Atomic Structure: Interpreting Mass Spectra

Question 1

What is a mass spectrum?

Answer 1

• A mass spectrum is a type of chart produced by a mass spectrometer
• Shows information about the sample that was passed through the mass spectrometer

Question 2

What does a mass spectrum tell us?

Answer 2

• If sample is an element, each line will represent a different isotope of each element
• Y-axis gives the abundance of ions, often as a %
• The height of each peak gives the relative isotopic abundance (relative amount of each isotope present in a sample)
• Line with highest peak shows that that isotope appears in the sample more than any other isotope

• X-axis units are given as a ‘mass/charge’ ratio

Question 3

What does the m/z ratio tell us about the relative mass of an isotope or a molecule?

Answer 3

• Assuming that z (charge of ion) equals 1

• If spectrum is produced using electron impact ionisation, the m/z ratio of a peak is equal to that particle’s relative mass
- Relative atomic mass doesn’t change using electron impact (electron removed) and when divided by 1 it would equal the same number

• If spectrum is produced using electrospray ionisation, the m/z ratio of a peak needs to be subtracted by 1 to gain the relative mass
- Electrospray ionisation increases relative mass by 1 by adding a H+ ion, so m/z must be subtracted by 1 to reverse this change

Question 4

Mass Spectra for Diatomic Molecules

Answer 4

• The mass spectrum for Cl2, O2 and H2 and other diatomic molecule will contain peaks for both the separate ions and the molecules
• Peaks with lower m/z ratio, may be single ions or may be diatomic molecules made of elements with isotopes of a lower relative isotopic mass

Question 5

Calculating Relative Atomic Mass

Answer 5

𝑨_𝐫= ∑(mass of isotope × relative abundances) / (sum of the abundances)
- ∑ = sum of