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Key Idea 4
Natural Isotopic Variability
Concept Question: How can samples containing atoms of the same element from different places or times have different atomic weights?

What do we know?

As mentioned earlier, the abundances of the isotopes of an element can vary slightly depending on the origin of the sample.

 Otzi image with reference

Because isotopic abundances depend on the origin of a sample, the isotope ratios of atoms found in a sample can sometimes be used to indicate the physical location from which the sample was taken. Forensic researchers have used this approach to analyze human mummies and determine where the person had lived. For example, in 1991, a melting glacier in the Alps revealed a well-preserved 5000-year-old mummy, who has since been named Ötzi (see the image on the right). By comparing the isotopic composition of strontium, lead and oxygen atoms in Ötzi’s body with the isotopic composition of these elements in the surrounding environment, researchers were able to determine that Ötzi had spent most of his life within 60 km of where his body was found.


How do we know?

Through the consumption of food and nutrients, the isotopic composition of substances in a person’s environment is reflected in the mineralized parts of the person’s body. Therefore, the isotopic composition of substances in a person’s body will be determined by the area where the person spent his or her life. The mass spectral data produced by samples taken from Ötzi’s body matched the data produced by samples from the surrounding environment. Isotope ratios produced by environmental samples further from Ötzi’s location did not correspond with the ratios found in Ötzi’s body. If isotopic abundances did not vary throughout the Earth, use of this analytical technique would not be possible, as the isotopic abundances of samples with different origins would all be the same.