Mass spectrometry is used to determine the relative atomic mass (\(A_r\)) of an element from its isotopic composition.
Interpreting the Spectrum
- x-axis: Mass-to-charge ratio (m/z). Since z is usually +1, this is effectively the Isotopic Mass.
- y-axis: % Abundance or Relative Intensity.
- Number of peaks: Tells you the number of isotopes.
- Height of peak: Tells you the abundance.
Calculating Relative Atomic Mass
Problem: Calculate the relative atomic mass of Chlorine using the spectrum data: ³⁵Cl (75%) and ³⁷Cl (25%).
1. Formula:
\( A_r = \frac{\sum (\text{mass} \times \text{abundance})}{100} \)
2. Substitute:
\( A_r = \frac{(35 \times 75) + (37 \times 25)}{100} \)
\( A_r = \frac{2625 + 925}{100} \)
3. Solve:
\( A_r = \frac{3550}{100} = 35.50 \)
Exam Tip: Diatomic Molecules
For diatomic elements like \(Cl_2\), you will also see peaks for the molecular ions \(M^+\) at higher
masses:
\( ^{35}Cl\text{-}^{35}Cl \) (m/z 70), \( ^{35}Cl\text{-}^{37}Cl \) (m/z 72), and \(
^{37}Cl\text{-}^{37}Cl \) (m/z 74).