Percent Abundance and Mass Spectrometry

Percent Abundance

• Percent abundance indicates the percentage of each isotope in a sample of an element.
• Example: In a sample of boron, if there are two isotopes, the percent abundance might show 80% of one isotope and 20% of another.

Determining Percent Abundance

• Tool Used: Mass spectrometer (Mass spectroscopy)
  • Analytical technique that distinguishes particles based on their charge-to-mass ratio
  • Consists of several stages: ionization, acceleration, and deflection through a magnetic field

Process in Mass Spectrometer

1. Ionization: Formation of Cations: (Region 1)
   • A gas sample is introduced into the chamber
   • The sample is hit with an electron beam, causing the formation of positively charged ions
2. Acceleration: (Region 2)
   • The cations are accelerated to high speeds
3. Deflection by Magnetic Field
   • Particles are deflected based on their charge-to-mass ratio
   • Larger particles are deflected less; smaller particles are deflected more

Bowling Analogy for Deflection

• Large Bowling Ball: Less deflection by crosswind
• Ping Pong Ball: More deflection by the same crosswind
• In a mass spectrometer, a magnetic field replaces the crosswind:
  • Smaller particles deflect more
  • Larger particles deflect less

Detection and Interpretation

• Detectors measure the number of particles hitting specific spots
• Relative abundance (%) is determined:
  • Example Abundance Data: 60% at one spot, 30% at another, 10% at another

Example Data Interpretation

• Tables of Percent Abundance: Provide stable isotopes and their relative abundances
• Sample Elements: Hydrogen, Carbon, Oxygen
  • Carbon Isotopes:
    • Carbon-12: Major isotope
    • Carbon-13: 1.11% abundance
  • Oxygen Isotopes:
    • Oxygen-16: 99.76% abundance
    • Oxygen-17: Trace amounts (very small, nearly negligible)
    • Oxygen-18: 2% abundance