1.2 Mass Spectroscopy

Mass Spectroscopy of Elements

What is Mass Spectrometry?

• Mass Spectrometry is used for various applications:

  • Identifying particular elements in a sample.

  • Determining isotopes present in the sample.

  • Assessing the percentage of each isotope.

  • Calculating the average atomic mass of the element.

How Mass Spectrometry Works

1. Sample Preparation:

   • The sample is vaporized and exposed to a high-energy electron beam.

   • This exposure causes atoms or molecules to become electrically charged (typically losing electrons).

2. Ion Deflection:

   • The positive ions created pass through an electric or magnetic field.

   • The degree of deflection is influenced by the charge and mass of the ion:

     • Heavier ions deflect less than lighter ions.

3. Mass Spectrum Generation:

   • Ions are detected, leading to a graph illustrating the relative number of ions against their mass-to-charge (m/z) ratio.

   • This graph is known as a mass spectrum.

Example: Mass Spectrum of Zirconium

• The mass spectrum shows peaks correlating to the relative abundance of isotopes in a natural sample.

• The x-axis represents the mass-to-charge ratio (m/z), which can be equated to the atomic mass of the isotope (number of protons + neutrons).

• The y-axis indicates the relative abundance, reported as a percentage of the total sample, totaling 100%:

  • Zirconium Isotopes:

    • Zirconium-90: 51.5%

    • Zirconium-91: 11.2%

    • Zirconium-92: 17.1%

    • Zirconium-94: 17.4%

    • Zirconium-96: 2.8%

Calculating Average Atomic Mass

• Average atomic mass is calculated using the formula:

  • Average mass = (Relative abundance × Isotopic mass)

• Using Zirconium data:

  • Average atomic mass = (0.515 × 90) + (0.112 × 91) + (0.171 × 92) + (0.174 × 94) + (0.028 × 96) = 91.318,

  • Rounded to two significant figures: 91.

Variability in Mass Spectra

• Reporting Differences:

  • Mass spectra may not always mirror the example above:

    • Peaks may be in percentages totaling 100%.

    • The tallest peak could be given a value of “100”.

    • Peaks could represent absolute atom counts, common in rare elements.

Further Example of Zirconium

• Zirconium features five isotopes; the one with the highest abundance is Zirconium-90 at 90 amu.

Strategy for Analyzing Isotopes

Interpreting Relative Abundance

• In a mass spectrum, the tallest peak signifies the most abundant isotope,

  • Example:

    • Unknown Isotope 10 has a relative abundance of 23.

    • Unknown Isotope 11 has a relative abundance of 100.

Calculating Percentages

• Convert relative abundances into percentages:

  • Total = 100 + 23 = 123.

  • For Unknown Isotope 10: 23/123 = 18.699%

  • For Unknown Isotope 11: 100/123 = 81.301% (round at end).

Finding Average Atomic Mass

• Using the relative abundance:

  • Average Mass = (Relative abundance × Isotopic mass)

  • Example calculation yields:

  • Average mass = (0.18699 × 10) + (0.81301 × 11) = 10.81301 amu,

  • Rounded to three significant figures: 10.8 amu.

Element Identification

• To determine the element, find the atomic mass closest in the periodic table, e.g., Boron with an atomic mass of 10.81.