Precise measurement of the relative abundances of stable isotopes of an element, typically as simple gases admitted to a magnetic-sector instrument, defines isotope ratio mass spectrometry (IRMS). Dual-inlet or continuous-flow sources ionize CO₂, N₂, H₂, SO₂, or related gases; Faraday cups simultaneously collect the isotopologues, and the ratio is reported on an international delta scale relative to a standard ^[1]. Hyphenation to gas or liquid chromatography (GC/C/IRMS, LC-IRMS) converts each eluting compound to a simple gas before the ratio measurement, so that compound-specific isotope ratios are obtained ^[2]. Conceptual boundaries exclude organic molecular mass spectrometry that reports nominal or exact masses of intact molecules, accelerator mass spectrometry of rare radionuclides, and optical isotope-ratio methods. Essential features are simultaneous or rapidly switched collection of two or more isotopic ion beams, high-precision electronics, and a calibration to certified isotopic reference materials ^[3]. Compound-specific measurements of steroids retain the same defining observable: a calibrated isotope-abundance ratio for an element in the isolated compound ^[4].