[2] 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 (however, to obtain more accurate determinations, carbon isotopic fractionation must be also taken into account, since several studies have reported significant 13C discrimination during biodegradation of simple and complex substrates).
The ratio of carbon-13 and carbon-12 isotopes in these types of plants is as follows:[11] Limestones formed by precipitation in seas from the atmospheric carbon dioxide contain normal proportion of 13C.
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.
[12] An extension of this research resulted in development of DNA-based stable-isotope probing, which allows examination of links between metabolic function and taxonomic identity of microorganisms in the environment, without the need for culture isolation.
[19] 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.
[21] Furthermore, elevated δ15N have been linked to the preferential excretion of 14N and reutilization of already enriched 15N tissues in the body under prolonged water stress conditions or insufficient protein intake.
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 is incorporated into the shells of calcium carbonate-secreting organisms, such sediments provide a chronological record of temperature and salinity of the water in the area.
[26] The oxygen isotopic signatures of solid samples (organic and inorganic) are usually measured with pyrolysis and mass spectrometry.
[27] Sulfur has four stable isotopes, 32S, 33S, 34S, and 36S, of which 32S is the most abundant by a large margin due to the fact it is created by the very common 12C in supernovas.
Differences in sulfur isotope ratios are thought to exist primarily due to kinetic fractionation during reactions and transformations.
As a very redox-active element, sulfur can be useful for recording major chemistry-altering events throughout Earth's history, such as marine evaporites which reflect the change in the atmosphere's redox state brought about by the Oxygen Crisis.
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.
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.
Uranium has a relatively constant isotope ratio in all natural samples with ~0.72% 235U, some 55 ppm 234U (in secular equilibrium with its parent nuclide 238U), and the balance made up by 238U.
Isotopic compositions that diverge significantly from those values are evidence for the uranium having been subject to depletion or enrichment in some fashion or of (part of it) having participated in a nuclear fission reaction.
In archaeological studies, stable isotope ratios have been used to track diet within the time span formation of analyzed tissues (10–15 years for bone collagen and intra-annual periods for tooth enamel bioapatite) from individuals; "recipes" of foodstuffs (ceramic vessel residues); locations of cultivation and types of plants grown (chemical extractions from sediments); and migration of individuals (dental material).
For example, the isotope signatures of plants can be to a degree influenced by the growth conditions, including moisture and nutrient availability.
Electronics coupled with scintillator detectors are routinely used to evaluate isotope signatures and identify unknown sources.
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.
[39] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body.
[47] Modern sulfate-reducing bacteria are known to favorably reduce lighter 32S instead of 34S, and the presence of these microorganisms can measurably alter the sulfur isotope composition of the ocean.
[51] Because it has not been proven that the sulfide and barite minerals formed in the absence of major hydrothermal input, it is not conclusive evidence of life or of the microbial sulfate reduction pathway in the Archean.