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What is isotope ratio mass spectrometry?

Although often presumed to be constant and stable, natural isotope abundance ratios show significant and characteristic variations when measured very precisely. Isotope ratio measurements are useful in a wide range of applications, for example, metabolic studies using isotopically enriched elements as tracers; climate studies using measurements of temperature-dependent oxygen and carbon isotope ratios in foraminifers; rock age dating using radiogenic isotopes of elements such as lead, neodymium or strontium; and source determinations using carbon isotope ratios (is a substance naturally occurring or is it a pertroleum-based synthetic?).

In isotope ratio mass spectrometry, element isotope ratios are determined very accurately and precisely. Typically, single focusing magnetic sector mass spectrometers with fixed multiple detectors (one per isotope) are used. Complex compounds are reduced to simple molecules prior to measurement, for example, organic compounds are combusted to CO₂, H₂O and N₂.

What is elemental mass spectrometry?

In elemental mass spectrometry, a technique used mostly for inorganic materials, the elemental composition of a sample is determined rather than the structural identities of its chemical constituents. Elemental mass spectrometry provides quantitative information about the concentrations of those elements. The ion source used in elemental MS is ordinarily an atmospheric-pressure discharge such as the inductively coupled plasma (ICP) or a moderate-power device such as the glow-discharge source. In either case, the decomposition of the sample into its constituent atoms and ionization of those atoms occurs in a specially designed source. The resulting atomic-ion beam is then separated or sorted by a mass spectrometer and the signal as a function of m/z used to determine the sample composition. With an ICP employed as an ion source, solution detection limits down to the part-per-trillion level are possible in favorable cases, while with the glow-discharge source, solid metal samples can be analyzed directly and their elemental composition determined over a million-fold range of concentrations. Isotopic information is readily available, and samples can be analyzed very rapidly.