Mass Spectrometry [Greenland] Flashcards
How is mass spectroscopy carried out?
- The sample is vapourised and ionised (+ve or -ve)
- The ions are subject to a filter to separate them according to their mass:charge ratio
- The stream of mass-separated ions are detected
- An electrical output is generated producing a plot of the signal intensity (number of ions in sample) for each mass:charge ratio
Sample - Ionisation - Mass/charge separation - Ion detection - Mass spectra
What is the base peak?
The highest peak in the mass spectrum set to 100% relative intensity
What is the molecular ion peak?
The signal corresponding to the mass of the target compound e.g. 80 = [+CH3CH2CH2 35Cl]+
What is the x axis units?
Mass/charge
What happens during Electron Impact (EI) ionisation?
- The vapourised sample is bombarded with a stream of high energy electrons
- This ejects an electron from the compound so generating a high energy charged species = M+1
- M+1 can breakdown into lower MW fragment ions and radicals
How are the ions separated?
- The ions are accelerated in an electric field
- In the magnetic sector (at right angles to the stream of ions), a circular flight path is adopted and this is dependent upon the mass/charge ratio (m/z)
- Changing the magnetic field strength allows only ions of the correct m/z ratio through the detector
Explain the circular flight path which is adopted by the ions in the magnetic sector
- The circular flight path which is adopted is dependent upon the mass/charge ratio (m/z) of the ions
- Large ions are deflected less by the magnetic field
- At a specific field strength (H) and accelerating voltage (V), only ions of a particular mass adopt a flight path which enables them to pass through the detector
- Spectrometer operates sweeping magnetic field to scan the mass range
What is the formula that allows the mass/charge ratio (m/z) to be calculated?
m/z = (H2r2)/2V
Define the terms of the mass/charge ratio formula
H = magnetic field strength
r = radius of the circular path which the ion travels
V = accelerating voltage
What is Matrix Assisted Laser Desorption/Ionisation (MALDI)?
- A softer, lower energy ionisation technique
- Useful for analysis of peptide-based therapeutics
- An energy-absorbing matrix material transfers energy from the laser to the sample
- The matrix must be a strong UV-absorber to absorb energy from a pulsed UV laser and transfer it
What does MALDI-TOF stand for?
Matrix assisted laser desorption/ionisation time-of-flight mass spectrometry
How are all ions subjected to the same accelerating force (F)?
Cations formed by ionisation are repelled by the positively charged anode and attracted towards the negative cathode before leaving the source through a hole in the cathods
Why will heavier ions to achieve as much velocity as the lighter ones?
- All ions are subjected to the same accelerating force (F) but their acceleration is also mass-dependent
- Therefore the heavier ions will not achieve as high a velocity as the lighter ones and so take longer to reach the detector (= TOF analysis)
Compare the EI and MALDI techniques
- MALDI - soft ionisation technique = little fragmentation
- EI - hard ionisation technique = lots of fragmentation
- MALDI - lots of sample prep and optimisation required
- EI - little sample prep required - standard conditions work for most samples
- MALDI - high mass ranges
- EI - not so high mass ranges, can 1/1000th of the range
How are fragment ions produced?
High energy impact from electrons required to ionise the compound
