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3.1.1 Atomic Structure

Time of Flight Mass Spectrometry

AQA A-level Chemistry

The four stages

1 Ionisation 2 Acceleration 3 Flight tube 4 Detection heated filament sample + an electron is knocked out − − − negative plates NO ELECTRIC FIELD HERE heavy, slow light, fast detector (negative) current becomes a 1+ ion all gain the same KE lower m/z arrives first current gives abundance every ion has the same kinetic energy, so the lighter ion moves faster and reaches the detector first
Every label is wording to use in an answer. Nothing here is deflected by a magnetic field.

Ionisation by electron impact

High energy electrons from an electron gun are fired at the sample and knock out an electron from each atom or molecule, leaving a 1+ ion.

M(g) + e− → M+(g) + 2e−

Everything is in the gas phase, so write (g) after every species, every time.

The ion is the whole particle, charge 1+

Bromine gives Br2+, never Br+: the electron gun removes an electron, it does not split the molecule. Nor do particles gain electrons to form positive ions; here they lose one.

Ionisation by electrospray

The sample is dissolved in a volatile polar solvent such as water or methanol, and injected through a needle, nozzle or capillary at high voltage. Each molecule gains a proton, H+; never write atoms gain a proton.

M + H+ → MH+

No state symbols needed here.

Electron impact against electrospray

ELECTRON IMPACT heated filament fires high energy electrons sample M M M + one electron is knocked out to accelerating plates m/z = Mr for elements and small molecules can cause fragmentation ELECTROSPRAY solvent is sprayed from a needle at high voltage M + H+ MH + droplets shrink as solvent evaporates m/z = Mr + 1 for large or fragile molecules, proteins no fragmentation electron impact gives m/z = Mr. electrospray gives m/z = Mr + 1, so subtract 1. a peak at m/z 556 made by electrospray means Mr = 555
Two samples of equal Mr are separated only by the method used on them, and the smaller m/z has the shorter time of flight.
Electron impactElectrospray
What happenshigh energy electrons knock out an electron the molecule gains a proton
Ion formedM+ MH+
Peak positionm/z = Mr m/z = Mr + 1, so subtract 1
State symbols(g) on every species not needed
Used forelements and small molecules large or fragile molecules, proteins
Fragmentationpossibledoes not break up or fragment

Why the sample must be ionised

  • Ions, not molecules, are accelerated by an electric field, never a magnetic field.
  • Only ions create a current when they hit the detector.

Acceleration to a constant kinetic energy

  • Positive ions are accelerated by an electric field, attracted to a negatively charged plate, to a constant kinetic energy.
  • The kinetic energy of the heavier ion equals the kinetic energy of the lighter ion, so equal energies and different masses give different speeds.
SAME KINETIC ENERGY, DIFFERENT SPEEDBOTH IONSsamekineticenergy1+ ionslighter ion, lower m/zmoves fasterheavier ion, higher m/zmoves more slowlydetectorsame kinetic energy, so the lower m/z ion has the greater speed and arrives first
Same kinetic energy, different mass, different speed.

Which ion reaches the detector first

Write the chain in this order: same kinetic energy, so the ion with the lower m/z moves faster, so it arrives at the detector first. The last to arrive has the highest mass to charge ratio, so it travels the slowest.

Name the ion with the mass number and the charge, 58Ni+, never the bare symbol Ni. Every ion here is 1+, so m/z is the mass of the ion.

Detection

  1. Each ion hits the detector, a negative plate, never a positive one.
  2. The ion gains an electron, so a current is generated.
  3. The current is proportional to the abundance.

The current and the proportionality are both needed, so never stop at the detector counts the ions.

The calculations

KE = ½mv2 and the Avogadro constant are given. v = d / t must be recalled and written down.

v = √(2KE / m)      m = 2KE / v2

Every ion carries the same kinetic energy, so t is proportional to √m; between two isotopes, m1 / t12 = m2 / t22. After electrospray, subtract 1.

THE CALCULATION ROUTE MAPGIVEN d AND tGIVEN THE MASS NUMBERv = d / tm = 2KE / v² in kg× 1000 gives grams× L gives the mass of one moleround to a whole numberm = (mass number / 1000) / Lv² = 2KE / mv = √(2KE / m)t = d / v or d = v tconvert cm to mdivide by 1000 to get kg
Choose the branch by what you are given. The red flags are the two conversions that are easiest to forget.
The two unit conversions

Mass to kilograms. The mass of one ion is (mass number / 1000) / L; the division by 1000 is what gets left out.

Flight tube to metres. Lengths are given in centimetres, so convert to metres before using d / t.

The mass spectrum of a diatomic element

Two isotopes give three peaks: light with light, the mixed molecule, heavy with heavy. The middle peak is doubled, because the mixed molecule forms two ways. Equal abundances give 1 : 2 : 1, a 3 : 1 ratio gives 9 : 6 : 1.

A DIATOMIC ELEMENT GIVES THREE PEAKStwo isotopes in equal abundance025507510015879 + 7916079 + 81and 81 + 7916281 + 81x-axis: m/zrelative abundancetwo isotopes in a 3 : 1 ratio7035 + 357235 + 37and 37 + 357437 + 37heights 9 : 6 : 1heights 1 : 2 : 1, drawn as 25, 50, 25y-axis: relative abundance or %
Both axis labels go on before a single peak is drawn.
Method: sketching or completing the spectrum
  1. Label both axes: y is relative abundance or %, x is m/z.
  2. Put a peak at the sum of each pair of isotope masses.
  3. Heights follow the square of the abundance ratio, the mixed peak doubled: equal isotopes give 25, 50 and 25.

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