Atomic Structure
Time of Flight Mass Spectrometry
OxfordAQA International AS and A-level Chemistry
The instrument
- The whole instrument is kept under vacuum.
- It measures the mass of each isotope in the sample and the abundance of each isotope.
- Mass spectrometry can be used to identify an element, and to determine a relative molecular mass.
Never answer to remove the air on its own. The reason is what the air would otherwise do: the ions would collide with particles in the air and so would not move in a straight line. Particles in the air could also be ionised themselves and produce a peak of their own.
Ionisation
The sample is ionised by electron impact. Each atom loses one electron and becomes a 1+ ion.
The atom and the ion are both gases, so write (g) after each of them, every time.
Why the sample must be ionised
- Ions, not molecules, will interact with and be accelerated by an electric field. Never write a magnetic field.
- Only ions will create a current when hitting the detector.
Acceleration
- The ions are accelerated by an electric field, by attraction towards a negatively charged plate.
- Every ion is given the same kinetic energy, so ions of different mass leave the plates at different speeds. Never write that they leave with the same speed.
Ion drift
There is no electric field in the flight tube. The ions drift along it and separate as they go. m/z is the ratio of the mass of an ion to its charge.
- Two isotopes of one element have the same kinetic energy, but the heavier ion has a lower velocity because it is heavier.
- Put the result as a comparison of the two ions: the lighter one has a shorter time of flight than the heavier one.
- Every ion here is 1+, so the m/z of a peak is the mass of that ion.
Ion detection
- Ions hit the detector and accept electrons causing current to flow.
- The size of the current gives the abundance: bigger current = higher abundance of that ion.
Data analysis
- Each isotope gives one bar, at its own m/z, and the height of that bar is the relative abundance of that isotope in the sample.
- A relative atomic mass measured this way can differ from the published value for the element because the sample has different isotopes or different abundances of isotopes.
The calculations
The five quantities are the mass of one ion m, its kinetic energy KE, the velocity of the ion v, the length d of the flight tube and the time of flight t. Two relations link them.
Learn v = d / t and write it out in full whenever a time of flight or a tube length is involved.
The mass of one ion
Work in kilograms, so that the kinetic energy comes out in joules. The mass of one ion is the mass number divided by the Avogadro constant and by 1000, with L for the Avogadro constant.
Going the other way, a mass in kilograms is multiplied by 1000 and by the Avogadro constant to give the mass of one mole, and the mass number is read off that.
Dividing the mass number by the Avogadro constant alone gives the mass of one ion in grams. Divide by 1000 as well, and never allow a mass left in grams being substituted into the kinetic energy relation: it needs the mass in kilograms, and the factor of 1000 is the one that gets left out.
Rearranging
The relations are used in every direction, so rearrange them rather than learning one route. A tube length given in centimetres is converted to metres first.
| To find | Write |
|---|---|
| v, from KE and m | v2 = 2KE / m, then v = √(2KE / m) |
| m, from KE and v | m = 2KE / v2 |
| KE, from m, d and t | KE = d2m / 2t2 |
| d, from m, KE and t | d = t√(2KE) / √m |
| t, from m, KE and d | t = d√(m / 2KE) |
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