Mass spectrometry

Question 1

Vacuum

Answer 1

The whole apparatus is kept under a high vacuum to prevent the ions that are produced from colliding with molecules in the air

Question 2

Electrospray ionisation

Answer 2

The sample is dissolved in a volatile solvent.

It is then injected through a fine, hypodermic needle that is connected to the positive terminal of a high-voltage supply.

This produces tiny positively charged droplets that have gained a proton from the solvent.

Question 3

Electron impact ionisation

Answer 3

The sample is vaporised.

High-energy electrons from an electron gun are fired at the sample.

This knocks off one electron from each particle, forming 1+ ions.

May cause fragmentation (2+ ions)

Question 4

Acceleration

Answer 4

The positive 1+ ions are accelerated by an electric field.

They all have the same kinetic energy, therefore velocity will depend on mass.

Lighter ions will move faster.

Question 5

Ion drift

Answer 5

The ions pass through a hole in the negatively charged plate and move into the flight tube.

The “time of flight” of each 1+ ion depends on their velocity.

Question 6

Detection

Answer 6

When ions hit the negatively charged detector plate, they gain an electron.

This discharges the ion and causes a current to be produced.

This size of the current is proportional to the abundance of those ions hitting the plate and gaining an electron.