Isotopic signature

An isotopic signature (also isotopic fingerprint) is a ratio of non-radiogenic 'stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample material are measured by isotope ratio mass spectrometry. See also isotope analysis.

Stable isotopes

The atomic mass of different isotopes affect their chemical kinetic behavior, leading to natural isotope separation processes.

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. In geochemistry, paleoclimatology and paleoceanography this ratio is called δ¹³C.

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, C4 carbon fixation, where the heavier ¹³C is less depleted, and Crassulacean Acid Metabolism (CAM) plants, where the effect is similar but less pronounced than with C₄ plants. 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.

The ratio of carbon-13 and carbon-12 isotopes in these types of plants is as follows:^[2]

• C₄ plants: -16 to -10 ‰
• CAM plants: -20 to -10 ‰
• C₃ plants: -33 to -24 ‰

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 tends to increase with trophic level, such that herbivores have higher nitrogen isotope values than plants, and carnivores have higher nitrogen isotope values than herbivores. Depending on the tissue being examined, there tends to be an increase of 3-4‰ at each trophic level. A number of other environmental and physiological factors can influence the nitrogen isotopic composition at the base of the food web (i.e. in plants) or at the level of individual animals. For example, in arid regions, the nitrogen cycle tends to be more 'open' and prone to the loss of nitrogen (¹⁴N in particular), and concentrates soils and plants in ¹⁵N ¹⁴N.^[3] This leads to relatively high δ¹⁵N values in plants and animals in hot and arid ecosystems relative to cooler and moister ecosystems.^[4]

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.^[5]

Oxygen isotopes

Oxygen comes in three variants, but the ¹⁷O is so rare that it is very difficult to detect (~0.04% abundant).^[6] 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.

Oxygen isotope ratio in atmosphere varies predictably with time of year and geographic location; e.g. there is a 2% difference between ¹⁸O-rich precipitation in Montana and ¹⁸O-depleted precipitation in Florida Keys. This variability can be used for approximate determination of geographic location of origin of a material; e.g. it is possible to determine where a shipment of uranium oxide was produced. The rate of exchange of surface isotopes with the environment has to be taken in account.^[7]

Radiogenic isotopes

Lead isotopes

Lead consists of four stable isotopes: ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb. Local variations in uranium/thorium/lead content cause a wide location-specific variation of isotopic ratios for lead from different localities. Lead emitted to the atmosphere by industrial processes has an isotopic composition different from lead in minerals. Combustion of gasoline with tetraethyllead additive led to formation of ubiquitous micrometer-sized lead-rich particulates in car exhaust smoke; especially in urban areas the man-made lead particles are much more common than natural ones. The differences in isotopic content in particles found in objects can be used for approximate geolocation of the object's origin.^[7]

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 spectrometry 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.

Applications

Forensics

With the advent of stable isotope ratio mass spectrometry, isotopic signatures of materials find increasing use in forensics, distinguishing 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, e.g. characterization of impurities, are not optimal. Electronics coupled with scintillator detectors are routinely used to evaluate isotope signatures and identify unknown sources. (one example - SAM Isotope Identifier )

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.^[8]

Measurement of carbon isotopic ratios can be used for detection of adulteration of honey. Addition of sugars originated from corn or sugar cane (C4 plants) skews the isotopic ratio of sugars present in honey, but does not influence the isotopic ratio of proteins; in an unadulterated honey the carbon isotopic ratios of sugars and proteins should match.^[9] As low as 7% level of addition can be detected.^[10]

Nuclear explosions form ¹⁰Be by a reaction of fast neutrons with ¹³C in the carbon dioxide in air. This is one of the historical indicators of past activity at nuclear test sites.^[11]

Solar system origins

Isotopic fingerprints are used to study the origin of materials in the Solar System.^[12] For example, the Moon's oxygen isotopic ratios seem to be essentially identical to Earth's.^[13] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body.^[14] Different oxygen isotopic signatures can indicated the origin of material ejected into space.^[15] The Moon's titanium isotope ratio (⁵⁰Ti/⁴⁷Ti) appears close to the Earth's (within 4 ppm).^[16][17] In 2013, a study was released that indicated water in lunar magma was 'indistinguishable' from carbonaceous chondrites and nearly the same as Earth's, based on the composition of water isotopes.^[12][18]

See also

• Isoscapes
• Isotope electrochemistry

References

Further reading

• Carbon isotopes: you are what you eat
• Hair-rising research
• Ayacucho Archaeo Isotope Project
• The pursuit of isotopic and molecular fire tracers in the polar atmosphere and cryosphere