Mass spectrometry (MS) is an analytical technique that ionizes chemical species and sorts the resulting ions based on their mass-to-charge ratio (m/z). It provides information about molecular weight, elemental composition, and structural fragments, making it indispensable in chemistry, biochemistry, and medicine.

Basic Workflow

1. Ionization: Sample molecules are converted into gas-phase ions using methods such as electron ionization (EI), electrospray ionization (ESI), or matrix-assisted laser desorption/ionization (MALDI).
2. Mass Analysis: Ions are separated by their m/z ratio in a mass analyzer such as a quadrupole, time-of-flight (TOF), ion trap, or Orbitrap.
3. Detection: A detector (e.g., electron multiplier) measures the abundance of each ion, producing a mass spectrum of intensity versus m/z.

Ionization Techniques

1. Electron Ionization (EI): Hard ionization that produces extensive fragmentation, yielding reproducible fingerprint spectra for library matching.
2. Electrospray Ionization (ESI): Soft ionization that generates mostly molecular ions, ideal for proteins, peptides, and polar compounds.
3. MALDI: Uses a laser and matrix to ionize large biomolecules with minimal fragmentation.
4. Chemical Ionization (CI): Uses reagent gas to produce ions with less fragmentation than EI.

Mass Analyzers

1. Quadrupole: Four parallel rods that filter ions by stable oscillation paths; used in GC-MS and LC-MS.
2. Time-of-Flight (TOF): Measures the time ions take to travel a fixed distance; offers high mass accuracy and resolution.
3. Ion Trap: Captures ions in a three-dimensional electric field and sequentially ejects them.
4. Orbitrap: Traps ions in an electrostatic field and measures their axial oscillation frequency for ultra-high resolution.

Tandem Mass Spectrometry (MS/MS)

1. Precursor ion selection in the first analyzer, fragmentation in a collision cell, and fragment ion analysis in the second analyzer.
2. Used for peptide sequencing, metabolite identification, and structural elucidation of unknowns.

Applications

1. Identification of organic compounds and reaction products.
2. Proteomics: protein identification and post-translational modification analysis.
3. Drug metabolism and pharmacokinetic studies.
4. Environmental monitoring of pollutants and contaminants in food and water.