Isotopic signature

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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 against an isotopic reference material. This process is called isotope analysis.

Contents

Stable isotopes

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

Carbon isotopes

Algal groupδ13C range [1]
HCO3-using red algae 22.5 to 9.6
CO2-using red algae34.5 to 29.9
Brown algae 20.8 to 10.5
Green algae 20.3 to 8.8

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. The ratio is calculated with respect to Pee Dee Belemnite (PDB) standard:

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. Isotopic fractionation in plants is caused by physical (slower diffusion of 13C in plant tissues due to increased atomic weight) and biochemical (preference of 12C by two enzymes: RuBisCO and phosphoenolpyruvate carboxylase) factors. [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). [3] [4] Within C3 plants processes regulating changes in δ13C are well understood, particularly at the leaf level, [5] but also during wood formation. [6] [7] Many recent studies combine leaf level isotopic fractionation with annual patterns of wood formation (i.e. tree ring δ13C) to quantify the impacts of climatic variations and atmospheric composition [8] on physiological processes of individual trees and forest stands. [9] The next phase of understanding, in terrestrial ecosystems at least, seems to be the combination of multiple isotopic proxies to decipher interactions between plants, soils and the atmosphere, and predict how changes in land use will affect climate change. [10] 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: [11]

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 layer of limestone deposited at the Permian extinction 252 Mya can be identified by the 1% drop in 13C/12C.

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

Nitrogen-15, or 15N, is often used in agricultural and medical research, for example in the Meselson–Stahl experiment to establish the nature of DNA replication. [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. [13] [14] Proteins can be isotopically labelled by cultivating them in a medium containing 15N as the only source of nitrogen, e.g., in quantitative proteomics such as SILAC.

Nitrogen-15 is extensively used to trace mineral nitrogen compounds (particularly fertilizers) in the environment. When combined with the use of other isotopic labels, 15N is also a very important tracer for describing the fate of nitrogenous organic pollutants. [15] [16] Nitrogen-15 tracing is an important method used in biogeochemistry.

The ratio of stable nitrogen isotopes, 15N/14N or δ15N, 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 parts per thousand with each increase in trophic level. [17] The tissues and hair of vegans therefore contain significantly lower δ15N than the bodies of people who eat mostly meat. Similarly, a terrestrial diet produces a different signature than a marine-based diet. Isotopic analysis of hair is an important source of information for archaeologists, providing clues about the ancient diets and differing cultural attitudes to food sources. [18]

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 14N, increasing δ15N in soils and plants. [19] This leads to relatively high δ15N values in plants and animals in hot and arid ecosystems relative to cooler and moister ecosystems. [20] 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. [21] [22]

δ15N also provides a diagnostic tool in planetary science as the ratio exhibited in atmospheres and surface materials "is closely tied to the conditions under which materials form". [23]

Oxygen isotopes

Oxygen comes in three variants, but the 17 O is so rare that it is very difficult to detect (~0.04% abundant). [24] 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 18 O/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. [25]

The oxygen isotopic signatures of solid samples (organic and inorganic) are usually measured with pyrolysis and mass spectrometry. [26] Researchers need to avoid improper or prolonged storage of the samples for accurate measurements. [26]

Sulfur Isotopes

Sulfur has four stable isotopes, 32 S, 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. Sulfur isotope ratios are almost always expressed as ratios relative to 32S due to this major relative abundance (95.0%). Sulfur isotope fractionations are usually measured in terms of δ34S due to its higher abundance (4.25%) compared to the other stable isotopes of sulfur, though δ33S is also sometimes measured. Differences in sulfur isotope ratios are thought to exist primarily due to kinetic fractionation during reactions and transformations.

Sulfur isotopes are generally measured against standards; prior to 1993, the Canyon Diablo troilite standard (abbreviated to CDT), which has a 32S:34S equal to 22.220, was used as both a reference material and the zero point for the isotopic scale. Since 1993, the Vienna-CDT standard has been used as a zero point, and there are several materials used as reference materials for sulfur isotope measurements. Sulfur fractionations by natural processes measured against these standards have been shown to exist between -72‰ and +147‰, [27] [28] as calculated by the following equation:

Natural sulfur isotope values
Natural Source δ34S range
Petroleum [29] -32‰ to -8‰
River water [30] -8‰ to 10‰
Lunar rocks [30] -2‰ to 2.5‰
Meteorites [30] 0‰ to 2‰
Ocean water [30] 17‰ to 20‰
Most relevant isotopes of sulfur
Isotope Abundance Half-life
32S94.99%Stable
33S0.75%Stable
34S4.25%Stable
35S<0.1%87.4 days
36S0.01%Stable

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. [31] [32]

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. [25]

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 60 Co and 152 Eu. The Chernobyl accident did not release these particles but did release 125 Sb and 144 Ce. Particles from underwater bursts will consist mostly of irradiated sea salts. Ratios of 152 Eu/155Eu, 154Eu/155Eu, and 238 Pu/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% 235
U
some 55 ppm 234
U
(in secular equilibrium with its parent nuclide 238
U
) and the balance made up by 238
U
. 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. While the latter is almost as universally due to human influence as the former two, the natural nuclear fission reactor at Oklo, Gabon was detected through a significant diversion of 235
U
concentration in samples from Oklo compared to those of all other known deposits on earth. Given that 235
U
is a material of proliferation concern then as now every IAEA-approved supplier of Uranium fuel keeps track of the isotopic composition of uranium to ensure none is diverted for nefarious purposes. It would thus become apparent quickly if another Uranium deposit besides Oklo proves to have once been a natural nuclear fission reactor.

Applications

Archaeological studies

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).[ citation needed ]

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.

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. [33]

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. [34] As low as 7% level of addition can be detected. [35]

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. [36]

Solar System origins

Isotopic fingerprints are used to study the origin of materials in the Solar System. [37] For example, the Moon's oxygen isotopic ratios seem to be essentially identical to Earth's. [38] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body. [39] Different oxygen isotopic signatures can indicate the origin of material ejected into space. [40] The Moon's titanium isotope ratio (50Ti/47Ti) appears close to the Earth's (within 4 ppm). [41] [42] 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. [37] [43]

Records of Early Life on Earth

Isotope biogeochemistry has been used to investigate the timeline surrounding life and its earliest iterations on Earth. Isotopic fingerprints typical of life, preserved in sediments, have been used to suggest, but do not necessarily prove, that life was already in existence on Earth by 3.85 billion years ago. [44]

Sulfur isotope evidence has also been used to corroborate the timing of the Great Oxidation Event, during which the Earth's atmosphere experienced a measurable rise in oxygen (to about 9% of modern values [45] ) for the first time about 2.3-2.4 billion years ago. Mass-independent sulfur isotope fractionations are found to be widespread in the geologic record before about 2.45 billion years ago, and these isotopic signatures have since ceded to mass-dependent fractionation, providing strong evidence that the atmosphere shifted from anoxic to oxygenated at that threshold. [46]

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. [31] Because the δ34S values of sulfide minerals is primarily influenced by the presence of sulfate-reducing bacteria, [47] the absence of sulfur isotope fractionations in sulfide minerals suggests the absence of these bacterial processes or the absence of freely available sulfate. Some have used this knowledge of microbial sulfur fractionation to suggest that minerals (namely pyrite) with large sulfur isotope fractionations relative to the inferred seawater composition may be evidence of life. [48] [49] This claim is not clear-cut, however, and is sometimes contested using geologic evidence from the ~3.49 Ga sulfide minerals found in the Dresser Formation of Western Australia, which are found to have δ34S values as negative as -22‰. [50] 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. [51]

See also

Related Research Articles

<span class="mw-page-title-main">Isotope analysis</span> Analytical technique used to study isotopes

Isotope analysis is the identification of isotopic signature, abundance of certain stable isotopes of chemical elements within organic and inorganic compounds. Isotopic analysis can be used to understand the flow of energy through a food web, to reconstruct past environmental and climatic conditions, to investigate human and animal diets, for food authentification, and a variety of other physical, geological, palaeontological and chemical processes. Stable isotope ratios are measured using mass spectrometry, which separates the different isotopes of an element on the basis of their mass-to-charge ratio.

Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth.

Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.

Isotopic labeling is a technique used to track the passage of an isotope through a reaction, metabolic pathway, or cell. The reactant is 'labeled' by replacing specific atoms by their isotope. The reactant is then allowed to undergo the reaction. The position of the isotopes in the products is measured to determine the sequence the isotopic atom followed in the reaction or the cell's metabolic pathway. The nuclides used in isotopic labeling may be stable nuclides or radionuclides. In the latter case, the labeling is called radiolabeling.

Mass-independent isotope fractionation or Non-mass-dependent fractionation (NMD), refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes. Most isotopic fractionations are caused by the effects of the mass of an isotope on atomic or molecular velocities, diffusivities or bond strengths. Mass-independent fractionation processes are less common, occurring mainly in photochemical and spin-forbidden reactions. Observation of mass-independently fractionated materials can therefore be used to trace these types of reactions in nature and in laboratory experiments.

The term bioarchaeology has been attributed to British archaeologist Grahame Clark who, in 1972, defined it as the study of animal and human bones from archaeological sites. Redefined in 1977 by Jane Buikstra, bioarchaeology in the United States now refers to the scientific study of human remains from archaeological sites, a discipline known in other countries as osteoarchaeology, osteology or palaeo-osteology. Compared to bioarchaeology, osteoarchaeology is the scientific study that solely focus on the human skeleton. The human skeleton is used to tell us about health, lifestyle, diet, mortality and physique of the past. Furthermore, palaeo-osteology is simple the study of ancient bones.

An isoscape is a geologic map of isotope distribution. It is a spatially explicit prediction of elemental isotope ratios (δ) that is produced by executing process-level models of elemental isotope fractionation or distribution in a geographic information system (GIS).

<i>δ</i><sup>13</sup>C Measure of relative carbon-13 concentration in a sample

In geochemistry, paleoclimatology, and paleoceanography δ13C is an isotopic signature, a measure of the ratio of the two stable isotopes of carbon—13C and 12C—reported in parts per thousand. The measure is also widely used in archaeology for the reconstruction of past diets, particularly to see if marine foods or certain types of plants were consumed.

In geochemistry, hydrology, paleoclimatology and paleoceanography, δ15N or delta-N-15 is a measure of the ratio of the two stable isotopes of nitrogen, 15N:14N.

<span class="mw-page-title-main">Stable isotope ratio</span> Ratio of two stable isotopes

The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundance of such stable isotopes can be measured experimentally, yielding an isotope ratio that can be used as a research tool. Theoretically, such stable isotopes could include the radiogenic daughter products of radioactive decay, used in radiometric dating. However, the expression stable-isotope ratio is preferably used to refer to isotopes whose relative abundances are affected by isotope fractionation in nature. This field is termed stable isotope geochemistry.

Clumped isotopes are heavy isotopes that are bonded to other heavy isotopes. The relative abundance of clumped isotopes (and multiply-substituted isotopologues) in molecules such as methane, nitrous oxide, and carbonate is an area of active investigation. The carbonate clumped-isotope thermometer, or "13C–18O order/disorder carbonate thermometer", is a new approach for paleoclimate reconstruction, based on the temperature dependence of the clumping of 13C and 18O into bonds within the carbonate mineral lattice. This approach has the advantage that the 18O ratio in water is not necessary (different from the δ18O approach), but for precise paleotemperature estimation, it also needs very large and uncontaminated samples, long analytical runs, and extensive replication. Commonly used sample sources for paleoclimatological work include corals, otoliths, gastropods, tufa, bivalves, and foraminifera. Results are usually expressed as Δ47 (said as "cap 47"), which is the deviation of the ratio of isotopologues of CO2 with a molecular weight of 47 to those with a weight of 44 from the ratio expected if they were randomly distributed.

The δ34S value is a standardized method for reporting measurements of the ratio of two stable isotopes of sulfur, 34S:32S, in a sample against the equivalent ratio in a known reference standard. Presently, the most commonly used standard is Vienna-Canyon Diablo Troilite (VCDT). Results are reported as variations from the standard ratio in parts per thousand, per mil or per mille, using the ‰ symbol. Heavy and light sulfur isotopes fractionate at different rates and the resulting δ34S values, recorded in marine sulfate or sedimentary sulfides, have been studied and interpreted as records of the changing sulfur cycle throughout the earth's history.

Isotopic reference materials are compounds with well-defined isotopic compositions and are the ultimate sources of accuracy in mass spectrometric measurements of isotope ratios. Isotopic references are used because mass spectrometers are highly fractionating. As a result, the isotopic ratio that the instrument measures can be very different from that in the sample's measurement. Moreover, the degree of instrument fractionation changes during measurement, often on a timescale shorter than the measurement's duration, and can depend on the characteristics of the sample itself. By measuring a material of known isotopic composition, fractionation within the mass spectrometer can be removed during post-measurement data processing. Without isotope references, measurements by mass spectrometry would be much less accurate and could not be used in comparisons across different analytical facilities. Due to their critical role in measuring isotope ratios, and in part, due to historical legacy, isotopic reference materials define the scales on which isotope ratios are reported in the peer-reviewed scientific literature.

<span class="mw-page-title-main">Marilyn Fogel</span> American geo-ecologist (1952–2022)

Marilyn L. Fogel was an American geo-ecologist and Professor of Geo-ecology at UC Riverside in Riverside, California. She is known for her research using stable isotope mass spectrometry to study a variety of subjects including ancient climates, biogeochemical cycles, animal behavior, ecology, and astrobiology. Fogel served in many leadership roles, including Program Director at the National Science Foundation in geobiology and low-temperature geochemistry.

The geochemistry of carbon is the study of the transformations involving the element carbon within the systems of the Earth. To a large extent this study is organic geochemistry, but it also includes the very important carbon dioxide. Carbon is transformed by life, and moves between the major phases of the Earth, including the water bodies, atmosphere, and the rocky parts. Carbon is important in the formation of organic mineral deposits, such as coal, petroleum or natural gas. Most carbon is cycled through the atmosphere into living organisms and then respirated back into the atmosphere. However an important part of the carbon cycle involves the trapping of living matter into sediments. The carbon then becomes part of a sedimentary rock when lithification happens. Human technology or natural processes such as weathering, or underground life or water can return the carbon from sedimentary rocks to the atmosphere. From that point it can be transformed in the rock cycle into metamorphic rocks, or melted into igneous rocks. Carbon can return to the surface of the Earth by volcanoes or via uplift in tectonic processes. Carbon is returned to the atmosphere via volcanic gases. Carbon undergoes transformation in the mantle under pressure to diamond and other minerals, and also exists in the Earth's outer core in solution with iron, and may also be present in the inner core.

<span class="mw-page-title-main">Fractionation of carbon isotopes in oxygenic photosynthesis</span>

Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.

<span class="mw-page-title-main">Kinetic isotope effects of RuBisCO</span>

The kinetic isotope effect (KIE) of ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) is the isotopic fractionation associated solely with the step in the Calvin-Benson cycle where a molecule of carbon dioxide is attached to the 5-carbon sugar ribulose-1,5-bisphosphate (RuBP) to produce two 3-carbon sugars called 3-phosphoglycerate. This chemical reaction is catalyzed by the enzyme RuBisCO, and this enzyme-catalyzed reaction creates the primary kinetic isotope effect of photosynthesis. It is also largely responsible for the isotopic compositions of photosynthetic organisms and the heterotrophs that eat them. Understanding the intrinsic KIE of RuBisCO is of interest to earth scientists, botanists, and ecologists because this isotopic biosignature can be used to reconstruct the evolution of photosynthesis and the rise of oxygen in the geologic record, reconstruct past evolutionary relationships and environmental conditions, and infer plant relationships and productivity in modern environments.

Sulfur isotope biogeochemistry is the study of the distribution of sulfur isotopes in biological and geological materials. In addition to its common isotope, 32S, sulfur has three rare stable isotopes: 34S, 36S, and 33S. The distribution of these isotopes in the environment is controlled by many biochemical and physical processes, including biological metabolisms, mineral formation processes, and atmospheric chemistry. Measuring the abundance of sulfur stable isotopes in natural materials, like bacterial cultures, minerals, or seawater, can reveal information about these processes both in the modern environment and over Earth history.

The stable isotope composition of amino acids refers to the abundance of heavy and light non-radioactive isotopes of carbon, nitrogen, and other elements within these molecules. Amino acids are the building blocks of proteins. They are synthesized from alpha-keto acid precursors that are in turn intermediates of several different pathways in central metabolism. Carbon skeletons from these diverse sources are further modified before transamination, the addition of an amino group that completes amino acid biosynthesis. Bonds to heavy isotopes are stronger than bonds to light isotopes, making reactions involving heavier isotopes proceed slightly slower in most cases. This phenomenon, known as a kinetic isotope effect, gives rise to isotopic differences between reactants and products that can be detected using isotope ratio mass spectrometry. Amino acids are synthesized via a variety of pathways with reactions containing different, unknown isotope effects. Because of this, the 13C content of amino acid carbon skeletons varies considerably between the amino acids. There is also an isotope effect associated with transamination, which is apparent from the abundance of 15N in some amino acids.

Isotope analysis has many applications in archaeology, from dating sites and artefacts, determination of past diets and migration patterns and for environmental reconstruction.

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