A Level Chemistry

Electron impact ionisation

• A Vaporised sample is injected at low pressure


• An electron gun fires high energy electrons at the sample

• This Knocks out an outer electron


• Forming positive ions with different charges

E.g. Ti(g) è Ti+(g) + e–

Acceleration

• Positive ions are accelerated by an electric field

• To a constant kinetic energy

• KE = kinetic energy of particle (J)

• m = mass of the particle (kg)

• v = velocity of the particle (ms–1)  ­

KE = ½ mv2

Ion Drift

Ions with a smaller mass will move with a higher velocity than those with a larger mass.  This is because all ions are accelerated

t = d/v

• t = time of flight (s)


• d = length of flight tube (m)

• v = velocity of the particle (m s–1)

Detection

• The ions hit the detector and gain an electron

• This causes a charge to flow

• The size of the current is proportional to the abundance of the ion

This is recorded by the computer