Mass spectrometry is all about turning molecules into ions and sorting them by mass. This section covers the different ways to make ions and the tools used to separate them.

You'll learn about zapping molecules with electrons, spraying them with charged droplets, and blasting them with lasers. Then, we'll explore how to sort these ions using electric fields, flight times, and traps.

Ionization Techniques

Electron and Chemical Ionization Methods

• Electron ionization (EI) bombards vaporized sample molecules with high-energy electrons
  • Typically uses 70 eV electrons to remove an electron from the molecule
  • Produces radical cations (M+•) and extensive fragmentation
  • Generates reproducible mass spectra useful for compound identification
  • Limited to volatile and thermally stable compounds
• Chemical ionization (CI) uses reagent gas ions to ionize sample molecules
  • Softer ionization technique compared to EI
  • Produces protonated molecules [M+H]+ with less fragmentation
  • Common reagent gases include methane, isobutane, and ammonia
  • Useful for determining molecular mass of compounds

Electrospray and MALDI Techniques

• Electrospray ionization (ESI) converts ions from solution to the gas phase
  • Applies high voltage to a liquid sample flowing through a capillary
  • Creates a fine mist of charged droplets that undergo desolvation
  • Produces multiply charged ions for large molecules (proteins, peptides)
  • Suitable for coupling liquid chromatography to mass spectrometry (LC-MS)
• Matrix-assisted laser desorption/ionization (MALDI) uses a laser to ionize samples
  • Mixes analyte with a matrix compound (sinapinic acid, α-cyano-4-hydroxycinnamic acid)
  • Applies laser pulses to vaporize and ionize the sample-matrix mixture
  • Generates predominantly singly charged ions [M+H]+
  • Ideal for analyzing large biomolecules and polymers
  • Often coupled with time-of-flight (TOF) mass analyzers

Mass Analyzers

Quadrupole and Time-of-Flight Analyzers

• Quadrupole mass analyzer uses oscillating electric fields to separate ions
  • Consists of four parallel metal rods with applied DC and RF voltages
  • Allows only ions with specific m/z ratios to pass through to the detector
  • Offers good sensitivity and resolution for targeted analysis
  • Can be used in tandem (triple quadrupole) for enhanced selectivity
• Time-of-flight (TOF) analyzer separates ions based on their flight times
  • Accelerates ions with the same kinetic energy through a flight tube
  • Lighter ions reach the detector faster than heavier ions
  • Provides high mass accuracy and resolution
  • Useful for analyzing a wide mass range simultaneously
  • Often combined with MALDI for biomolecule analysis (MALDI-TOF)

Ion Trapping and High-Resolution Mass Analyzers

• Ion trap confines ions in a three-dimensional space using electric fields
  • Can store, isolate, and fragment ions for MS/MS experiments
  • Types include 3D ion traps (Paul trap) and linear ion traps
  • Offers high sensitivity and MSn capabilities
  • Useful for structural elucidation of compounds
• Fourier transform ion cyclotron resonance (FT-ICR) uses a strong magnetic field
  • Traps ions in a circular motion (cyclotron motion)
  • Measures the frequency of ion oscillations to determine m/z ratios
  • Provides ultra-high resolution and mass accuracy
  • Enables analysis of complex mixtures (petroleomics, metabolomics)
• Orbitrap uses electrostatic fields to trap ions in an orbital motion
  • Consists of a spindle-shaped central electrode and outer barrel-like electrode
  • Measures the frequency of ion oscillations along the central electrode
  • Offers high resolution and mass accuracy without a superconducting magnet
  • Widely used in proteomics and small molecule analysis