Mass Spectrometry

Question 1

What derivatisation is used to reduce H-bonding?

Answer 1

1. Acetylation
2. Trimethylsilation
3. Ketone to Ketal
4. Methylation
5. Esterification of acids

Question 2

What is Electron Impact Ionisation?

Answer 2

• Hard: fragments molecule
• gas phase ions bombarded with e- from filament
• e- beam expels 1 valence electron
• ion repeller (+4kV) repels cation radical into detector

Question 3

What is Electrospray Ionisation?

Answer 3

• Soft: requires collisional activation
• ionise from liquid phase
• large potential (4kV) between gold-tip and electrode
• highly charged droplets spray out
• solvent evaporates w/ nitrogen gas
• droplet explodes at critical limit for like-ion proximity
• naked gas phase ions drift into analyser by voltage gradient

Question 4

What is MALDI?

Answer 4

• Soft: requires collisional activation
• ionise from solid phase
• conjugated matrix absorbs laser energy and transfers it to sample
• sample ionised relative to location in crystal
• delay before applying large potential ensures ions closer to potential enter analyser at same velocity (delayed extraction)

Question 5

Types of Mass Analysers

Answer 5

1. Quadrupole
2. Time-of-flight
3. Ion trap
4. FT Ion Cyclotron Resonance (FT-ICR)
5. Orbitrap

Question 6

Types of Detectors

Answer 6

1. Photomultiplier (PM)
   - conversion dynode emits secondary electrons
   - secondary electrons strike screen releasing photons
   - photons pass into multiplier where amplification occurs in a cascade
   - electrical signal from photons becomes m/z peaks
2. Electron multiplier (EM)
   - ions strike dynode which emits e-
   - e- strike side causing more e- to be emitted (amplifying signal)
3. Micro-channel plate array detectors (MCP)
   - multiple electron multipliers
   - simultaneous detection of multiple m/z values

Question 7

Fragmentation Pathways

Answer 7

1. Alpha Cleavage
   - radical from carbonyl oxygen to R group
   - double to triple C-O bond
2. Beta Cleavage
   - radical from N or O to terminal group
   - single to double bond to N or O
3. Methyl Elimination in TMS
   - radical from O to CH3
   - single to double Si-O bond
4. Inductive Cleavage
   - initiated by +ve charge by attraction of e- pair
5. Neutral Elimination
   - loss of stable neutral molecule
   - 1,2 elimination
6. McLafferty Rearrangement
   - alpha/beta bond attacks beta/gamma bond
   - transfer of H from gamma atom to carbonyl O
   - double bond moves to carbonyl C/C alpha

Question 8

Ideal Ionisation and Detection Techniques

Answer 8

1. EI + quadrupole

2. ES + Triple quadrupole/ion trap/orbitrap/Q-TOF

Question 9

MS/MS

Answer 9

• Triple Quadrupole
• Q-TOF
• MALDI TOF-TOF

(1. MALDI TOF-TOF
2. Ion-trap + Orbitrap
3. MALDI quadrupole + Ion trap + TOF
4. Quadrupole + Orbitrap + Ion trap
5. ES/MALDI Quadrupole + Ion trap +FT-ICR)

Question 10

What is a quadrupole analyser?

Answer 10

• EM field in centre of 4 rods
• resonant ions pass through
• scan strength of EM field to allow different m/z through
• Upper mass limit: ~4000m/z
• Resolution: low
• low cost, rapid scanning
• ideal for gas-chromatography mass spec
• EI and ES

Question 11

What is a TOF analyser?

Answer 11

• different velocities separates ions
• larger ions take longer to travel
• Ion mirror (reflectron) gives ions with higher kinetic energies a longer flight path
• So ions with different kinetic energies but same m/z ratios detected simultaneously

Question 12

What is an Ion Trap analyser?

Answer 12

• stores ions with RF voltages applied to electrodes
• ions within selected m/z range enter midspace, become trapped and rotate (some may become annihilated)
• changing B field allows ejection of ions with different m/z
• doesn’t work as a filter
• linear ion traps reduce ion interference and increase storage capacity

Question 13

What is an FT Ion Cyclotron Resonance analyser?

Answer 13

• ions trapped in magnetic field at liquid helium temperature
• spiral flight paths
• ions absorb energy from RF field
• flight radius increases and change in electric current translated to m/z ratios
• scanning RF detects different m/z ratios

Question 14

What is an orbitrap?

Answer 14

• ions trapped by static electrostatic fields
• ions orbit central electrode and oscillate in axial direction
• oscillation and trajectory related to m/z ratio

Question 15

What is triple quadrupole?

Answer 15

MS/MS mode:

Selected by Q1
Fragmented by Q2 (collision gas)
Analysed by Q3