User:Benjah-bmm27/degree/3/PJG

Mass spectrometry, PJG

Major topics

EI

• Summary:
  • Best suited to highly volatile, thermally stable analytes
  • Low-mass limit
  • Forms radical ions
  • Usually observe loads of fragment ions
• Electron ionisation

CI

• 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

• 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

• Mass spectrometry
• Mass spectrum analysis
• Mass-to-charge ratio
• Ion sources
• Gas phase ion chemistry
• Mass spectrometry data format, e.g. JCAMP-DX

Resolution

• Resolution (mass spectrometry)
• Isobar (nuclide)
• Isotope

Dissociation

• Collision-induced dissociation
• Electron capture dissociation

Types of MS

• Inductively coupled plasma mass spectrometry
• Secondary ion mass spectrometry
• Time-of-flight mass spectrometry
• Tandem mass spectrometry

Ionization techniques

• Ionization
• Chemical ionization
• Atmospheric-pressure chemical ionization
• Electrospray ionization
• Matrix-assisted laser desorption/ionization
• Ion attachment mass spectrometry

Analysers, traps and detectors

• Quadrupole mass analyzer
• Ion trap
  • Quadrupole ion trap