Mass Spectroscopy

Question 1

What are the basic mass spec principles?

Answer 1

1. Sample
2. Ionisation (+ve or -ve)
3. Mass/Charge Separation
4. Ion Detection
5. Mass Spectra

Question 2

What is the base peak?

Answer 2

The highest peak in a mass spectrum that is set to 100% relative intensity

Question 3

What is the molecular ion peak?

Answer 3

The signal corresponding to the mass of the target compound

Question 4

Describe the method of ionisation steps?

Answer 4

1. Electron impact (EI) ionisation vaporised sample is bombarded with a stream of high energy electrons
2. Impact of high energy electron with a molecule serves to eject an electron from the compound which generates high electron charges species (M+dot)
3. M+dot can and frequently breakdown into lower molecular weight fragment ion and radicals (C6H5 can become C4H3)

Question 5

Describe what the magnetic sector mass spectrometer is?

Answer 5

1. Sample is vaporised and ionised
2. Ions are accelerated in an electric field (many KV)
3. Magnetic sector (right angles to stream of ions), circular flight path is adopted and this is dependent upon mass to charge ratio (m/z)
4. Changing the magnetic field strength allows only ions of the correct m/z ratio through detector

Question 6

What are the mass/charge filters?

Answer 6

1. Large ions are deflected less by the magnetic field
2. H= magnetic field strength, r=radius of the circular path where ion travels V=accelerating voltage at specific H and V values
3. These values are only adopted at ions of a particular mass adopt a flight path that enables them to pass through to the detector
4. Spectrometer operates by sweeping the magnetic field to scan the mass range

Question 7

Describe what the MALDI is?

Answer 7

1. Matrix Assisted Laser Desorption- a softer and lower energy ionisation
2. This transfers energy from the laser to the sample- this sample is not directly energised- this gave rise to Matrix-assisted laser-desorption/ionisation time of flight (MALDI-TOF)
3. The matrix must be a strong UV absorber to absorb energy from a pulsed ultraviolet laser (typically 337nm) and transfer it to the sample

Question 8

What is the mass/charge filters in relation to the Maldi-TOF mass spectrometry?

Answer 8

1. Cations formed by ionisation are repelled by the positively charged anode
2. They are then attracted towards the negative cathode before leaving the source through a hole in the cathode
3. Ions are subjected to the F the accelerating force.
4. This is also mass independent and shown by F=ma the heavier ions will not achieve as high velocity as the lighter ones
5. The heavier ions therefore take longer to reach the detector and are detected later (hence the time of flight analysis)

Question 9

What is the calibrant used in MALDI?

Answer 9

Bovine Serum Albumin (BSA)

Question 10

Describe MALDI vs EI?

Answer 10

1. MALDI- soft ionisation with little fragmentation- lots of sample preparation and optimisation (choice of matrix, laser power)
2. EI- Hard ionisation with lots of fragmentation- standard conditions work for most samples
3. MALDI is for high mass ranges of 1000 to 100,000 Da Polymer protein size molecules
4. EI is typically only for 1000 Da (drug size molecules)

Question 11

Describe the Mass Spectra- fragmentation methods?

Answer 11

1. EI employs high energy electrons to ionise compounds
2. This forms molecular ions M+dot and ions of lower molecular weight (fragment ions)
3. This can lead to forming of a Homolytic Alpha Cleavage- often affords the most abundant ion (base peak) in El mass spectra
4. The radical of the molecular ion is used to form a double bond with the heteroatom
5. Loss of the largest radical is most favoured
6. Heterolytic Alpha Cleavage- - not as common as homiletic

Question 12

What is Mclafferty rearrangement?

Answer 12

1. Where Aliphatic ketones and aldehydes feature γ-hydrogens which undergo a characteristic rearrangement
2. Can also occur for carboxylic acids, esters or amides- must possess an alkyl chain 3 carbons in length

Question 13

What are isotopologues?

Answer 13

Molecules that differ only in isotopic composition

Question 14

What are the two Chlorine Istopologues?

Answer 14

1. Cl^35 and Cl^37
2. Signals at 78 [CH3CH2CH2^35Cl]+
3. Signals at 80
   [CH3CH2CH2^37Cl]+

Question 15

What are the two Bromine Istopologues?

Answer 15

1. Br^79 and Br^81
2. Signals at 122
   [CH3CH2CH2^79Br]+
3. Signals at 124
   [CH3CH2CH2^81Br]+

Question 16

What are the hybrid mass spectroscopy techniques available?

Answer 16

1. HPLC system that uses a liquid mobile phase like solvents such as water, methanol, acetonitrile are used in reverse phase HPLC