A gas chromatograph combined with a combustion interface that burns the separated compounds to CO2, and with a special mass spectrometer that can then measure the relative abundance of isotopes. Used to determine the isotopic composition of individual molecular species.The acronym is sometimes also written in its all-uppercase form GC-IRM-MS. Synonymously, the acronyms irm-GC/MS and GC-IRMS are in use.
 Susan M. Gaines, Geoffrey Eglington, and Jürgen Rullkötter: Echoes of Life • What Fossil Molecules Reveal about Earth History. Oxford University Press, New York, 2009; see, for example, page 157 and glossary.
Further references to GC-irm-MS applications and results
 The graphical representation on page 158 in  gives a nice overview of differences of 13C isotopic composition found in various groups of organisms, the environment and geologic deposits.
 Group of 9 authors: Terrestrial vegetation change inferred from n-alkane σ13C analysis in the marine environment. Geochimica et Cosmochimica Acta 1995, 59, pp. 2853-2857.
 Bart E. van Dongen, Stefan Schouten and Jaap S. Sinninghe Damsté: Gas chromatography/combustion/isotope-ratio-monitoring mass spectrometeric analysis of methylboronic derivatives of monosaccharides: a new method for determining 13C abundances of carbohydrates. Rapid Communications in Mass Spectrometry 2001, 15 (7), pp. 496-500.