User:Benjah-bmm27/degree/3/PJG
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Mass spectrometry, PJG
[edit]Major topics
[edit]EI
[edit]- Summary:
- Best suited to highly volatile, thermally stable analytes
- Low-mass limit
- Forms radical ions
- Usually observe loads of fragment ions
- Electron ionisation
CI
[edit]- Summary:
- Like EI, best suited to highly volatile analytes
- Low-mass limit
- Form protonated molecules
- Unlike EI, see only a few fragment ions
- Chemical ionisation
MALDI
[edit]- Summary – best suited to:
- Involatile or thermally unstable analytes
- Highly functionalised or protected analytes
- Biologically important analytes – proteins, peptides, sugars, etc.
- Matrix-assisted laser desorption/ionization (MALDI)
- Evolved from an older technique called laser desorption
- MALDI involves blasting a solid mixture of analyte and UV-absorbing matrix with a UV laser
- The analyte is mixed with a UV-active matrix compound and the mixture is co-crystallized
- Common matrix compounds are 2,5-dihydroxybenzoic acid, sinapic acid, dithranol and alpha-cyano-4-hydroxycinnamic acid
- UV laser excites the chromophores of matrix molecules, with the heat produced causing the matrix and analyte to evaporate (desorb)
- Ion-molecule reactions transfer charge (H+ or metal cation, e.g. Na+) to analyte, resulting in [M+H]+ or e.g. [M+Na]+ ions
- These analyte cations pass through an extraction grid and focussing lens into the mass spectrometer
- MALDI is a low energy ionisation technique since the analyte becomes charged via a chemical reaction – generates low energy [M+H]+ ions
- Analyte remains intact, no fragmentation
- MALDI is not suitable for small molecules
- Matrix molecules are present and also have low masses, so they interfere with the mass spectrum
- Molecules with mass < 1000 Da will be lost in the matrix signal
- MALDI is ideal for ionising large biomolecules (e.g. peptides, proteins, DNA) and synthetic polymers, which might otherwise fragment and prevent the molecular ion from being observed
- Can very easily obtain the mass each protein in a mixture of proteins by MALDI-MS
- In fact, MALDI is the only way to detect the masses of proteins directly
- The technique is very fast and very sensitive
General topics
[edit]- Mass spectrometry
- Mass spectrum analysis
- Mass-to-charge ratio
- Ion sources
- Gas phase ion chemistry
- Mass spectrometry data format, e.g. JCAMP-DX
Resolution
[edit]Dissociation
[edit]Types of MS
[edit]- Inductively coupled plasma mass spectrometry
- Secondary ion mass spectrometry
- Time-of-flight mass spectrometry
- Tandem mass spectrometry
Ionization techniques
[edit]- Ionization
- Chemical ionization
- Atmospheric-pressure chemical ionization
- Electrospray ionization
- Matrix-assisted laser desorption/ionization
- Ion attachment mass spectrometry