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Organic Chemistry

Organic Chemistry Question of the Day

Practice Organic Chemistry with the production-style question-of-the-day selection for this public URL.

Question 1

Why is the mass-to-charge ratio (m/z) discerned in the mass spectrometer representative of the molecular mass of the molecular ion, or its fragments?

  1. The charge of the molecular ion is most likely to be +1 because of the ionization process by bombardment of single electrons. This charge of +1 is maintained in all the reactions wherein the molecular ion spontaneously fragments, so the mass / charge ratio of each molecular ion or molecular ion fragment corresponds to mass/1 which is equivalent to the mass itself.
  2. Because the charge of the molecular fragments is always 0 (neutral).
  3. The mass to charge ratio reflects the mass of each fragment since when the fragment is larger, the charge will be larger, and the ratio will come out to be the mass itself.
  4. The mass to charge ratio reflects the mass because the mass of an electron is negligible.
  5. The mass to charge ratio reflects the mass because the molecular ion and fragments thereof are being passed through an oxidation potential which neutralizes the charge caused by the ionization chamber.
Explanation: The correct answer is "The charge of the molecular ion is most likely to be +1 because of the ionization process by bombardment of single electrons. This charge of +1 is maintained in all the reactions wherein the molecular ion spontaneously fragments, so the mass / charge ratio of each molecular ion or molecular ion fragment corresponds to mass / 1 which is equivalent to the mass itself." Conservation of charge results in all fragments from the original molecular ion maintaining the +1 charge. A number divided by 1 is the same number, therefore m/z is representative of m when z = 1. Incorrect answers: The statement "Because the charge of the molecular fragments is always 0 (neutral)" is false because a) the fragments have a charge of +1 due to being ionized in the ionization chamber, and b) if z = 0, then the m/z quotient would not compute to m (it would be undefined). The statement "The mass to charge ratio reflects the mass of each fragment since when the fragment is larger, the charge will be larger, and the ratio will come out to be the mass itself" is incorrect because the charge does not vary based on molecular size, but rather is a result of spontaneous homolytic cleavage of the parent cation and therefore always maintains the charge of +1 (it is thermodynamically unlikely for a cation to be ionized once further while still in the ionization chamber where electron bombardment occurs). The statement "The mass to charge ratio reflects the mass because the mass of an electron is negligible" is not an accurate logical consequent -- the mass of an electron is very small compared to the mass of a proton, but that has nothing to do with the mass of the molecular ion itself or its charge (which is +1). The statement "The mass to charge ratio reflects the mass because the molecular ion and fragments thereof are being passed through an oxidation potential which neutralizes the charge caused by the ionization chamber" is false because a) the ions are not being passed through an oxidation potential but an electric field, and b) if the charge were neutralized, z would be 0, which would result in m/z being undefined, which would not be experimentally useful.