Mass spectrometer – isotopes and relative atomic mass

What it shows

Most elements are mixtures of isotopes: atoms with the same number of protons but different numbers of neutrons. In a magnetic-sector mass spectrometer, atoms are ionised by an electron beam, accelerated through 2 kV and bent by a magnetic field. Each ion follows a circle of radius r = √(2mV/e)/B, so heavier ions are deflected less and land further along the detector. The height of each peak gives the relative abundance of that isotope. The relative atomic mass is the weighted mean: Ar = Σ(m/z × abundance)/100, for example 35.48 for chlorine from the mass numbers 35 and 37.

How to use

Choose an element button or Custom mix and watch ions travel from 1. Ionisation through 2. Acceleration to 3. Deflection. Press +1000 ions to let the measured bars settle on the natural abundances. Move the Magnetic field B slider, or press Auto-set B, so that every peak lands on the detector. The table sets out the Ar calculation; the quiz option hides it so you can type your answer in Your Ar and press Check.

Parameters you can change

• Element Boron (B), Neon (Ne), Magnesium (Mg), Chlorine (Cl), Copper (Cu), Bromine (Br), Custom isotope mix
• Magnetic field B 0.1–1 T
• Sample rate 5–200 ions/s
• Students calculate (hide answer)
• Custom mix: mass number of isotope 1 1–250
• Custom mix: abundance of isotope 1 0–100 %
• Custom mix: mass number of isotope 2 1–250
• Custom mix: abundance of isotope 2 0–100 %
• Custom mix: mass number of isotope 3 1–250
• Custom mix: abundance of isotope 3 0–100 %

Questions to explore

1. Why is the relative atomic mass of chlorine 35.45 rather than a whole number?
2. If you decrease the magnetic field B, which way do the peaks move on the detector, and why?
3. How far do the measured abundances stray from the natural values when only a few ions are counted?