Time of Flight Mass Spectrometry Flashcards
What is a mass spectrometer?
A machine used to…
1) Find Mr of molecules in a sample
2) Determine the abundance and relative mass of each isotope in an element
How are mass spectrums used in the real world?
Drug tests in urine before sport events
Carbon dating
Drug swabs on luggage in the airport
What are the 5 stages of Time of Flight Mass Spectrometry?
1) Injection of sample
2) Ionisation
> Electron impact
> Electrospray
3) Acceleration
4) Drift
5) Detection
Describe Stages 1 & 2 of electron impact ionisation in time of flight mass spectrometry
STAGE 1
1) Sample molecules (M) injected into machine
2) Sample is vaporised
STAGE 2
3) Gaseous sample is passed through the path of an electron gun
4) Sample is BOMBARDED with high energy electrons
5) Molecules in sample form M⁺(g) ion
Electron impact is better for SMALL molecules
Why does the electron gun cause molecules to ionise in electron impact ionisation?
High energy electrons “knock” off one of the electrons in the sample.
What are the limitations of electron impact ionisation?
May cause FRAGMENTATION…
…parts of molecules may also be knocked off by high energy electrons
…such as methyl groups or side chains are often removed.
Write an equation for the electron impact ionisation of molecule X
X(g) + e⁻ ━━> X⁺(g) + 2e⁻
Describe Stages 1 & 2 of electrospray ionisation in time of flight mass spectrometry
STAGE 1
1) Sample molecules (M) dissolved in VOLATILE solvent
2) Injected through HYPODERMIC needle to give a fine aerosol
STAGE 2
3) Tip of needle is attached to the positive terminal of a high voltage POWER supply
4) Molecules ionised by gaining a PROTON from the solvent as they leave the needle
Electrospray ionisation is better for HIGH Mr compounds as it does not often cause fragmentation
What is meant by volatile?
Easily vaporised
What is meant by an aerosol?
Vapour - sprayed liquid
Write an equation for the electrospray ionisation of molecule X
X(g) + H⁺(g) ━━> XH⁺(g)
Why is electrospray ionisation known as a soft ionisation technique?
Limited fragmentation
What are the limitations of analysing data from electrospray time of flight mass spectrometry?
Data given will be Mr+1 because of the mass from the proton gained.
Must subtract mass of proton to find Mr of sample
Describe Stage 3 Acceleration of time in flight mass spectrometry
ACCELERATION
1) Positive ions are accelerated by a negatively charged plate
2) All ions in the sample have the SAME kinetic energy
Why is the point of accelerating positive ions, and why are they accelerating towards the negative plate?
To push them through the machine.
Ions are positively charged - opposite charges attract.
Describe Stage 4 Drift in time of flight mass spectrometry
DRIFT
1) Sample enters flight tube
2) All ions have same kinetic energy
3) As velocity is indirectly proportional to mass, heavier ions have a lower velocity than lighter ions
4) Lower molecular mass ions reach the detector first
If H₂O⁺ and ³⁷Cl⁺ were in the sample of a mass spectrometer, which would reach the detector first?
H₂O⁺ would reach the detector because it has a smaller Mr and therefore mass, so it has a higher velocity.
Mr H₂O⁺ = 18
Mr of ³⁷Cl⁺ = 37
Describe Stage 5 Detection in time of flight mass spectrometry
DETECTION
1) Detector is a negatively charged electric plate
2) Positive ions gain an electron
3) Movement of electrons means an electric current is flowing
4) Abundance of species is determined by the size of current flowing in the detector
Write an equation to show what happens to molecule X at the detector
X⁺(g) + e⁻ ━━> X(g)
Why is the abundance of a molecule an important measurement?
If synthesising a product, you need to see the amount you have made relative to other products
Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer. [2 marks] MS
Ions hit the detector / negative plate and gains an electron. [1]
(relative) abundance is proportional to the size of the current. [1]
