Just like in GCMS, where compounds are separated based on mass differences, in IRMS, or isotope ratio mass spectrometry, different isotopes of the same compound are separated based on minute mass differences. Depending on the type of compound being analysed, the ion stream is prepared through either a gas chromatagraph, an elemental analyzer or the gas bench. Details are on our IRMS page.
Once the analyte stream is prepared, like the GCMS, it is ionized by an ion source and concentrated into a beam by a magnet, which separates the heavy isotopes from the light isotopes. The electron multiplier detects the ions of different isotope masses over a range of mass/charge ratios.
This method is called compound-specific isotope analysis because prior to analyzing the isotope ratios, we have separated the bulk compound into biomarker compounds, such as phytane and pristane, for example. Thus, we get specific source information from the biomarker, combined with isotope information, which we use to more specifically reconstruct ancient environments as well as modern metabolic pathways.