Time of Flight Mass Spectrometry (AQA A Level Chemistry): Revision Note

Time of Flight (TOF) Mass Spectrometry

• Mass spectrometry is a powerful analytical technique

• It is the most useful instrument for the accurate determination of:

  • The relative atomic mass (A_r) of an element

  • The relative molecular mass (M_r) of a molecule

• As a sample passes through the mass spectrometer, a spectrum is produced

  • The spectrum plots the abundance of ions against their mass-to-charge ratio (m/z)

• The peak with the highest m/z value is often the molecular ion peak (M⁺)

• The tallest peak in the spectrum is called the base peak

Principles of TOF mass spectrometry

• The entire apparatus is kept under a high vacuum to prevent ions from colliding with air molecules.

• There are 4 key stages:

  • Ionisation

  • Acceleration

  • Ion drift

  • Detection

Ionisation

• The sample is converted into positive ions.

• The two main methods are electron impact and electrospray ionisation

Electron impact:

• This is used for elements and low-mass compounds

• High-energy electrons bombard the vaporised sample from an electron gun

• The high-energy electrons knock an electron off each particle to form a 1+ ion:

X (g) → X⁺ (g) + e^-

• The 1+ ions formed are called molecular ions (M⁺)

• This high-energy process can also cause the M⁺ ion to break into smaller pieces, known as fragments

  • Fragment ions also pass through a TOF mass spectrometer and appear on the final mass spectrum

Electrospray ionisation (ESI):

• This is used for higher-mass compounds (like proteins) to prevent fragmentation

• This is a 'soft ionisation' technique

• The sample is dissolved in a volatile solvent and injected through a high-voltage needle

• This causes the particles to gain a proton (H⁺) from the solvent:

M (g) + H⁺ → MH⁺ (g)

Acceleration

• The positive ions are:

  • Attracted towards a negatively charged plate

  • Accelerated by an electric field

• The key principle is that all ions are accelerated to have the same kinetic energy (KE)

• Since KE = ½mv²:

  • Ions with a lower mass have a higher velocity

  • Ions with a higher mass have a lower velocity

Ion drift

• The accelerated ions pass through a hole in the negatively charged plate and travel down a flight tube

  • The flight tube is a region with no electric field

• The time it takes for an ion to travel this distance is its time of flight

• Ions with a lower mass-to-charge ratio (m/z):

  • Travel faster

  • Have a shorter time of flight

Detection

• The ions arrive at a detector (an electron multiplier)

• When an ion hits the detector, it gains an electron, which generates a small electric current

• The size of this current is directly proportional to the abundance of that specific ion

• A computer records the time of flight and relative abundance for each ion to produce the mass spectrum

Example mass spectrum of boron

Key calculations for TOF mass spectrometry

• TOF mass spectrometry uses two key equations:

1. Kinetic energy:

KE = ½mv²

• Where:

  • KE = kinetic energy of the particles (J)

  • m = mass of the particles (kg)

  • v = velocity of the particles (ms^-1)

2. Velocity:

v =

• Where:

  • v = velocity of the particles (ms^-1)

  • d = the length of the flight tube (m)

  • t = time of flight of the particles (s)

• The kinetic energy and velocity equations are often combined

• KE = ½mv² rearranges to:

v =

• v = rearranges to:

t =

• Substituting v into the t = equation gives:

t = d

• Unit conversions are crucial:

  • The mass (m) of the ion must be in kilograms (kg)

  • The length of the flight tube (d) must be in metres (m)

• Calculating the mass of a single ion:

  • To convert the relative isotopic mass to the correct mass in kg:

    1. Divide the relative isotopic mass by the Avogadro constant (L) (6.022 x 10²³)

    2. Divide the result by 1000 to convert it from grams to kilograms