1.1 Atomic Structure

Question 1

what are the stages of TOF mass spec

Answer 1

1. ionisation
2. acceleration
3. ion drift
4. detection
5. mass spectrum analysis

Question 2

what happens in ionisation stage?

Answer 2

sample is ionised to create positive ions
two types of ionisation

Question 3

what happens in electron impact ionisation

Answer 3

• a high energy electron beam is fired at the sample using an electron gun
• this knocks off an electron from the sample molecule, forming a positive ion

X (g) + e- > X+(g) + 2e-

Question 4

what happens in electrospray ionisation

Answer 4

• sample is dissolved in a volatile solvent and injected through a fine needle to produce a mist
• a high voltage is applied to the tip of the needle, causing the sample molecules to gain a proton, forming positive ion

X(g) + H+ > XH+(g)

Question 5

what are the differences between electrospray and electron impact

Answer 5

• electron impact used for smaller molecules and atoms
• causes fragmentation, so smaller pieces of molecules may appesr in the spectrum
• electrospray used for large, fragile molecules like proteins and polymers
• produces less fragmentation than electron impact

Question 6

what happens in the acceleration stage

Answer 6

• positive ions are accelerated by an electric field
• at a constant kinetic energy

Question 7

what happens in the flight tube

Answer 7

• the positive ions with smaller m/z values will move faster, as same KE
• the heavier the particles take longer to move through
• ions distinguished by different flight times

Question 8

what happens at the detection

Answer 8

• ions reach detector and generate a small current, fed to computer for analysis
• current is proportional to the abundance of species

Question 9

define relative atomic mass

Answer 9

average mass of an atom of an element relative to 1/12th of the mass of one atom of carbon 12

Question 10

explain the first anomaly in ionisation energy across the period

Answer 10

outer electron is in a higher energy orbital than the other, so it is easier to remove ( less energy )

Question 11

explain the drop between groups 5 and 6 for ionisation energy

Answer 11

• shielding is identical, but the electron being removed exists in a pair of electrons.
• hence, pairing causes repulsion so it is easier to remove