3

Exact Mass: 44.0626Mol. Wt.: 44.0956

29

Base peakMolecular ion (M+)

Base Peak

Molecular ion-15

-CH3

CH4

Exact Mass: 16.0313Mol. Wt.: 16.0426

Fig. 12-4, p. 413

Mass Spectrum of Hexane

Chemical Formula: C6H14

Exact Mass: 86.1095Molecular Weight:

86.1753

-15

-CH3

Fig. 12-5, p. 413

Fig. 12-6, p. 414

methylcyclohexane or ethylcyclopentane ?

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

• -cleavage (cleavage of bond one removed from radical or radical cation)– Carbonyl compounds

CH3 H

O

C OH +CH3 H

OCH3

Fragmentation by -cleavage

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

OH

HH

OH

HH

• -cleavage (cleavage of bond one removed from radical or radical cation)– Carbonyl compounds

– Amines and alcohols

– Alkenes

CH3 H

O

C OH +CH3 H

OCH3

Fragmentation by -cleavage

Fragmentation by -cleavage• -cleavage (cleavage of bond one

removed from radical or radical cation)– Carbonyl compounds

– Amines and alcohols

– Alkenes

CH3 H

O

C OH +CH3 H

OCH3

OH

HH

OH

HH

CH3

CH3 H

R

HH

CH3

CH3 H

R

HH

Fragmentation by -cleavage• -cleavage (cleavage of bond one

removed from radical or radical cation)– Methyl benzene (toluene) compounds

CH

H

H

CH

H

H

Fig. 12-8, p. 416

m/z = 102

Determination of Formula:Rule of 13

Assumes CnHn and amu equivalent (13 for n=1) is present in all molecular fragment ions

Step 1: Divide M+ mass by 13, this gives n

Step 2: Any remainder represents count of additional Hʼs

example 1: for M+ = 7878 ÷ 13 = 6 → n = 6 → C6H6

example 2: for M+ = 9292 ÷ 13 = 7.077 → n = 77 x 13 = 91 → 1 extra H is presentFormula is C7H7+1 = C7H8

Determination of Formula:Rule of 13

example 3: for M+ = 161161 ÷ 13 = 12.385 → n = 1212 x 13 = 156 → 5 extra Hʼs are presentFormula is C12H12+5 = C12H17

What about heteroatoms?Step 1: First derive formula as aboveStep 2: Next, modify using CnHm equivalents

Problem

An unknown compound subjected to mass spectrometry gave the parent ion at m/z = 114. The parent ion quickly undergoes homolytic -cleavage to give fragment ions with m/z = 99 (100 %) and m/z = 45 (45 %). No other significant peaks are seen the in spectrum. The unknown does not react when treated with H2, Pd/C. The infrared spectrum of the unknown shows a strong, broad peak at 3500 cm-1. Show the reaction and the electron pushing mechanism that accounts for the fragmentation of the parent ion to give either of the observed fragments.

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