T1 L2 - Time of flight mass spectrometry

Question 1

What can mass spectrometry be used to find?

Answer 1

• the abundance and mass of each isotope allow us to determine relative atomic mass
• find relative molecular mass of substances

Question 2

What are the two ways a sample can be ionised?

Answer 2

• Electron impact
• Electrospray ionisation

Question 3

Give an overview of Electron impact.

Answer 3

• Sample is vaporised
• High energy electronat it
• Comes from electron gun with a current
• Electron knocks off an electron and particle forms a +1 ion
• Ions then attracted and accelerated towards negative electric plate

Question 4

What frequently happens during Electron impact?

Answer 4

The molecular ion often breaks down into smaller fragments - fragmentation

Question 5

Give an overview of Electrospray Ionisation.

Answer 5

-Sample is dissolved in a volatile solution
-Injected through a fine needle to give a fine mist
-End of needle is positively charged
-Particles are ionised and gain a proton (H+) from the solvent
-Leave the needle producing XH+ ions
-Solvent evaporates away, XH+ attracted towards negative plate, then accelerated.

Question 6

Which technique is used for elements with low formula mass?

Answer 6

Electron Impact

Question 7

Which technique is used for many substances with a higher molecular mass?

Answer 7

Electrospray Ionisation

Question 8

Give an example of electron impact with Methane, CH4

Answer 8

CH4(g) -> CH4+(g) + e-

Question 9

Give an example of Electrospray ionisation with Methanol (CH3OH)

Answer 9

CH3OH + H+ -> CH3OH2 +

Question 10

The positive ions are ________ using an _______ ______ so that they’ll all have the same __________ _______

Answer 10

1. Accelerated
2. Electric field
3. Kinetic energy

Question 11

Which particle, Cl35 or Cl37 will reach the end of the tube first? And why?

Answer 11

Cl35 as it has a lower mass, meaning lighter particles have a faster velocity