6 Mass Spectrometry pt1

Organic Chemistry: Structure and Function

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1 Mass Spectrometry Part 1 Lecture Supplement: Take one handout from the stage
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2 Spectroscopy Why bother with spectroscopy? Determine structure of unknown substance Verify purity/identity of known substance
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3 Spectroscopy What methods are commonly used? *Not rigorously a type of spectroscopy Mass spectrometry (MS)* molecular formula Infrared spectroscopy (IR) functional groups Nuclear magnetic resonance (NMR) C/H molecular skeleton X-ray crystallography* spatial position of atoms
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4 Spectroscopy Example: unidentified white powder •MS: C 10 H 15 N •IR: benzene ring, secondary amine (R 2 NH) •NMR: has CH 2 -CH-CH 3 X-ray: not necessary in this case CH 3 CH 3 N H
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5 Detector quiet m/z too small Mass Spectrometry The Mass Spectrometer Fundamental operating principle Determine mass by manipulating flight path of an ion in a magnetic field sample introduction Measure ion mass-to-charge ratio (m/z) Detector Ionization Electron gun + - Accelerator plates Magnet m/z just right Detector fires Ionization : X + e - X +. + 2 e - m/z too large Detector quiet
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6 Isotopes Aston mass spectrum of neon (1919) Ne empirical atomic weight = 20.2 amu Ne mass spectrum: predict single peak at m/z = 20.2 Results m/z relative intensity 20.2 no peak 20.0 90% 22.0 10% Conclusions Neon is a mixture of isotopes Weighted average: (90% x 20.0 amu) + (10.0% x 22.0 amu) = 20.2 amu Isotopes : atoms with same number of protons and same number of electrons but different numbers of neutrons Nobel Prize in Chemistry 1922 to Aston for discovery of stable element isotopes
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7 The Mass Spectrum Example : methane CH 4 + e - CH 4 +. + 2 e - mass-to-charge ratio (m/z) Relative ion abundance (%) Base peak : most abundant ion m/z = (1 x 12) + (4 x 1) = 16 C H
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8 The Mass Spectrum Alternate data presentation... m/z (amu) Relative abundance (%) 1 < 0.5 17 1.1 16 100.0 1 85.0 2 9.2 3 3.0 12 1.0 M+2 M+1 M Molecular ion (M) : intact ion of substance being analyzed Fragment ion : formed by cleavage of one or more bonds on molecular ions 12 C 1 H 4 13 C 1 H 4 or 12 C 2 H 1 H 3 14 C 1 H 4 or 12 C 3 H 1 H 3 or... M - H M - 2H M - 3H M - 4H
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9 The Mass Spectrum Origin of Relative Ion Abundances M contributors M+1 contributors M+2 contributors Isotope Natural Abundance Isotope Natural Abundance Isotope Natural Abundance 1 H 99.9855% 2 H 0.015% 3 H ppm 12 C 98.893 13 C 1.107 14 C ppm 14 N 99.634 15 N 0.366 16 O 99.759 17 O 0.037 18 O 0.204 19 F 100.0 32 S 95.0 33 S 0.76 34 S 4.22 35 Cl 75.77 37 Cl 24.23 79 Br 50.69 81 Br 49.31 127 I 100.0 This table will be provided on an exam. Do not memorize it.
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10 The Mass Spectrum Relative Intensity of Molecular Ion Peaks Imagine a sample containing 10,000 methane molecules...
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