Time Of Flight Mass Spectrometry

Question 1

TOF

Answer 1

(TOF) mass spectrometry is a sophisticated technique used to measure the mass-to-charge ratio (m/z) of ions, providing information about the isotopes and structure of molecules. It is commonly used in A-level chemistry to measure the masses of ions and deduce the composition of substances.

9.1 Principle of TOF Mass Spectrometry

In TOF mass spectrometry, ions are accelerated through an electric field and then allowed to travel down a flight tube. The time it takes for ions to reach the detector depends on their mass-to-charge ratio (m/z). Lighter ions travel faster, while heavier ions take longer to reach the detector.

Question 2

What is the purpose of mass spectrometry?

Answer 2

To determine the relative atomic mass (Ar) or relative molecular mass (Mr) by measuring the mass-to-charge ratio (m/z) of ions.

Question 3

What are the four stages of TOF mass spectrometry?

Answer 3

1. Ionisation
   
   2. Acceleration
   3. Drift / Flight tube
   4. Detection

Question 4

Describe electron impact ionisation.

Answer 4

• Sample is vaporised and bombarded with high-energy electrons which are fired from an electron gun.
• An electron is knocked off, forming a 1+ ion (M⁺).
• These are attracted towards a negatively charged plate , accelerating them through the mass spectrometer.
- The ion can be broken down further, or fragmented
- The fragments are also accelerated through the sample and hit the detector, causing different peaks to show on the mass spectrum which is produced

M(g) —> M+(g) + e-

Question 5

Describe electrospray ionisation.

Answer 5

Sample dissolved in volatile solvent and sprayed through a fine hyoerdermic charged needle to form aerosol ( a fine mist)
• Gains a proton to form MH⁺.
• Produces fewer fragments.
Equation: M(g) + H⁺ → MH⁺(g)

Question 6

Which ionisation method is used for large, polar molecules like proteins?

Answer 6

Electrospray ionisation, because it is a “soft” ionisation technique and avoids fragmentation.

Question 7

What happens during acceleration in TOF MS?

Answer 7

Positive ions are accelerated by an electric field.
• All ions gain the same kinetic energy (KE = ½mv²).
• Lighter ions move faster than heavier ones.

Question 8

What is the relationship between time, mass and velocity in TOF?

Answer 8

T=d/v and KE =1/2mv2 so t is directly proportional to the square root of M

→ Lighter ions have shorter time of flight

Question 9

What happens in the drift region?

Answer 9

• Ions move at constant speed through a field-free tube.
• Their time of flight is recorded.

Question 10

How are ions detected in TOF MS?

Answer 10

Ions hit a negatively charged detector, gaining an electron.
• This causes a current.
• The time taken and current are used to calculate m/z and abundance.

Question 11

What does the x-axis and y-axis of a mass spectrum show?

Answer 11

• x-axis: Mass-to-charge ratio (m/z)
• y-axis: Relative abundance

Question 12

How is relative atomic mass (Ar) calculated from a spectrum?

Answer 12

Ar = sum of (the mz value*abundance)/ sum of abundances

Question 13

What is the molecular ion peak?

Answer 13

The peak with the highest m/z value, representing the intact molecular ion (M⁺ or MH⁺).

Question 14

What is the base peak?

Answer 14

The tallest peak, representing the most abundant ion detected.

Question 15

Why are only ions detected in MS?

Answer 15

Because only charged particles can be accelerated and generate a current at the detector.

Question 16

Give three uses of TOF mass spectrometry.

Answer 16

• Determining Ar or Mr
• Identifying isotopes
• Structural analysis through fragmentation patterns