TOF mass spectrometer

Physical chemistry - mass number and isotopes

What is mass spectrometry used for?

To determine relative atomic mass (Ar) or relative molecular mass (Mr)

What are the main steps in a mass spectrometer?

1. Ionisation

2. Acceleration

3. Ion drift

4. Detection

What is TOF mass spectrometry carried out in and why?

A vacuum to prevent ions colliding with molecules in the air

Why is it important for the substance to be ionised?

So it can be accelerated by the electric field

What are the two different ways an ion can be formed in a TOF mass spectrometer?

Electrospray ionisation

Electron impact ionisation

What happens in electrospray ionisation?

Sample is dissolved in a volatile polar solvent

Sample is then passed through a fine needle connected to a high voltage supply

At the tip of the needle, the sample, M, gains a proton, H+, from the solvent forming MH+

e.g. M(g) + H+ —> MH+

What happens in electron impact ionisation?

Vaporised sample is injected at low pressure

Electron gun fires high energy electrons at sample

This knocks out an outer electron

Which forms positive ions with different charges

e.g. Ti(g) —> Ti+(g) + e-

What type of substance is electron impact used for?

For elements and substances with a low formula mass

What can electron impact cause?

Fragmentation

What type of substance is electrospray ionisation used for?

Larger molecules

Doesn’t cause fragmentation

What happens during acceleration?

Positive ions are accelerated by an electric field towards a negatively charged plate

All ions reach a constant kinetic energy

What does the velocity of each particle depend on and why?

The mass of the particle because all particles have the same kinetic energy

So, use KE = ½ mv²

What are the units for KE = ½ mv² ?

KE = (J) kinetic energy of particle

m = (kg) mass of particle

v = (ms^-1) velocity of particle

What is the velocity like for lighter and heavier particles?

Lighter particles have a faster velocity

Heavier particles have a slower velocity

What happens during ion drift?

Heavier particles take longer to move

Ions are distinguished by different flight times:

t = d/v

What are the units for t=d/v ?

t = (s) time of flight

d = (m) length of flight tube

v = (ms^-1) velocity of the particle

What happens during detection?

Ions reach detector and generates a current

Which is fed to computer for analysis

How is the current produced?

By electrons transferring from detector to positive ions

What determines the size of the current?

Current is proportional to abundance of species