Time of Flight Mass Spectrometry

Question 1

What is a mass spectrometer?

Answer 1

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

Question 2

How are mass spectrums used in the real world?

Answer 2

Drug tests in urine before sport events

Carbon dating

Drug swabs on luggage in the airport

Question 3

What are the 5 stages of Time of Flight Mass Spectrometry?

Answer 3

1) Injection of sample

2) Ionisation
> Electron impact
> Electrospray

3) Acceleration

4) Drift

5) Detection

Question 4

Describe Stages 1 & 2 of electron impact ionisation in time of flight mass spectrometry

Answer 4

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

Question 5

Why does the electron gun cause molecules to ionise in electron impact ionisation?

Answer 5

High energy electrons “knock” off one of the electrons in the sample.

Question 6

What are the limitations of electron impact ionisation?

Answer 6

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.

Question 7

Write an equation for the electron impact ionisation of molecule X

Answer 7

X(g) + e⁻ ━━> X⁺(g) + 2e⁻

Question 8

Describe Stages 1 & 2 of electrospray ionisation in time of flight mass spectrometry

Answer 8

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

Question 9

What is meant by volatile?

Answer 9

Easily vaporised

Question 10

What is meant by an aerosol?

Answer 10

Vapour - sprayed liquid

Question 11

Write an equation for the electrospray ionisation of molecule X

Answer 11

X(g) + H⁺(g) ━━> XH⁺(g)

Question 12

Why is electrospray ionisation known as a soft ionisation technique?

Answer 12

Limited fragmentation

Question 13

What are the limitations of analysing data from electrospray time of flight mass spectrometry?

Answer 13

Data given will be Mr+1 because of the mass from the proton gained.

Must subtract mass of proton to find Mr of sample

Question 14

Describe Stage 3 Acceleration of time in flight mass spectrometry

Answer 14

ACCELERATION

1) Positive ions are accelerated by a negatively charged plate
2) All ions in the sample have the SAME kinetic energy

Question 15

Why is the point of accelerating positive ions, and why are they accelerating towards the negative plate?

Answer 15

To push them through the machine.

Ions are positively charged - opposite charges attract.

Question 16

Describe Stage 4 Drift in time of flight mass spectrometry

Answer 16

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

Question 17

If H₂O⁺ and ³⁷Cl⁺ were in the sample of a mass spectrometer, which would reach the detector first?

Answer 17

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

Question 18

Describe Stage 5 Detection in time of flight mass spectrometry

Answer 18

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

Question 19

Write an equation to show what happens to molecule X at the detector

Answer 19

X⁺(g) + e⁻ ━━> X(g)

Question 20

Why is the abundance of a molecule an important measurement?

Answer 20

If synthesising a product, you need to see the amount you have made relative to other products

Question 21

Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer. [2 marks] MS

Answer 21

Ions hit the detector / negative plate and gains an electron. [1]

(relative) abundance is proportional to the size of the current. [1]