Isotopic signature

An isotopic signature (also isotopic fingerprint) is a ratio of stable or unstable isotopes of particular elements found in an investigated material. The atomic mass of different isotopes affect their chemical kinetic behavior, leading to natural isotope separation processes.

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Carbon isotopes

For example, different sources and sinks of methane have different affinity for the ¹²C and ¹³C isotopes, which allows distinguishing between different sources by the ¹³C/¹²C ratio in methane in the air.

Similarly, carbon in inorganic carbonates shows little isotopic fractionation, while carbon in materials originated by photosynthesis is depleted of the heavier isotopes. In addition, there are two types of plants with different biochemical pathways; the C3 carbon fixation, where the isotope separation effect is more pronounced, and C4 carbon fixation, where the heavier ¹³C is less depleted. The different isotope ratios for the two kinds of plants propagate through the food chain, thus it is possible to determine if the principal diet of a human or an animal consists primarily of C3 plants (rice, wheat, soybeans, potatoes) or C4 plants (corn, or corn-fed beef) by isotope analysis of their flesh and bone collagen. Similarly, marine fish contain more ¹³C than freshwater fish, with values approximating the C4 and C3 plants respectively.

Limestones formed by precipitation in seas from the atmospheric carbon dioxide contain normal proportion of ¹³C. Conversely, calcite found in salt domes originates from carbon dioxide formed by oxidation of petroleum, which due to its plant origin is ¹³C-depleted.

The ¹⁴C isotope is important in distinguishing biosynthetized materials from man-made ones. Biogenic chemicals are derived from biospheric carbon, which contains ¹⁴C. Carbon in artificially made chemicals is usually derived from fossil fuels like coal or petroleum, where the ¹⁴C originally present has decayed below detectable limits. The amount of ¹⁴C currently present in a sample therefore indicates the proportion of carbon of biogenic origin.

Nitrogen isotopes

The ratio of ¹⁵N/¹⁴N presents a characteristic distinction between herbivores and carnivores, as the movement up along the food chain tends to concentrate the ¹⁵N isotope, by 3-4‰ with each step of the food chain (terrestrial plants, with the exception of legumes, has the isotopic ratio 2-6‰ of N). The tissues and hair of vegans therefore contain significantly lower percentage of ¹⁵N than the bodies of people who eat mostly meat. Isotopic analysis of hair is an important source of information for archaeologists, providing clues about the ancient diets; a terrestrial diet produces a different signature than a marine-based diet and this phenomenon has been used in analysing differing cultural attitudes to food sources.

Oxygen isotopes

Oxygen comes in two variants as well. The ratio of ¹⁸O/¹⁶O in water depends on the amount of evaporation the water experienced (as ¹⁸O is heavier and therefore less likely to vaporize). As the vapor tension depends on the concentration of dissolved salts, the ¹⁸O/¹⁶O ratio shows correlation on the salinity and temperature of water. As oxygen gets built into the shells of calcium carbonate secreting organisms, such sediments prove a chronological record of temperature and salinity of the water in the area.

Radioactive isotopes

Hot particles, radioactive particles of nuclear fallout and radioactive waste, also exhibit distinct isotopic signatures. Their radionuclide composition (and thus their age and origin) can be determined by mass spectroscopy or by gamma spectrometry. For example, particles generated by a nuclear blast contain detectable amounts of ⁶⁰Co and ¹⁵²Eu. The Chernobyl accident did not release these particles but did release ¹²⁵Sb and ¹⁴⁴Ce. Particles from underwater bursts will consist mostly of irradiated sea salts. Ratios of ¹⁵²Eu/¹⁵⁵Eu, ¹⁵⁴Eu/¹⁵⁵Eu, and ²³⁸Pu/²³⁹Pu are also different for fusion and fission nuclear weapons, which allows identification of hot particles of unknown origin.

Forensics use

With the advent of stable isotope ratio mass spectrometry, isotopic signatures of materials find increasing use in forensics, allowing disguising the origin of otherwise similar materials and tracking the materials to their common source. For example the isotope signatures of plants can be to a degree influenced by the growth conditions, including moisture and nutrient availability. In case of synthetic materials, the signature is influenced by the conditions during the chemical reaction. The isotopic signature profiling is useful in cases where other kinds of profiling, eg. characterization of impurities, are not optimal.

A study was published demonstrating the possibility of determination of the origin of a common brown PSA packaging tape by using the carbon, oxygen, and hydrogen isotopic signature of the backing polymer, additives, and adhesive[1].