AQA A Level Chemistry: Time of Flight Mass Spectometry

Question 1

What is the purpose of mass spectrometry?

Answer 1

To determine the relative atomic mass and molecular mass of a sample by measuring the mass-to-charge ratio (m/z) of ions.

Question 2

What are the four stages of time of flight (TOF) mass spectrometry?

Answer 2

1. Ionisation
   
   2. Acceleration
   3. Ion Drift
   4. Detection

Question 3

What are the two types of ionisation in TOF mass spectrometry?

Answer 3

• Electron Impact: High-energy electrons knock off an electron from the sample, forming a positive ion (M⁺).
  
  • Electrospray Ionisation: Sample is dissolved, passed through a needle at high voltage, and gains a proton (H⁺) to form MH⁺ ions.

Question 4

What happens during the acceleration stage?

Answer 4

Ions are accelerated by an electric field, giving them constant kinetic energy.

KE = 1/2mv^2

Since they have the same kinetic energy, lighter ions travel faster than heavier ions.

Question 5

What is ion drift in TOF mass spectrometry?

Answer 5

Ions pass through a flight tube with no electric field.
* Lighter ions travel faster and reach the detector first.
* Heavier ions travel slower and take longer to reach the detector.

Question 6

What happens during the detection stage?

Answer 6

Ions hit the detector, gain an electron, and produce a current. The size of the current is proportional to the abundance of the ion.

Question 7

How is the mass-to-charge ratio (m/z) calculated?

Answer 7

m/z= mass of ion / charge of ion

For singly charged ions, m/z is equal to the mass of the ion.

Question 8

What does the mass spectrum show?

Answer 8

• m/z ratio on the x-axis.
  
  • Relative abundance on the y-axis.
  • The peak with the highest m/z value corresponds to the molecular ion (M⁺), giving the molecular mass.

Question 9

What is the molecular ion peak?

Answer 9

The peak with the highest m/z ratio, representing the molecular mass of the sample.