«The Concentration and Isotopic Abundances of Atmospheric Carbon Dioxide in Rural Areas.» (realclimate.org)
However, he cautions that the natural isotopic abundances might be different on Mars than on Earth, which would complicate the analysis. (newscientist.com)
Measurement techniques today are so sensitive that samples of food, water or pollutants can often be traced to their sources based on isotopic abundance. (scientificamerican.com)