Isotope Ratio Mass Spectrometry (IRMS).
Isotope ratio MS is capable of very precise determination of ¹³C/¹²C rations and can be combined with GC-MS. It is exploited principally in examining trace enrichment of 13C in small molecular-weight analytes (e.g. protein-derived amino acids) after biosynthetic incorporation of a ¹³C-labelled precursor. Applications of IRMS include the study of substrate disposition in humans after infusion of ¹³C-labelled precursors. Other elemental rations (e.g. ²H/¹H, and ¹⁸O/¹⁶O) can also be measured although it is mainly used for detection of ¹³C/¹²C and ¹⁵N/¹⁴N ratios, that are useful in proteomics research.