13C

13C is a stable isotope of carbon with six protons and seven neutrons, giving it a mass number of 13. It is one of the two stable carbon isotopes commonly found in nature, the other being 12C. About 1.1% of carbon atoms on Earth are 13C. The nucleus has spin I = 1/2, which makes 13C detectable by nuclear magnetic resonance (NMR). Its atomic mass is 13.003355 u.

Natural abundance of 13C varies slightly among reservoirs due to isotopic fractionation during biological and geological processes. The 13C/12C ratio is routinely measured to study biogeochemical cycles, plant physiology, and paleoenvironmental conditions. The ratio is often reported as delta13C (δ13C), a per mil (‰) value relative to a standard.

Analytical methods and standards: Isotope ratios are measured with isotope ratio mass spectrometry (IRMS) or high-resolution NMR. δ13C values are reported relative to the Vienna Pee Dee Belemnite (VPDB) standard. Typical δ13C values span broad ranges depending on material and source, from negative values for many organic matter to near 0‰ for marine carbonates.

Applications: In chemistry, 13C NMR is a powerful tool for determining carbon skeletons, aided by higher sensitivity through enriched 13C samples and various multidimensional experiments. In biochemistry and physiology, 13C labeling is used to trace metabolic pathways and reaction mechanisms. In ecology and earth science, δ13C values help distinguish carbon sources (for example, C3 versus C4 plant material) and inform paleoclimate and ecosystem studies.

Safety and handling: 13C is non-radioactive and generally safe to handle; enrichment materials can be costly and require standard laboratory safety practices for organic compounds.