Atomic Structure Flashcards
How does a flight mass spectrometer work?
(1) Vacuum
- entire machine is a vacuum to prevent any particles being tested colliding with molecules in the air
(2) Ionisation
- two methods of ionisation: electron impact and electron spray ionisation
(3) Acceleration
- positive ions are attracted to a negatively charged plate & accelerate towards it, once accelerated all ions have the same KE
(4) ion drift
- some ions will pass through hole in negatively charged plate - form a beam of particles & travel along ‘flight tube’ towards detector - particles are travelling at different speeds so drift apart
(5) - detection
- as each ion hits detector gains an electron - generates a current, the size of which is proportional to the number of each type of ion
(6) - data analysis
- signal from detector is passed to a computer which generates a mass spectrum
Why are the sample particles ionised?
- so they can be accelerated towards negatively charged plate
- so they generate a current when they hit the detector
How is the ion accelerated?
- positive ions attracted to negatively charged plate
- all ions have the same KE
How are ions separated in the flight tube?
- ions travelling at higher speeds move ahead of those travelling slowly
How are the ions detected?
- each ions hit the detector
- ion gains an electron
- generates a current
- size of current is proportional to the abundance of the ion
Describe electron impact ionisation
- sample being analysed is vaporised and then high energy electrons are fired it
- high energy electrons come from an ‘electron gun’
- this knocks off one electron from each particle forming 1+ ions
Describe electron spray ionisation
- sample is dissolved in volatile solvent & injected through a fine hypodermic needle to give a fine mist
- tip of the needle is attached to positive terminal of a high voltage power supply
- particles are ionised by gaining a proton
