Isotopes of the same element show subtle differences in chemical behavior.  They occur in the same compounds, but one isotope may be slightly enriched during the formation of the compound.  For example, the crepe myrtle in your backyard is enriched in ¹²C relative to the atmospheric CO₂ by about 2% (or as is commonly used, 20‰ [per mil]).  In one application of stable isotopes, researchers study the ¹³C/¹²C ratios of fossil shells to examine changes in the global rate of photosynthesis and burial of plant matter.   This provides a critical understanding of how the Earth's carbon cycle functions.  In related research, scientists make ¹³C/¹²C measurements of recent fossils to study the impact of the burning of fossil fuel on the atmosphere.  

       Researchers use a mass spectrometer to measure isotope ratios, and can measure carbon isotopic variation with a precision of better than 0.05‰.  This makes stable isotopes a very sensitive tool for studying global change.  Other stable isotope ratios used in studies of Earth processes and history (as well as biology, medicine, forensics, chemistry, among others) are ²H/¹H, ¹⁸O/¹⁶O, ¹⁵N/¹⁴N, and ³⁴S/³²S.  For an example of how carbon isotope ratios vary in nature, click here.