Ionization methods and mass analyzers

There are several ionization methods and mass analyzers used in mass spectrometry (MS):

Ionization methods:

  1. Electron ionization (EI): This method uses a beam of high-energy electrons to ionize the sample. It is typically used for gas-phase samples and produces predominantly singly charged ions.
  2. Chemical ionization (CI): This method uses a reagent gas, such as methane or ammonia, to ionize the sample via proton transfer reactions. It is typically used for volatile and polar compounds and can produce both singly and multiply charged ions.
  3. Electrospray ionization (ESI): This method uses an electrically charged spray to create a stream of charged droplets from a solution of the sample. The droplets evaporate, leaving behind charged ions that are typically singly or multiply charged. It is commonly used for biomolecules, such as proteins and peptides.
  4. Matrix-assisted laser desorption/ionization (MALDI): This method uses a matrix material, typically a small organic molecule, to assist in the ionization of the sample. A laser is used to vaporize the matrix, causing the sample to become ionized. MALDI is commonly used for large biomolecules, such as proteins and peptides.

Mass analyzers:

  1. Magnetic sector analyzer: This analyzer separates ions based on their m/z ratio by applying a magnetic field perpendicular to the ion beam. The magnetic field causes the ions to follow a curved path, with the radius of curvature depending on their m/z ratio.
  2. Quadrupole analyzer: This analyzer separates ions based on their m/z ratio using a combination of radio frequency and direct current electric fields. The RF field oscillates rapidly, causing the ions to move back and forth between the quadrupole rods, while the DC field stabilizes the motion.
  3. Time-of-flight (TOF) analyzer: This analyzer measures the time it takes for ions to travel a certain distance along a flight path, which is proportional to their m/z ratio. TOF analyzers are typically used in MALDI-MS.
  4. Ion trap analyzer: This analyzer uses an electric field to trap and store ions, and then selectively ejects ions based on their m/z ratio. It can be used for both MS and tandem MS (MS/MS) experiments.
  5. Fourier transform ion cyclotron resonance (FT-ICR) analyzer: This analyzer uses a strong magnetic field to trap and store ions, and then applies a radio frequency electric field to excite the ions in a cyclotron motion. The time it takes for the ions to complete a full cyclotron orbit is measured, which is proportional to their m/z ratio. FT-ICR analyzers are used for high-resolution MS and MS/MS experiments.