Mass spectrometry

Question 1

What does the mass spectrometer do?

Answer 1

Determines all the isotopes present in a sample of an element and to therefore identify elements

Question 2

Why must it be in a vacuum?

Answer 2

Air particles would ionise and register on the detector

Question 3

What is the first stage of the mass spectrometer?

Answer 3

Ionisation - electrospray or electron impact

Question 4

Outline the process of electrospray ionisation

Answer 4

Sample dissolved in a volatile, polar solvent. Then injected through a fine needle at high pressure (fine mist or aerosol). High voltage is applied, causing each particle to gain an H+ ion (proton)

The sample is turned into a gas made up of positive ions

Question 5

What is the equation for electrospray ionisation?

Answer 5

X(g) + H+ –> XH+(g)

Question 6

Outline the process of electron impact ionisation

Answer 6

Sample vaporised and an electron gun fires high speed electrons at it.

This knocks one electron off each particle, so they become +1 ions

Question 7

What is the equation for electron impact ionisation?

Answer 7

X(g) –> X+(g) + e-

Question 8

How are ions accelerated?

Answer 8

Positively charged ions are accelerated by a negatively charged plate so they all have the same kinetic energy (lighter ions will move faster than heavier ions)

Question 9

What happens during ion drift?

Answer 9

Ions enter a region with no electric field, so they drift through it.

Lighter ions drift faster than heavier ions

Question 10

How are ions detected?

Answer 10

As lighter ions travel at higher speeds in drift region, they reach the detector faster than heavier ions.

Ions hit a metal plate (detector) and gain an electron, causing an electrical current that’s recorded and plotted on a mass spectrum

Question 11

How is a mass spectrum plotted?

Answer 11

Mass/charge on the x axis, abundance on the y axis