Molecular Mass Spectroscopy
• Molecular structure
• Composition of mixtures
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Molecular mass spectra
Ion Source
Mass Spectrometers
Applications
12-1
Molecular Mass Spectra
• Removal of electron by electron bombardment

In vapor phase

Charged species is the molecular ion
• Electron causes excitation and fragmentation

Major product is base peak
 Assigned 100% relative abundance

Smaller fragments also form
12-2
Ion Sources
• Ion source has profound effect on spectra
 Gas phase source
 Vaporized then ionized
 Desorption source
 Conversion of liquid or solid to gas
• Hard source
 Ion in excited state
 Fragments produced
• Soft source
 Little fragmentation
 Mainly ion of molecule
12-3
12-4
12-5
Electron-Impact Source
• Sample as vapor
• Ionized by beam of electrons

W or Re filament

70 V potential

1E-6 effective

M+e- ->M++2e
High potentials in
accelerating region
 1E3 to 1E4 volts
• KE of ion in 1000 V

KE=qV=zeV

KE=1*1.6E-19 C*1000 V
 1.6E-16 J
* KE independent of
mass
* Velocity varies with
mass
* KE=0.5mv2
12-6
Electron Impact Spectra
• Energy from e- accelerated by 70V
 Find in J/mol to compare bond energy
 KE=eV
1.6E-19 C *70 V=1.12E-17 CV/eFor a mole
* 1.12E-17 J*6.02E23 =6.7E6 J/mol
 Bond energy 200 to 600 kJ/mol
12-7
Electron Impact Spectra
12-8
Electron Impact Spectra
12-9
Electron Impact Spectra
• Sensitive method
• Fragments useful in identification
• Lack of molecular ion peak
 M+, difficult to identify specie
 Molecules must be in vapor phase
Stability issues in vapor phase
• MW<1000 dalton
12-10
Isotopics
• Isotopic variation can impact spectra
12-11
Chemical Ionization
• Sample ionized by secondary ionization
 Reagent gas ionized by electrons, then
ionized reagent gas reacts with sample gas
 Reagent to sample ratio
1E3 to 1E4
• Methane most common reagent gas
 CH4+ and CH3+ (about 90%), CH2+
12-12
Chemical Ionization
• Produces ions that are 1 proton more or 1
proton less than molecule
 Transfer of C2H5+ give M+29 peak
• Field Ionization
 Large electric field
1E8 V/cm
* Mainly produces M and M+1 peaks
12-13
Comparison of spectra
• a-electron impact
• b-field ionization
• c-desorption
12-14
Large molecule desorption
• Solid or liquid samples directly energized into gas
phase
 Molecular or protonated ion
• Matrix Assisted Laser Desorption/Ionization (MALDI)
 Soft Ionization
 Sample dissolved in solution containing UVabsorber and solvent
 Solution evaporated and precipitate formed
 Pulsed laser used to excite precipitate
 Molecular ion desorbed from surface of precipitate
12-15
12-16
Electrospray Ionization
• Solution pumped through a
needle

Needle is at kV potential
compared to
surrounding electrode

Droplets become
charged

Solvent evaporates,
droplets shrink and
charge density increases
• Can be combined with a
number of methods
• Useful for large molecules
• M+, M2+
12-17
Electrospray MS spectra
12-18
Fast Atom Bombardment
• Samples in glycerol
• Bombarded by Xe or Ar
atoms
 Several keV
• Atoms and ions
sputtered from surface
• Production of fragments
12-19
Mass Spectrometers
12-20
Magnetic sector
12-21
Ion Trap Analyzer
• Variable radio frequency voltage applied to the
ring electrode
• Ions of appropriate m/z circulate in stable orbit
• scan radio frequency
 heavier particles stable
 lighter particles collide with ring electrode
• ejected ions detected by transducer as an ion
current
12-22
Ion Trap
12-23
Applications
• Identification of Pure Compounds:

Nominal M+ peak (one m/z resolution) (or (M+1)+ or (M1)+)
• gives MW (not EI)

Exact m/z (fractional m/z resolution) can give stoichiometry
but not structure (double-focusing instrument)

Fragment peaks give evidence for functional groups

(M-15)+ peak methyl

(M-18)+ OH or water

(M-45)+ CO2H

series (M-14)+, (M-28)+, (M-42)+..sequential CH2
• loss in alkanes

Isotopic peaks can indicate presence of certain atoms
 Cl, Br, S, Si
* Isotopic ratios can suggest plausible molecules from
M+,
• Comparison with library spectra
12-24