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Chemistry Stoody Guide

Mass Spectrometry

  1. Describe the operation of mass spectrometer. Identify elements and perform calculations involving a mass spectrum and average atomic mass.

Spectrometer (How It Works)

  1. Atoms are ionized
  2. Ionized atoms are shot through a narrow tube, with each atom having the same KE
  3. Since the KE is the same, while the different isotopes are different, different isotopes travel at different veolcities, allowing them to be separated.

spectrometer

Reading/Analyzing Specetrometer Readings

  1. Estimating Average AMU:

    • For each mass point, multiply it by the relative abundance (i.e. if some 90 M/Z has an abundance of 50%, multiply 90*50%).
    • Add all the values together and divide by the sum of the relative abundances, to get the average AMU.

  2. Identifying Elements:

    • Find the average AMU
    • The element with the closest AMU to the AMU you just found is probably the element

  3. M/Z:

    • Mass to Change ratio
    • Occurs on the x-axis because relative abundance depends on M/Z, and it seperates isotopes by mass.

AMU to % Composition

Ex: It is found that a sample of uranium from a power plant in Germany has average atomic mass of 236.8 AMU. Find percentage composition of U235 and U238.

Note that % comp of both elements must add up to 1 (or 100%). Thus, assign one of the elements, suppose U235, to be x. Then, U238 is 1-x.

Since U235 and U238 have masses of 235 and 238 AMU, respectively, the average atomic mass is the weighted average of the two isotopes. Thus, the equation is:

Solving, we get that x = 0.4, and thus U235 is 40% and U238 is 60%.