Mass spectrometry is used to determine the relative atomic mass (\(A_r\)) of an element from its isotopic composition.
Mass Spectrometer
Vary the magnetic field to detect isotopes
50
DETECTOR
ION
SOURCE
Isotope 35 (Light)
Isotope 37 (Heavy)
Detector IntensityMagnetic Field Strength (B)
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
Paper 2 StyleCalculate the relative atomic mass of Chlorine using the spectrum data:
³⁵Cl (75%) and ³⁷Cl (25%).
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:
\( ^{35}Cl-^{35}Cl \) (m/z 70), \( ^{35}Cl-^{37}Cl \) (m/z 72), and \( ^{37}Cl-^{37}Cl \) (m/z 74).
Student Practice Set
1.
Boron has two isotopes: B-10 (20%) and B-11 (80%). Calculate the Ar.