Linked from
The 14 pages that link to Isotope-ratio mass spectrometry, each with the reason it gives.
IsobarRelated: Its measurements must distinguish nuclides that may have closely spaced masses.
Isotopes of nitrogenRelated: It quantifies nitrogen-15 relative to nitrogen-14 in samples.
Semiheavy waterRelated: It can quantify deuterium enrichment in water samples containing HDO.
δ18ORelated: It is a standard technique for measuring oxygen isotope ratios.