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DESCRIPTION
Basics of Mass SpectroscopyTRANSCRIPT
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Mass Spectrometry• Molecular weight can be obtained from a
very small sample.• It does not involve the absorption or emission
of light.• A beam of high-energy electrons breaks the
molecule apart.• The masses of the fragments and their
relative abundance reveal information about the structure of the molecule.
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Mass Spectrometry
THE MAIN USE OF MS IN ORG CHEM IS:
• DETERMINE THE MOLECULAR MASS OFORGANIC COMPOUNDS
• DETERMINE THE MOLECULAR FORMULA OF ORGANIC COMPOUNDS
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HOW DO WE ACHIEVE THIS?
• PERSUADE THE MOLECULE TO ENTER THE VAPOR PHASE (CAN BE DIFFICULT)
• PRODUCE IONS FROM THE MOLECULES THAT ENTER THE GAS PHASE
• SEPARATE THE IONS ACCORDING TO THEIR MASS-TO-CHARGE RATIOS (m/z))
• MEASURE AND RECORD THESE IONS
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IONIZING METHODS
• ELECTRON IMPACT - HIGH ENERGY ELECTRONS ABOUT 70 EV!!
• CHEMICAL IONIZATION LOW ENERGY
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ELECTRON IMPACT
H-C:HH
H+ e H-C
H
HH + 2e
RADICAL CATION
CH4
H-CH
HH
H-CH
H+ + H
ONLY CATIONS ARE CARRIED TO DETECTOR
H-CH
HH++
CATIONRADICALBOND-BREAKING
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MS of METHANE
1 15 16
Molecular ion
fragments
base peak
mass measurement takes about 20 secsmany fragmentations occur
So what does a typical ms looks like????
m/z
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C
H
H H
H
H.. H
C. . . . . . H H
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. . . . . H.. H
C. . . . . . H H +
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Electron Impact Ionization
A high-energy electron can dislodge an electron from a bond, creating a radical cation (a positive ion with an unpaired e-).
e- + H C
H
H
C
H
H
H
H C
H
H
C
H
H
H
H C
H
H
C
H
H
+ H
H C
H
H
C
H
H
H
+=>
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Separation of Ions
• Only the cations are deflected by the magnetic field.
• Amount of deflection depends on m/z.
• The detector signal is proportional to the number of ions hitting it.
• By varying the magnetic field, ions of all masses are collected and counted. =>
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Mass Spectrometer
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The Mass Spectrum
Masses are graphed or tabulated according to their relative abundance.
=>
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M.S. Spectrum
142 g/mol – 113 g/mol = 29 g/mol
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M.S. Spectrum
Why is there a small peak at 109 g/mol?
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The GC-MS
=>
A mixture of compounds is separatedby gas chromatography, then identifiedby mass spectrometry.
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High Resolution MS• Masses measured to 1 part in 20,000.
• A molecule with mass of 44 could be C3H8, C2H4O, CO2, or CN2H4.
• If a more exact mass is 44.029, pick the correct structure from the table:
C3H8 C2H4O CO2 CN2H4
44.06260 44.02620 43.98983 44.03740
=>
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Molecules with Heteroatoms
• Isotopes: present in their usual abundance.
• Hydrocarbons contain 1.1% C-13, so there will be a small M+1 peak.
• If Br is present, M+2 is equal to M+.
• If Cl is present, M+2 is one-third of M+.
• If iodine is present, peak at 127, large gap.
• If N is present, M+ will be an odd number.• If S is present, M+2 will be 4% of M+. =>
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Isotopic Abundance
=>
81Br
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Mass Spectrum with Sulfur
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Mass Spectrum with Chlorine
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Mass Spectrum with Bromine
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Mass Spectra of Alkanes
More stable carbocations will be more abundant.
=>
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Mass Spectra of Alkenes
Resonance-stabilized cations favored.
=>
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End of Chapter 12