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Introduction to Mass Spectrometry (MS)

Introduction to Mass Spectrometry (MS) A mass spectrometer produces a spectrum of masses based on the structure of a molecule. The x-axis of a mass spectrum represents the masses of ions produced (m/z) The y-axis represents the relative abundance of each ion produced

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Introduction to Mass Spectrometry (MS)

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  1. Introduction to Mass Spectrometry (MS) • A mass spectrometer produces a spectrum of masses based on the structure of a molecule. • The x-axis of a mass spectrum represents the masses of ions produced (m/z) • The y-axis represents the relative abundance of each ion produced • The pattern of ions obtained and their abundance is characteristic of the structure of a particular molecule 1

  2. Ionization (the formation of ions) • A molecule is bombarded with a beam of high energy electrons • An electron is dislodged from the molecule by the impact, leaving a positively charged ion with an unpaired electron (a radical cation) • This initial ion is called the molecular ion (M+.) because it has the same molecular weight as the analyte 2

  3. Fragmentation • Excess vibrational energy is imparted to the molecular ion by collision with the electron beam - this causes fragmentation • The fragmentation pattern is highly characteristic of the structure of the molecule 3

  4. Common Isotope Abundances 5

  5. 3 Classes of Isotopes • M - only a single isotope • EX: F, P, I • M+1 - two isotopes with significant relative abundance differing by 1 mass unit • EX: H, C, N • M+2 - two isotopes with significant relative abundance differing by 2 mass units • EX: Cl, S, O

  6. Determination of Molecular Formulas & Weights • The Molecular Ion and Isotopic Peaks • The presence of heavier isotopes one or two mass units above the common isotope yields small peaks at M+.+1 and M+.+2 • Example: In the spectrum of methane one expects an M+.+1 peak of 1.17% based on a 1.11% natural abundance of 13C and a 0.016% natural abundance of 2H 6

  7. Four Basic Rules • If M+ is even, then the unknown contains an even number of Nitrogen atoms (zero is an even number) • The abundance of M++1 indicates the number of Carbon atoms: # of C = relative abundance/1.1 • The abundance of the M++2 peak indicates the presence of S (4.4%), Cl (33%) or Br (98%) • The remaining unknown mass can be attributed to Hydrogen 7

  8. C – C – C - Cl H H H H H H H C3H7Cl • Is the molecular ion even? • Yes, there must be either an even number of N, or no Nitrogen atoms. • How many Carbon atoms are there? • # Carbons = 3 / 1.1 ≈ 3 carbon atoms • Is a S, Cl or Br present? • A M++2 peak of 33% indicates the presence of chlorine • How many Hydrogen atoms are there? • 78 = (1 * 35) + (3 * 12) + (H * 1) • 78 = 71 + H • # of Hydrogen atoms = 7 8

  9. (M-1) % Intensity 86 (M+) 87 (M+1) 88 (M+2) H H H H H H H H H H H C – C – C – C – C = O C5H10O • Is the molecular ion even? • Yes, there must be either 0, 2, 4 … Nitrogen atoms • How many Carbon atoms are there? • # Carbons = 5.6 / 1.1 ≈ 5 carbon atoms • if there are 2 N atoms then the FW would be (5*12) + (2*14) = 88 • Therefore, there are no nitrogen atoms • Is a S, Cl or Br present? • A M++2 peak of .4% indicates no S, Cl or Br • How many Hydrogen atoms are there? • 86 = (5 * 12) + (H * 1) • 86 = 60 + H • # of Hydrogen atoms = 26 • C5H26 ~not possible~ • What if there is Oxygen in the molecule? • 86 = (1 * 16) + (5 * 12) + (H * 1) • 86 = 76 + H • # of Hydrogen atoms = 10 8

  10. Determination ofMolecular Formula distinguish between compounds of same MW C5H10O4 or C10H14

  11. Determination ofMolecular Formula distinguish between compounds of same MW C5H10O4 13C 5 * 1.11% = 5.55% 2H 10 * 0.016% = 0.16% 17O 4 * 0.04% = 0.16% ------- 135peak/134peak 5.87%

  12. Determination ofMolecular Formula distinguish between compounds of same MW C10H14 13C 10 * 1.11% = 11.1% 2H 14 * 0.016% = 0.22% ------- 135peak/134peak 11.32%

  13. The Numbers Approach • If compound with formula CwHxNyOz , relative intensities of M, M+1, and M+2 ions will be given by:

  14. High-Resolution Mass Spectrometry • Low-resolution mass spectrometers measure m/z values to the nearest whole number • High-resolution mass spectrometers measure m/z values to three or four decimal places • The high accuracy of the molecular weight calculation allows accurate determination of the molecular formula of a fragment • Example • One can accurately pick the molecular formula of a fragment with a nominal molecular weight of 32 using high-resolution MS 9

  15. The exact mass of certain nuclides is shown below 10

  16. http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htmhttp://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htm

  17. Fragmentation by Cleavage at a Single Bond • Cleavage of a radical cation gives a radical and a cation but only the cation is observable by MS • In general the fragmentation proceeds to give mainly the most stable carbocation • In the spectrum of propane the peak at 29 is the base peak (most abundant) 100% and the peak at 15 is 5.6% 11

  18. Fragmentation Equations • The M+. Ion is formed by loss of one of its most loosely held electrons • If nonbonding electron pairs or pi electrons are present, an electron from one of these locations is usually lost by electron impact to form M+. • In molecules with only C-C and C-H bonds, the location of the lone electron cannot be predicted and the formula is written to reflect this using brackets 12

  19. Example: The spectrum of hexane 13

  20. Example: spectrum of neopentane • Fragmentation of neopentane shows the propensity of cleavage to occur at a branch point leading to a relatively stable carbocation • The formation of the 3o carbocation is so favored that almost no molecular ion is detected 14

  21. Carbocations stabilized by resonance are also formed preferentially • Alkenes fragment to give resonance-stabilized allylic carbocations • Carbon-carbon bonds next to an atom with an unshared electron pair break readily to yield a resonance stabilized carbocation • Z=N, O, or S R may be H 15

  22. Carbon-carbon bonds next to carbonyl groups fragment readily to yield resonance stabilized acylium ions 16

  23. Alkyl substituted benzenes often lose a hydrogen or alkyl group to yield the relatively stable tropylium ion • Other substituted benzenes usually lose their substitutents to yield a phenyl cation 17

  24. Fragmentation by Cleavage of 2 Bonds • The products are a new radical cation and a neutral molecule • Alcohols usually show an M+.-18 peak from loss of water 18

  25. Cycloalkenes can undergo a retro-Diels Alder reaction (section 13.11) to yield an alkadienyl radical cation 19

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