Gas Source Isotope Ratio Mass Spectrometry (IRMS) of 1947
In 1947, Alfred Nier developed the first dual-inlet, double-collector gas-source mass spectrometer. The double collector allowed the simultaneous measurement of two isotopes and the dual inlet allowed ratio measurement on both a sample and a standard by alternating between inlets. It has since become the measurement technique of preference for isotope ratios of most of the light elements (e.g. H, C, N, O and S) because of its relative simplicity and because the use of international standards allows comparison of databases from different laboratories.
(See Environmental Isotopes in Hydrogeology [I. Clark and P. Fritz, 1997] for more information)

In the last few years, a new methodology to measure d¹³C contents of individual compounds in complex organic mixtures has been developed. This technique, which uses a capillary column gas chromatograph (GC), a combustion interface (C) to produce CO₂ , that is linked to a modified conventional gas mass-spectrometer (MS, as described above). This GC-C-MS method has the capability to measure individual carbon in mixtures of subnanogram samples with a precision of better than ±0.5%.