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

The mass spectrometry workflow involves three main stages. First, 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). Next, ions are separated by their m/z ratio in a mass analyzer such as a quadrupole, time-of-flight (TOF), ion trap, or Orbitrap. Finally, a detector (e.g., electron multiplier) measures the abundance of each ion, producing a mass spectrum of intensity versus m/z.

Ionization Techniques

Several ionization techniques are available. Electron Ionization (EI) is a hard ionization method that produces extensive fragmentation, yielding reproducible fingerprint spectra for library matching. Electrospray Ionization (ESI) is a soft ionization technique that generates mostly molecular ions, ideal for proteins, peptides, and polar compounds. MALDI uses a laser and matrix to ionize large biomolecules with minimal fragmentation, while Chemical Ionization (CI) uses reagent gas to produce ions with less fragmentation than EI.

Mass Analyzers

Several mass analyzers are used in MS. The quadrupole consists of four parallel rods that filter ions by stable oscillation paths and is commonly used in GC-MS and LC-MS. Time-of-Flight (TOF) analyzers measure the time ions take to travel a fixed distance, offering high mass accuracy and resolution. Ion traps capture ions in a three-dimensional electric field and sequentially eject them, while the Orbitrap traps ions in an electrostatic field and measures their axial oscillation frequency for ultra-high resolution.

Tandem Mass Spectrometry (MS/MS)

Tandem MS involves precursor ion selection in the first analyzer, fragmentation in a collision cell, and fragment ion analysis in the second analyzer. It is used for peptide sequencing, metabolite identification, and structural elucidation of unknowns.

Applications

Mass spectrometry is used for identification of organic compounds and reaction products, proteomics for protein identification and post-translational modification analysis, drug metabolism and pharmacokinetic studies, and environmental monitoring of pollutants and contaminants in food and water.