Mass Spectroscopy Flashcards
1
Q
What are the basic mass spec principles?
A
- Sample
- Ionisation (+ve or -ve)
- Mass/Charge Separation
- Ion Detection
- Mass Spectra
2
Q
What is the base peak?
A
The highest peak in a mass spectrum that is set to 100% relative intensity
3
Q
What is the molecular ion peak?
A
The signal corresponding to the mass of the target compound
4
Q
Describe the method of ionisation steps?
A
- Electron impact (EI) ionisation vaporised sample is bombarded with a stream of high energy electrons
- Impact of high energy electron with a molecule serves to eject an electron from the compound which generates high electron charges species (M+dot)
- M+dot can and frequently breakdown into lower molecular weight fragment ion and radicals (C6H5 can become C4H3)
5
Q
Describe what the magnetic sector mass spectrometer is?
A
- Sample is vaporised and ionised
- Ions are accelerated in an electric field (many KV)
- Magnetic sector (right angles to stream of ions), circular flight path is adopted and this is dependent upon mass to charge ratio (m/z)
- Changing the magnetic field strength allows only ions of the correct m/z ratio through detector
6
Q
What are the mass/charge filters?
A
- Large ions are deflected less by the magnetic field
- H= magnetic field strength, r=radius of the circular path where ion travels V=accelerating voltage at specific H and V values
- These values are only adopted at ions of a particular mass adopt a flight path that enables them to pass through to the detector
- Spectrometer operates by sweeping the magnetic field to scan the mass range
7
Q
Describe what the MALDI is?
A
- Matrix Assisted Laser Desorption- a softer and lower energy ionisation
- 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)
- The matrix must be a strong UV absorber to absorb energy from a pulsed ultraviolet laser (typically 337nm) and transfer it to the sample
8
Q
What is the mass/charge filters in relation to the Maldi-TOF mass spectrometry?
A
- Cations formed by ionisation are repelled by the positively charged anode
- They are then attracted towards the negative cathode before leaving the source through a hole in the cathode
- Ions are subjected to the F the accelerating force.
- This is also mass independent and shown by F=ma the heavier ions will not achieve as high velocity as the lighter ones
- The heavier ions therefore take longer to reach the detector and are detected later (hence the time of flight analysis)
9
Q
What is the calibrant used in MALDI?
A
Bovine Serum Albumin (BSA)
10
Q
Describe MALDI vs EI?
A
- MALDI- soft ionisation with little fragmentation- lots of sample preparation and optimisation (choice of matrix, laser power)
- EI- Hard ionisation with lots of fragmentation- standard conditions work for most samples
- MALDI is for high mass ranges of 1000 to 100,000 Da Polymer protein size molecules
- EI is typically only for 1000 Da (drug size molecules)
11
Q
Describe the Mass Spectra- fragmentation methods?
A
- EI employs high energy electrons to ionise compounds
- This forms molecular ions M+dot and ions of lower molecular weight (fragment ions)
- This can lead to forming of a Homolytic Alpha Cleavage- often affords the most abundant ion (base peak) in El mass spectra
- The radical of the molecular ion is used to form a double bond with the heteroatom
- Loss of the largest radical is most favoured
- Heterolytic Alpha Cleavage- - not as common as homiletic
12
Q
What is Mclafferty rearrangement?
A
- Where Aliphatic ketones and aldehydes feature γ-hydrogens which undergo a characteristic rearrangement
- Can also occur for carboxylic acids, esters or amides- must possess an alkyl chain 3 carbons in length
13
Q
What are isotopologues?
A
Molecules that differ only in isotopic composition
14
Q
What are the two Chlorine Istopologues?
A
- Cl^35 and Cl^37
- Signals at 78 [CH3CH2CH2^35Cl]+
- Signals at 80
[CH3CH2CH2^37Cl]+
15
Q
What are the two Bromine Istopologues?
A
- Br^79 and Br^81
- Signals at 122
[CH3CH2CH2^79Br]+ - Signals at 124
[CH3CH2CH2^81Br]+
