Time of flight mass spectrometry

What is mass spectrometry used for?

Instrumental method of analysis used to find abundance and mass of each isotope in an element/ relative molecular mass of substances made of molecules

Conditions needed for ToFMS

High vacuum to prevent ions produced colliding with molecules present in air (would register on detector)

Quick summary

• particles of substance ionised to 1+ ions
• ions accelerated so all have same kinetic energy
• time taken to travel fixed distance used to find mass of each ion

Stages

1- Ionisation
2- Acceleration
3- Flight Tube
4- Detection

What are the two techniques of ionisation?

Electron impact and electrospray ionisation

What happens in electron impact?

• sample vaporised and injected at low pressure
• electron gun fires high energy electrons at sample- knocks off an outer electron
• forms positive ions

What type of substances is electron impact used for?

Low formula mass- could cause larger organic molecules to fragment

What happens in electro-spray ionisation?

• sample dissolved in a volatile, polar solvent
• then injected through a fine needle to produce a fine mist/aerosol
• tip of needle has a high voltage where sample is ionised by gaining a proton- H+ ion from solvent
  -solvent evaporates away whilst XH+ ions move to negative plate

What type of substances is electro-spay ionisation used for?

Larger organic molecules as is a soft ionise technique- no fragmentation

Why is ionisation necessary?

To allow ions to be:

• Accelerated by an electric field

• Deflected by a magnetic field

• Detected

2- Acceleration stage

Positive ions accelerated by electric field so all have same kinetic energy towards negatively charged plate

Velocity of ions

As all have same kinetic energy, velocity depends on mass
Lighter particles = faster velocity
Heavier particles = slower velocity

3- Flight tube stage

Positive ions pass though hole in negatively charged plate to negative detection plate

4- Detection stage

At negatively charged detection plate positive ions gain an electron and produce a current (discharged when gain electrons)

Current produced

Is caused by the flow of electrons from plate to ions
Amount of current produced is proportional to abundance of species

Data analysis

The current produced an time of flight are analysed to produce a spectra with the relative abundance produced by ions with varying mass/charge (m/z) ratios

Explain how the detector in a mass spectrometer enables the abundance of an isotope to be measured

Ions hit detector and gain electrons producing a current

Current is proportional to abundance

State how mass spectrometry can be used to identify elements

The calculated RAM can be compared to known data