Isotope investigation

Isotope studies determine the proportion of isotopes of a chemical element within a sample. Most chemical elements have multiple isotopes. With a mass spectrometer , this isotope composition (the isotope ) can be determined very precisely (up to nanograms of sample quantity and, depending on the element and isotope, up to ppt accuracy).

Measurement method

The isotope investigation is carried out by mass spectrometry . In the mass spectrometer, the isotopes are deflected from their trajectory to different degrees depending on their mass and charge and recorded as peaks. The higher the concentration of an isotope, the larger the peak output. To calculate the isotopic composition, international standards (different for the various elements) are used, which are measured together with the samples and have a defined isotopic composition. Application examples for isotope studies:

Measuring radiogenic isotopes provides conclusions about the age of a particular mineral - or rock sample, see geochronology and Radiometric dating .
Isotopes of lighter elements (especially carbon , oxygen , nitrogen , sulfur , hydrogen ) serve, for example, as evidence of the regional and climatic origin of foods, such as fruit juices, or of the environmental conditions that occur during the formation of mussels or foraminifera -Shells have ruled the ocean.
In organic chemistry , isotope studies with lighter elements are used to clarify reaction mechanisms .
Oxygen and hydrogen isotopes can be used in plant ecology to determine the water sources of plants.
The causes, effects and applications of isotope effects can be investigated.
By examining the distribution or the proportion of deuterium in an organic molecule with ² H-NMR spectroscopy , a statement can be made about the origin of the substance.
Different ore deposits of a metal often differ in the proportions of the isotopes they contain, so that conclusions can be drawn about the deposit from the determination of the proportions. This is particularly important in the case of metallic archaeological finds and is used, among other things, to reconstruct early trade routes.

Overview

Isotope ratio	Fractionation	use
δ18O	biological, climatic	for petrological , stratigraphic or paleoclimatological investigations
δ2H	biological, climatic	z. B. Water and Wine Studies
δ13C	biological, anthropogenic	for studies in geochemistry , paleoclimatology and paleoceanography
δ15N	biological, anthropogenic	for studies in geochemistry , paleoclimatology and paleoceanography
δ34S	anthropogenic, geological
δ208Pb	anthropogenic, geological
δ87Sr	geological
δ143Nd	geological

terminology

The results of an isotope measurement are given as the ratio of heavy to light isotopes and listed as delta values ( ). All isotopes are measured as the relative difference to an international standard and are given in per thousand . For example is ${\ displaystyle \ delta}$

{\ displaystyle \ delta ^ {18} \ mathrm {O} = {\ frac {\ left ({\ frac {{} ^ {18} \ mathrm {O}} {{} ^ {16} \ mathrm {O} }} \ right) _ {\ text {sample}} - \ left ({\ frac {{} ^ {18} \ mathrm {O}} {{} ^ {16} \ mathrm {O}}} \ right) _ {\ text {Standard}}} {\ left ({\ frac {{} ^ {18} \ mathrm {O}} {{} ^ {16} \ mathrm {O}}} \ right) _ {\ text {Standard}}}} \ cdot 1000 \, {} ^ {0 \!} \! / \! _ {00}}

.

Hydrogen isotopes

Hydrogen (H) has two stable isotopes: protium or protons ( ¹ H) and deuterium ( ² H or D) and the radioactive (unstable) isotope tritium ( ³ H or T) with a half-life of 12.3 years. There are only a few kilograms of tritium as a natural occurrence on earth. It is created by cosmic rays in the upper layers of the atmosphere .

Deuterium is also referred to as heavy and tritium as super-heavy hydrogen.

Tritium method

Rainwater contains tritium, which was created by cosmic rays in the atmosphere. Since tritium decays over time, the tritium method can be used to determine the age of spring water, for example.

Because tritium is so rare in nature, the smallest contamination from technical applications can be easily identified.

Oxygen isotopes

Oxygen has 3 stable isotopes: ¹⁶ O, ¹⁷ O and ¹⁸ O.

The ratio ¹⁸ O / ¹⁶ O is usually measured for investigations , because ¹⁷ O occurs in amounts that are difficult to detect. The isotope ratio of the Vienna Standard Mean Ocean Water (VSMOW) is mainly used as the standard for calculating the ratio (see terminology) . ${\ displaystyle \ delta ^ {18} \ mathrm {O}}$

The isotope ratio of ¹⁸ O / ¹⁶ O in water vapor in the atmosphere and in the water of all bodies of water differs from region to region. As with condensation, isotope fractionation occurs in the evaporation of water . During evaporation the lighter isotope preferentially changes into the vapor, during condensation (e.g. cloud formation and rain) the heavier isotope preferentially changes into the liquid phase. The isotope fractionation is temperature dependent, so that precipitation in cool regions has a lower ¹⁸ O / ¹⁶ O ratio (and also lower D / H ratio) than in regions with a hot climate. Seasonal temperature fluctuations are also reflected in changes in the isotope ratio in rainwater.

This fact is used in archaeometry for palaeo-temperature reconstruction. Mammals build oxygen isotopes into their bones and teeth. The ratio depends on the value of the drinking water. The relationships are species-specific and can be applied to bone and tooth finds from archaeological excavations. From the analysis of tooth enamel , which does not change in the course of the life of an adult mammal, conclusions can be drawn about the climate in which the animal grew up. The higher the value, the higher the temperature. ${\ displaystyle \ delta ^ {18} \ mathrm {O}}$ ${\ displaystyle \ delta ^ {18} \ mathrm {O}}$

¹⁸ O is used in organic and biochemical reactions to elucidate the reaction mechanism . Either elemental oxygen or oxygen bound in water ( ¹⁸ O ₂ , H ₂¹⁸ O) is used here. Well-known examples of this are the formation or hydrolysis of esters . In the case of biochemical reactions, dehydrogenation in particular can be clarified in enzymatic reactions.

Carbon isotopes

Carbon (C) has two stable isotopes : ¹² C (98.89%), ¹³ C (1.11%) and the unstable ¹⁴ C isotope (0.000 000 000 1%). The latter is the basis for the best known application of isotope studies, radiocarbon dating, in which the ¹⁴ C content is measured to determine the age of organic samples .

The ratio of the two stable isotopes is also used for scientific questions. Natural isotope fractionation takes place between ¹² C and ¹³ C during photosynthesis . C ₃ plants , such as wheat, only have the photosynthetic enzyme RuBisCO (ribulose 1,5-bisphosphate carboxylase oxygenase) to fix CO ₂ . It discriminates against the heavier δ ¹³ C isotope and preferentially fixes lighter CO ₂ molecules. The δ ¹³ C values of C ₃ plants are in the range of −26.5 ‰. In C ₄ plants such as millet and maize, CO ₂ fixation takes place differently, and there, in addition to RuBisCO, phosphoenolpyruvate carboxylase (PEP carboxylase) takes place, through which CO ₂ in the form of hydrogen carbonate (HCO ₃^- ) is much higher Affinity is pre-fixed. The PEP carboxylase does not discriminate against the heavier δ ¹³ C isotope, which is expressed in a more positive δ ¹³ C ratio of approx. −12.5 ‰ of the C ₄ plants . The deviation is set in relation to the Pee Dee Belemnite standard and this ratio is stated. Due to the special CO ₂ fixation of the CAM plants, even more positive δ ¹³ C ratios can be observed in nature. Plankton and marine animals are even more valuable. This enables anthropologists, for example , to draw conclusions about nutrition based on the δ ¹³ C value of human bones. This is particularly interesting in connection with the δ ¹⁵ N.

¹³ C nuclear magnetic resonance spectroscopy , which is also used in organic chemistry to elucidate chemical structures , is suitable for the investigation .

Nitrogen isotopes

Nitrogen (N) has the two stable isotopes ¹⁴ N (99.634%) and ¹⁵ N (0.366%). The isotope ratio of air is used as the standard for calculating the δ ¹⁵ N ratio (see terminology).

Isotope fractionation takes place in the nitrogen cycle primarily in the interaction between plants and microorganisms in the soil. Dry savannah and desert soils contain more ¹⁵ N than moist, cool forest soils in temperate regions. Biological materials enrich the heavy isotope in relation to the atmosphere. Further accumulations are observed within the food chain . Carnivores, as the last link in the food chain, show the highest values. In archaeometry, the analysis of the N isotope ratio is used to draw conclusions about the diet of animals and humans from bone finds. A predominantly meat diet was also determined for the Neanderthals due to its δ ¹⁵ N value.

Potassium-argon system

The argon method makes use of the fact that the usually solid element potassium ⁴⁰ K with a half-life of 1.3 billion years breaks down to the gaseous ⁴⁰ Ar, which can escape from a melt but not from a solid. In geology it is used to date the solidification time of volcanic materials.

Rubidium-Strontium System

Strontium has four stable, naturally occurring isotopes : ⁸⁴ Sr (0.56%), ⁸⁶ Sr (9.86%), ⁸⁷ Sr (7.0%), and ⁸⁸ Sr (82.58%).

⁸⁷ Sr is a decay product of ⁸⁷rubidium , which has a half-life of 48.8 billion years. Therefore one can determine the age of some rocks with the help of their rubidium and strontium isotope ratios.

In living beings (e.g. humans), strontium is built into bones and tooth enamel instead of calcium. Unlike in bones, Sr is no longer exchanged in tooth enamel after the age of four. Therefore the isotope ratio there remains identical to that at the place where the child lives. Strontium isotope analysis is used for archaeological investigations of skeletal finds. If one compares the Sr isotope ratio in the bones with that in the chewing teeth, a different ratio proves a migratory movement that occurred after the age of four.

Thorium-uranium-lead method

With the Th-U-Pb method, the concentrations and isotope ratios of the elements thorium, uranium and lead are determined. Each of the three isotopes ²³⁸ U, ²³⁵ U and ²³² Th decays radioactively through complicated series of decays into exactly one lead isotope:

{\ displaystyle {\ frac {^ {206} \ mathrm {Pb}} {^ {238} \ mathrm {U}}}, {\ frac {^ {207} \ mathrm {Pb}} {^ {235} \ mathrm {U}}} \ quad {\ text {and}} \ quad {\ frac {^ {208} \ mathrm {Pb}} {^ {232} \ mathrm {Th}}}}

Since the isotopy of three independent decay series is determined, a three-dimensional representation of the results is theoretically possible. Usually, however, a two-dimensional representation is used and the ²⁰⁷ Pb / ²³² Th system is used for fractionation correction .

Applications

Dating of minerals: apatite ( tooth enamel , see above), monazite or zircon
The origin of clothing, people, animals, food can be differentiated on a global scale between different continents. Given a given regional limitation, for example butter from Germany, more precise distinctions are also possible by analyzing the water in the food.
By examining Th-U-Pb isotopes, one can, for example, distinguish between different types of nuclear reactors or nuclear weapons.

Other methods

Sulfur system
Sm-Nd system
U-Pb system
- U-He method (historical)
- Pb method (historical)
- Pb-Pb method

Individual evidence

^ Brian Fry: Stable isotope ecology . 2006. Springer. ISBN 0387305130
↑ Elisabeth Stephan: Stable isotopes in fossil fauna finds: Research into the climate, environment and nutrition of prehistoric animals . In: Andreas Hauptmann (Ed.): Archaeometry. Methods and application examples . Stuttgart 2008, ISBN 978-3-510-65232-7 , pp. 51-58.
↑ MJ Schoeninger, MJ Kohn, JW Valley: Tooth oxygen isotope ratios as paleoclimate monitors in arid ecosystems. In: SH Ambrose, MA Katzenberg (Ed.): Biogeochemical Approches to Paleodietary Analysis. Advances in Archeological and Museum Science 5, New York 2000, pp. 117-140.
↑ ^a ^b H. L. Schmidt, E. Schmelz: Stabile Isotopes in Chemistry and Life Science, Chemistry in our Time, 14th year 1980, No. 1, p. 25
^ Wilhelm Nultsch: General botany . Thieme (Ed.) 2001
↑ Elisabeth Stephan: Stable isotopes in fossil fauna finds: Research into the climate, environment and nutrition of prehistoric animals . In: Andreas Hauptmann (Ed.): Archaeometry. Methods and application examples . Stuttgart 2008, ISBN 978-3-510-65232-7 , pp. 58-60.
↑ Elisabeth Stephan: Stable isotopes in fossil fauna finds: Research into the climate, environment and nutrition of prehistoric animals . In: Andreas Hauptmann (Ed.): Archaeometry. Methods and application examples . Stuttgart 2008, ISBN 978-3-510-65232-7 , pp. 60-64.

[1] Brian Fry: Stable isotope ecology . 2006. Springer. ISBN 0387305130

[2] Elisabeth Stephan: Stable isotopes in fossil fauna finds: Research into the climate, environment and nutrition of prehistoric animals . In: Andreas Hauptmann (Ed.): Archaeometry. Methods and application examples . Stuttgart 2008, ISBN 978-3-510-65232-7 , pp. 51-58.

[3] MJ Schoeninger, MJ Kohn, JW Valley: Tooth oxygen isotope ratios as paleoclimate monitors in arid ecosystems. In: SH Ambrose, MA Katzenberg (Ed.): Biogeochemical Approches to Paleodietary Analysis. Advances in Archeological and Museum Science 5, New York 2000, pp. 117-140.

[ChiuZ-4] H. L. Schmidt, E. Schmelz: Stabile Isotopes in Chemistry and Life Science, Chemistry in our Time, 14th year 1980, No. 1, p. 25

[5] Wilhelm Nultsch: General botany . Thieme (Ed.) 2001

[6] Elisabeth Stephan: Stable isotopes in fossil fauna finds: Research into the climate, environment and nutrition of prehistoric animals . In: Andreas Hauptmann (Ed.): Archaeometry. Methods and application examples . Stuttgart 2008, ISBN 978-3-510-65232-7 , pp. 58-60.

[7] Elisabeth Stephan: Stable isotopes in fossil fauna finds: Research into the climate, environment and nutrition of prehistoric animals . In: Andreas Hauptmann (Ed.): Archaeometry. Methods and application examples . Stuttgart 2008, ISBN 978-3-510-65232-7 , pp. 60-64.