Mass Spectrometry

Mass Spectrometry

1. Ionization Technique

   • Hard Ionization Source: High Fragmentation
     • Electron Ionization (EI)
       • Molecules are converted to ions by bombardment with e-. The M+ ion retains excess energy from bombardment & will fragment further.
   • Soft Ionization Source: Low Fragmentation
     • Chemical Ionization (CI)
       • Reagent gas will become reactant ions that will ionize the analyte
     • Electrospray Ionization (ESI)
       • Ions formed through spraying the sample solution as droplets under ↑ voltage. As solvent evaporates, the charged solute molecules pack together and undergo coulombic explosion to give gas phase ions.
     • Matrix-Assisted Laser Desorption/Ionization Method (MALDI)
       • UV laser ionizes matrix-coated sample and the ionized matrix carries the sample and continues to collide with neutral molecules with the sample plate and therefore, ionizing everything

2. Detector

   • Electron Multiplier
     • A single e- bombards a secondary emission material and emits 1-3 e-. This process can be repeated by applying an electric potential between another metal plate until the signal detected at the metal anode is sufficient
   • Microchannel Plate
     • Functions similar to the electron multiplier, but has many separate channels, so can proved additional spatial resolution

3. Mass Analyzer

   • Time-of-Flight (TOF)
     • The time taken for the kicked ions to pass through a drift region (no electric field in this area) is measured
       • Has high sensitivity and scans in 1-50µs (typical flight times)
   • Quadrupole
     • Has 4 rods that have a combination of DC and AC which allow ions with appropriate m/z to pass through. To allow more ions to pass, the AC can be scanned.
       • Scan time is <100ms
   • Ion Trap
     • Gaseous anions/cations can be formed and stored using E & B fields, where ions are sequentially ejected in order of mass by changing radio-frequency
   • Magnetic Sector
     • Ions pass through an applied magnetic field . Field strength is specific to certain m/z ratio and can be varied to allow more ions to pass.

4. WHAT? Measures mass for identification, verification and quantitation of analytes by analyzing masses of molecules or its fragments

   • Sample preparation: Must be either a gas/species removed from a condensed phase
     • HOW? ~Sample introduced and then ionized to charged species ~Separation of ions based on m/z with mass analyzer ~Detection of ions
       • APPLICATION: ~Great for isotopic analysis ~Biomolecule characterization - metabolites, recombinant proteins, proteins, oligonucleotides, peptides (ESI/MALDI with tandem MS) ~Pharmacokinetics- Active ingredients in drugs (LC-MS with Triple Quad) ~Forensic/Toxicology investigations - identification of illegal drugs, explosive compounds, exotics or to confirm substance drug abuse (GC-MS)

5. Chromatographic Separation Technique

   • GC-MS
     • For small, volatile, non-polar and thermally stable compounds. Compounds that do not fall under this criteria can undergo pyrolysis, thermochemolysis and derivatization
       • Usually smaller than proteins
       • Ex: Can perform acylation/silylation: To ↓ polarity & ↑ stability and volatilty
     • SAMPLE STATE: Gaseous ions
       • 1. Eluted gaseous compounds are ionized 2. Separates ions in vacuum based on m/z ratio 3. Intensity of each ions is measured
     • ~The interface must be gas-tight with ↓ pressure ~Carrier gases will be removed ~Temp is kept ↑ so only volatile compounds can enter MS
     • Potential interference from carrier gas ions
   • LC-MS
     • Suitable for large, polar, ionic, thermally unstable or involatile compounds
     • CHALLENGES INTERFACING LC-MS: ~ ↑ pressure to separate a liquid phase and produce ↑ gas load but MS needs to be in a vacuum and can only accept ↓ gas load ~ MS operates at high temp, but LC needs ambient temp ~ LC has no mass range limitation but MS analyzer has ~ LC can use inorganic buffer but MS prefers voltaile buffer
     • Potential interference from buffer ions

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