Mass spectrometry is an instrumental technique in analytical chemistry that measures the mass-to-charge ratios of gas-phase ions. It is used to identify compounds, determine molecular masses, investigate chemical structures, and quantify substances in mixtures. A mass spectrometer converts sample components into ions, analyzes them according to their mass-to-charge ratios, and records their signals. The resulting mass spectrum provides information about the ions present, rather than directly measuring every neutral molecule in the original sample. (broadinstitute.org)
Measurement principles
The central quantity, written m/z, relates an ion’s mass to its charge number. In conventional mass-spectral notation, its numerical value is obtained by dividing the ion’s mass expressed in unified atomic mass units by the magnitude of its charge number. Consequently, singly and doubly charged ions of approximately the same mass appear at different m/z values. Charge-state assignment is therefore essential when reconstructing molecular masses. (publications.iupac.org)
A mass spectrum plots ion signal intensity against m/z. Peaks can represent intact molecular ions, protonated or deprotonated molecules, adducts, or fragments. Different isotopes also produce characteristic groups of peaks. Peak intensity describes the detected ion signal; it is not automatically equivalent to the concentration of the corresponding substance in the sample. (publications.iupac.org)
Two important performance characteristics are mass accuracy and resolving power. Mass accuracy concerns agreement between measured and reference values. Resolving power describes the ability to distinguish closely spaced signals and is commonly expressed as , with the peak-width criterion specified. Accurate mass measurements constrain possible elemental compositions, but do not necessarily establish a unique molecular structure. (publications.iupac.org)
Instrument components and ionization
The principal components are an ion source, mass analyzer, detector, and data-processing system. Ionization makes sample components electrically charged so that electric or magnetic fields can manipulate them. The analyzer distinguishes ions, while the detection system measures their signals. Instrument configurations differ according to sample properties and the analytical information required. (broadinstitute.org)
Electron ionization (EI) exposes gas-phase molecules to energetic electrons, commonly at 70 electronvolts. Removing an electron creates a positive ion and often produces extensive fragmentation. EI is especially suitable for relatively volatile, thermally stable compounds. Its reproducible fragmentation patterns support identification through comparison with reference spectral libraries. (thermofisher.com)
Electrospray ionization (ESI) generates ions from a solution dispersed into electrically charged droplets. It can produce multiply charged ions, allowing large biomolecules to be analyzed within an instrument’s available m/z range. Matrix-assisted laser desorption/ionization (MALDI) uses a laser and an absorbing matrix to assist the release and ionization of analytes. These relatively gentle approaches made intact protein measurements practicable, although “soft” ionization does not imply that fragmentation is entirely absent. (nobelprize.org)
Mass analyzers and tandem measurements
Common analyzer families include quadrupoles, time-of-flight instruments, ion traps, magnetic sectors, and Orbitraps. They offer different combinations of acquisition speed, mass range, resolving power, and ion-handling capabilities. Hybrid instruments combine analyzer types to perform complementary tasks within one system. (thermofisher.com)
A quadrupole mass analyzer filters ions through electric fields, transmitting a selected m/z range. Time-of-flight analysis distinguishes ions by their travel times through an analyzer. Ion traps store ions and permit controlled manipulation before analysis. Analyzer choice influences both the measurement strategy and the kinds of structural or quantitative information obtained. (publications.iupac.org)
In tandem mass spectrometry (MS/MS), a precursor ion is selected, induced to fragment, and its product ions are analyzed. The relationship between precursor and product ions supplies information beyond an initial mass spectrum. Successive fragmentation stages can investigate structural features; targeted precursor–product measurements also provide selective signals for quantification. (thermofisher.com)
Separation and applications
Mass spectrometry is frequently coupled to chromatography. Gas chromatography–mass spectrometry separates suitable volatile compounds before ionization, commonly by EI. Liquid chromatography–mass spectrometry analyzes separated solution-phase components and commonly uses ESI. Separation reduces mixture complexity and adds retention information to the mass-spectral evidence. (thermofisher.com)
Applications include environmental contaminant screening, food analysis, forensic investigations, and characterization of industrial chemicals. Proteomics uses mass spectrometry to investigate proteins and their constituent peptides, while metabolomics examines metabolites and related small molecules. Reference libraries contain experimentally evaluated EI and tandem spectra that support compound identification across these fields. (nist.gov)
Quantification and limitations
Quantitative measurements require calibration because compounds can differ in ionization efficiency and detector response. Isotope dilution mass spectrometry adds a known amount of an isotopically labeled analogue and measures the ratio of analyte to labeled standard. Under appropriate conditions, this approach compensates for losses during sample processing. (nist.gov)
Coexisting sample constituents can cause ion suppression, reducing the ion signal independently of analyte concentration. Other limitations include interferences, incomplete separation, and uncertainty in structural assignments. Internal standards can compensate for some matrix effects, but severe suppression may leave insufficient signal for reliable measurement. (tsapps.nist.gov)
Historical development
Early mass spectrographs established mass spectrometry as a tool for investigating isotopic composition. Francis William Aston received the 1922 Nobel Prize in Chemistry for discovering isotopes in numerous nonradioactive elements using his mass spectrograph and for formulating the whole-number rule. (nobelprize.org)
During the 1980s, developments in electrospray and soft laser desorption extended mass spectrometry to large biological molecules. John B. Fenn and Koichi Tanaka shared half of the 2002 chemistry prize for developing soft desorption ionization methods; the other half recognized Kurt Wüthrich’s work on nuclear magnetic resonance methods for biological macromolecules. (nobelprize.org)