Analysis of gaseous bromine vapourised in a mass spectrometer shows that it consists of 51% of Br-79 and 49% of Br-81...?

  • In the mass spectrometer ionizing process, what charge will each bromine atom have?
  • Sketch a mass spectrogram for the vapourised bromine atoms
    • Explain how many peaks it has and why.
    • What would be the mass value for each peak?

1 Answer
Jul 21, 2018
  • Preferably +1 for all atoms.
  • Five discrete peaks, one for each isotopic atom, and one for each isotopic molecule.
  • Percent abundance is indicated by their relative heights.

https://upload.wikimedia.org/https://upload.wikimedia.org/

Explanation:

The ionizer in an "electron impact" mass spectrometer would knock an electron off of each bromine atom.

It would preferably remove only one electron from each atom because it is rare that the same particle gets ionized twice.

Ions with a single positive charge can be directly sorted from other ions as they pass through an electric or magnetic field based on their molar mass, as it gives a different m//z ratio.

Two isotopes of "Br" can make up to three "Br"_2 molecules of unique mass numbers. Each of the three molecules corresponds to a separate peak in the mass spectrometric graph; the probability for a random "Br"_2 molecule to be of a particular makeup is determined by the binomial distribution.

  • Two color(white)(.)_79"Br" make a "Br"_2 molecule of mass number 158,
    which is of mass 2 * 78.92 ~~ 157.84 color(white)(l) g * mol^(-1)
    This configuration is found in p(79)^2 ~~ 26% of all "Br"_2 molecules

  • One color(white)(.)_79"Br" and one color(white)(.)_81"Br" make a "Br"_2 molecule of mass number 160,
    which is of mass 78.92 + 80.92 ~~ 159.84 color(white)(l) g * mol^(-1)
    This configuration is found in 2 * (p(79) * p(81)) ~~ 50% of all "Br"_2 molecules

  • Two color(white)(.)_81"Br" makes a "Br"_2 molecule of mass number 162,
    which is of mass 2 * 80.92 ~~ 161.84 color(white)(l) g * mol^(-1),
    This configuration is found in p(81)^2 ~~ 24% of all "Br"_2 molecules

Reference
"Isotopes of Bromine," https://en.wikipedia.org/wiki/Isotopes_of_bromine