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Mass Spectrometry (MS)

Mass Spectrometry (MS). In MS a molecule is vaporized and ionized by bombardment with a beam of high-energy electrons. E = 1600 kcal (or 70 eV). C-C BDE = 100 kcal. MS principles and terms. “molecular ion” fragments

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Mass Spectrometry (MS)

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  1. Mass Spectrometry (MS) • In MS a molecule is vaporized and ionized by bombardment with a beam of high-energy electrons. E = 1600 kcal (or 70 eV). C-C BDE = 100 kcal

  2. MS principles and terms • “molecular ion” fragments • The mass spectrometer separates the cations based on their mass to charge ratio (m/z). • A mass spectrum is a plot of the amount of each cation (its abundance) versus m/z. • z is almost always +1 so m/z represents the mass.

  3. An Example Mass Spectrum • The most abundant peak is called the base peak and assigned a relative abundance of 100%. • The highest m/z value peak is usually the M+. Peak • M + 1 peak. 12C = 98.9%, 13C = 1.1%. For a molecule having n carbons the probability of one 13C is n x 1.1%.

  4. Alkyl Halides • Most common atoms (C, H, O, N, I, P) are monoisotopic. • Chlorine is diisotopic: 35Cl = 75% and 37Cl = 25%. • Thus, the molecular ion of an alkyl chloride shows two peaks separated by two m/z units in a 3:1 ratio. • The larger peak corresponds to molecules of the compound containing the 35Cl. The smaller peak corresponds to the molecules containing 37Cl. • Br is also diisotopic. 79Br and 81Br occur in a ratio of ~1:1. • Thus, the molecular ion of an alkyl bromide shows two peaks separated by two m/z units in a 1:1 ratio. • We can use these facts to confirm or disprove the presence of Cl or Br in a compound.

  5. An example alkyl chloride

  6. Lit Spectrum This is the MS of 2-chloropropane off from the ISDBS website. Note the peak at m/z = 65!

  7. An example alkyl bromide

  8. High Resolution MS (HRMS) • Low resolution mass spectrometers report m/z values to the nearest whole number. High resolution mass spectrometers measure m/z ratios to four (or more) decimal places. • This is valuable because no two molecular formulas have the same exact molecular weight. • Exact MWs can be calculated using exact isotope masses. Table 14.1 • Thus HRMS is a way to determine chemical formula!

  9. GC-MS

  10. Interpretation of fragmentation patterns. Example C8H18 isomers MW 114 Look for ways bond cleavage can occur to give stable carbocations. Calculate mass of neutral fragments lost. (Focus on right side of spectrum.)

  11. Example RBr Spectrum

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