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Understanding Mass Spectrometry: Isotopes, Natural Abundance, and Molecular Analysis

This introduction to mass spectrometry covers fundamental concepts, including the determination of isotopes like Carbon-12, Hydrogen-1, and others, as well as their natural abundances. Detailed analyses of molecular weights for common hydrocarbons and elements are provided. The text explores mass spectra, focusing on methane and hexanes, illustrating fragmentation patterns. Insights into the biosynthesis of squalene and the conversion to cholesterol reveal the applications of mass spectrometry in biochemical processes. This foundational knowledge is vital for chemists and researchers.

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Understanding Mass Spectrometry: Isotopes, Natural Abundance, and Molecular Analysis

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  1. An Introduction to Mass Spectrometry e-

  2. Carbon 12C = 12.000000 Hydrogen 1H = 1.007825 Nitrogen 14N = 14.003050 Oxygen 16O = 15.994914 Masses of Common Isotopes CO MW = 27.994914 N2 MW = 28.006100 CH2=CH2 MW = 28.031300

  3. CO N2 CH2=CH2 27.994914 28.006100 28.031300 28 28 1/100 28 28.0 28.0 1/1,000 28.0 1/10,000 27.99 28.01 28.03 Resolution of Masses Resolution

  4. Hydrogen 1H = 99.985% 2H =0.015% Carbon 12C = 98.90% 13C = 1.10% Nitrogen 14N = 99.63% 15N =0.37% Oxygen 16O = 99.762% 17O =0.038% 18O =0.200% Sulfur 32S = 95.02% 33S =0.75% 34S = 4.21% 36S =0.02% Chlorine 35Cl = 75.77% 37Cl = 24.23% Bromine 79Br = 50.69% 81Br = 49.31% Natural Abundance of Common Elements

  5. The Mass Spectrometer Focusing slits Accelerating voltage V Focused ion M+ Ion M+ Ionizing voltage Radius r Detector Sample inlet Low vacuum m/z = B2r2e/2V Where m = mass, e = electron charge, z = # of charges/ion r = radius of curvature Magnetic field B Increase B

  6. For carbon, One in ~90 atoms of carbon is C-13 For every 90 molecules of methane ...

  7. only one molecule contains C-13

  8. Where’s Waldo?

  9. … or for the C30 triterpene squalene

  10. one carbon in 3 molecules is C-13

  11. The Mass Spectrum of Methane M+ = 16 Molecular ion [C12H4]+. M+ = 15 C12H3+ [C12H2]+. C12H+ 16 M +1 = 17 [C13H4]+. [C12]+. 8 3 m/z 12 13 14 15 16 17 100 Base peak 86 1.11

  12. M = 43; M-43 base peak M = 57; M-29 n-hexane; MW 86 M = 29; M-57 molecular ion M+ 86 M = 71; M-15 M = 15; M-71 SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/

  13. There are five constitutional isomers of C6H14 Which one of the remaining four has the following mass spectrum?

  14. M = 57; M-29 weak signal base peak M = 43; M-43 M = 29; M-57 weak signal M = 71; M-15 molecular ion M+ 86 M = 15; M-71 C6H14, MW 86 SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/

  15. M = 57 M = 57 M = 57 M = 29 M = 29 M = 29 3-methylpentane 2-methylpentane M = 43 M = 71 M = 15 2,2-dimethylbutane 2,3-dimethylbutane The Four RemainingHexanes

  16. n -butyl radical BDE = 98 kcal/mol methyl radical  = +104 kcal/mol M = 87 M = 45 2-hexanol MW 102 SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/

  17. Ethyl Bromide C2H5Br81 M = 29 C2H5+ C2H5Br79 SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/

  18. Isopropyl Chloride i-PrCl37 i-PrCl35 M-15 M-15 SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/

  19. Back to squalene (C30H50) or (6E, 10E, 14E, 18E)- 2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene

  20. Pentenylpyrophosphate Farnesylpyrophosphate Squalene On the Mechanism of the Biosynthesis of Squalene 3 CH3CO2H

  21. CH3 CH3 Squalene C30 CH3 HO Cholesterol C27 On the Biochemical Conversion of Squalene to Cholesterol

  22. Farnesylpyrophosphate - d2 Squalene - d3 A Deuterium Labeling Experiment on the Biosynthesis of Squalene

  23. Oxidative Ozonolysis of Deuterium Labeled Squalene and succinic acid - d3 levulinic acid acetone

  24. -H2O -H2O Formation of Succinic Anhydride Succinic acid Succinic anhydride Succinic acid - d3 Succinic anhydride - d3

  25. Succinic Anhydride Succinic Anhydride-d3 M = 28 M = 56 M-31; C2D3H M = 59 M+ = 100 M - 44; CO2 M - 28; C2H4 M - 44; CO2 M+ = 103

  26. The End e-

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