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Electromagnetic Mass Spec

Just 0.1 -1.0 mg of solid or liquid sample is required.

  1. Vaporisation
    The sample is heated to produce a vapour of atoms or molecules.
  2. IONISATION & ACCELERATION
    An electron is knocked off the atoms or molecules using high speed electrons 
    Be (g) + e–  →  Be+ (g) + 2e-
    C2H5OH (g) + e–  → C2H5OH+ (g) + 2e-
    Now that positive ions have been formed, it is possible to accelerate them using an electric field. A narrow beam of positive ions pass through a slit (marked 6)  in a negative plate. They enter a magnetic field and all possess the same kinetic energy. 
  3. DEFLECTION
    Lighter ions are deflected most by the magnetic field, so ions of different mass are separated – registering in different locations on the detection plate.
  4. DETECTION
    The positive ions strike a detection plate at the end of the magnetic field. Each  ion takes an electron from the detection plate and electrons flow in to replace them. These events produce a tiny electric current that can be amplified to form peaks on a spectrum. A peak will be twice as intense if there are twice as many ions of a particular mass to charge ratio.

See my mass spectra presentations below.

  1. Chlorine
  2. Ethanol
  3. Ethanoic Acid
  4. 1-chloropropane
  5. Propanal vs Propanone
  6. Methyl ethanoate

AQA Question.
The mass spectrum of the isotopes of element X is shown in the diagram.

(a) Define the term relative atomic mass .
(b) Use data from the diagram to calculate the relative atomic mass of X 
     Give your answer to one decimal place.
(c) Identify the ion responsible for the peak at 72
(d) Identify which one of the isotopes of X is deflected the most in
     the magnetic field of a mass spectrometer.
     Give a reason for your answer.
(e) In a mass spectrometer, the relative abundance of each isotope is
     proportional to the current generated by that isotope at the detector.
     Explain how this current is generated.
(f) X and Zn are different elements.
    Explain why the chemical properties of 70X and 70Zn are different.

Answer. (a)
Average/mean mass of 1 atom (of an element)
1/12 mass of one atom of 12C
OR
Average/mean mass of atoms of an element
1/12 mass of one atom of 12C
OR others
(b) [(70×3) + (72×4) + 73 + (74×5)] / 13 = 941 = 72.4
(c) 72Ge + or germanium +
(d) 70
      Lowest mass / lowest m/z
(e) Electron(s) transferred / flow (at the detector) 
     (From detector / plate) to the (+) ion
(f) They do not have the same electron configuration / they
     have different number of electrons (in the outer shell)

Calculating more Relative Atomic Mass (Ar) from mass spectra. Click here.

Examination Questions

iQuiz

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