Mass Spectrometry
Mass spectrometry identifies and quantifies molecules and elements by ionizing them and measuring the mass-to-charge ratios of the resulting ions.
Definition
Mass spectrometry is an analytical technique that converts analytes into gas-phase ions and separates and detects them according to their mass-to-charge ratio, yielding spectra used for identification, structural elucidation, and quantitation.
Scope
This area covers the principles and instrumentation of mass spectrometry as an analytical method: the production of gas-phase ions, their separation by mass-to-charge ratio in various analyzers, their detection, and the interpretation of the resulting spectra. It treats both molecular and elemental mass spectrometry, fragmentation, accurate-mass measurement, and the coupling of mass spectrometers to separation techniques. The atomic-emission and absorption methods used for elements are covered separately under analytical spectroscopy.
Sub-topics
Core questions
- How are analytes converted into gas-phase ions suitable for mass analysis?
- How do different mass analyzers achieve their resolving power and mass range?
- How do fragmentation patterns and accurate mass support identification?
- How is mass spectrometry made quantitative, and how is it coupled to separations?
Key theories
- Separation by mass-to-charge ratio
- Once analytes are ionized, their motion in electric or magnetic fields depends on their mass-to-charge ratio; measuring this ratio for each ion produces a mass spectrum whose peak positions identify species and whose intensities, suitably calibrated, quantify them.
- Soft ionization of biomolecules
- Electrospray and matrix-assisted laser desorption ionization transfer intact large and fragile molecules into the gas phase as ions with little fragmentation, extending mass spectrometry from small volatile molecules to proteins and other biopolymers.
Mechanisms
An analyte is ionized—by electron impact, electrospray, laser desorption, or a plasma—to form charged species in the gas phase. A mass analyzer then separates these ions by mass-to-charge ratio using electric and magnetic fields, time of flight, or ion trapping, and a detector records the abundance of ions at each ratio. The resulting spectrum provides molecular or elemental masses, isotope patterns, and, through controlled fragmentation, structural information; calibrated peak areas give quantitation.
Clinical relevance
Mass spectrometry underpins proteomics and metabolomics, clinical newborn screening and therapeutic drug monitoring, environmental and food contaminant analysis, forensic toxicology, and elemental trace analysis by inductively coupled plasma mass spectrometry, valued for its sensitivity, specificity, and breadth.
History
Mass spectrometry originated with J. J. Thomson's positive-ray experiments and Francis Aston's mass spectrograph, which revealed isotopes in the early 20th century. Mid-century instruments served organic structure elucidation, and the development of soft ionization methods—electrospray by John Fenn and laser desorption advanced by Koichi Tanaka—in the 1980s, recognized with the Nobel Prize, opened mass spectrometry to large biomolecules.
Key figures
- J. J. Thomson
- Francis Aston
- John Fenn
- Koichi Tanaka
Related topics
Seminal works
- gross2017
- deHoffmann2007
- skoog2017
Frequently asked questions
- What does a mass spectrometer actually measure?
- It measures the mass-to-charge ratio of ions produced from a sample and how many ions occur at each ratio, giving a spectrum used to determine molecular or elemental masses, isotope patterns, and structure.
- Why was soft ionization a breakthrough?
- Earlier ionization methods fragmented or could not vaporize large molecules; electrospray and matrix-assisted laser desorption gently produce intact ions of proteins and other biopolymers, extending mass spectrometry into biology and medicine.