Isotope analysis

Isotope studies to determine the proportion of isotopes of a chemical element within a sample. Most chemical elements are composed of several isotopes. With a mass spectrometer can these isotope composition ( isotope ) determined very accurately (up to nanogram sample size and depending on the element and isotope to ppt accuracy).

The isotopic investigation carried out by mass spectrometry. In the mass spectrometer the isotopes are differentially deflected depending on the mass and charge of their trajectory and recorded as peaks. The higher the concentration of the isotope, the greater is the output peak. For calculating the isotopic composition of international standards are used (different for the various elements ), which are measured with the samples together and have a defined isotopic composition.

Examples of isotope studies:

• 8.1 applications

Terminology

The results of isotopic measurements are given for light isotopes as the ratio of heavy and listed as delta values ​​(). All isotopes are measured as the relative difference to an international standard and expressed in parts per thousand. For example,

Hydrogen isotopes

Hydrogen (H) has two stable isotopes: protium or protons (1H) and deuterium (2H or D) and the radioactive (unstable ) isotope, tritium ( 3 H, or T) with a half life of 12.3 years. On Earth, there are only a few kilograms of tritium as a natural occurrence. It is caused by cosmic radiation in the upper layers of the atmosphere.

Also referred to as heavy deuterium and tritium as a heavy hydrogen.

Tritium method

Rain water contains tritium, which is caused by cosmic radiation in the atmosphere. Since tritium decays with time, one can determine the tritium method, for example, the age of the source water.

Because tritium in nature is so rare to see the smallest contamination from technical applications are easy to find.

Oxygen isotopes

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

For most studies, the ratio 18O/16O is measured, because 17O occurs in difficult detectable amounts. As a standard for the calculation of the ratio ( see terminology) is predominantly used the isotope ratio of Vienna Standard Mean Ocean Water ( VSMOW ).

The isotopic ratio 18O/16O of water vapor in the atmosphere and in the water all waters varies regionally. In the evaporation of water as well as in the condensation occurs isotope. Upon evaporation of the lighter isotope is preferably above the steam in the condensation (for example, clouds and rain), the heavier isotope is preferably in the liquid phase. The isotopic fractionation is temperature dependent, so that precipitates in cool regions a lower 18O/16O ratio (and lower D / H ratio ) than in areas with hot climates. Also seasonal temperature changes are reflected in changes in the isotope ratio in rain water.

This fact is used in Archaeometry for paleotemperature reconstruction. Mammals build their bones and teeth an oxygen isotopes. The ratio is dependent on value of drinking. The relations are species specific and can be applied to bone and tooth finds from archaeological excavations. From the analysis of tooth enamel, not remodels over the course of the life of an adult mammal, certain conclusions can be drawn on the climate in which the animal was raised. The higher the value, the higher the temperature.

18O is used in the organic and biochemical reactions to elucidate the reaction mechanism. This serves either elemental or in water -bound oxygen ( 18O2, H218O ). Well-known examples for this are the formation or hydrolysis of esters. In biochemical reactions, especially Dehydrogenierungen in enzymatic reactions can explain.

Carbon isotopes

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

The ratio of the two stable isotopes is also used for scientific questions. Between 12C and 13C natural isotope fractionation during photosynthesis takes place. C3 plants, such as wheat, for the fixation of CO2 have only the photosynthetic enzyme RuBisCO ( ribulose -1 ,5- bisphosphate carboxylase - oxygenase ). It discriminates against the heavier isotope δ13C and fixed preferred lighter CO2 molecules. δ13C value of C3 plants are -26.5 ‰ in the area. C4 plants, such as sorghum and maize, however, have a different mechanism of CO2 fixation and possess not only the RuBisCO nor the phosphoenol (PEP carboxylase ), which pre-fixed CO2 in the form of bicarbonate (HCO3-) with much higher affinity. The PEP carboxylase does not discriminate against the heavier δ13C isotope which is expressed in a more positive δ13C ratio of about -12.5 ‰ of C4 plants. The deviation is set Belemnite standard in relation to the Pee Dee and indicated this ratio. Due to the special CO2 fixation of CAM plants more positive δ13C ratios in nature can be observed. An even higher value plankton and marine animals. This allows, for example, anthropologists based on the δ13C value of human bones, to draw conclusions on the diet. This is especially interesting in connection with the d15N.

For the study, the 13C nuclear magnetic resonance spectroscopy, which is particularly used in organic chemistry for the elucidation of chemical structures suitable.

Nitrogen isotopes

Nitrogen (N) has two stable isotopes 14 N ( 99.634 %), and 15N ( 0.366 %). As a standard for the calculation of the ratio d15N (see terminology) the isotopic ratio of the air is used.

The isotope fractionation takes place predominantly in the nitrogen cycle in the interaction between plants and microorganisms in the soil. Dry savannas and desert soils contain more 15N than moist, cool forest soils of temperate regions. To the atmosphere enrich biological materials to the heavy isotope. Within the food chain further enrichments are observed. 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 inferences from bones found in the diet of animals and humans. A predominant diet through meat was found due to its d15N value of both Neanderthal man.

Potassium -argon system

→ Main article: Potassium - argon dating

The argon method makes use of the fact that the usually solid element potassium 40K decays with a half life of 1.3 billion years for gaseous 40Ar, which can not but from a melt escape from a solid. In geology, so is the solidification time of volcanic materials dated.

Rubidium, strontium system

→ Main article strontium isotope analysis

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

87Sr is Zerfallsprodult 87Rubidium of 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 organisms ( eg, humans ) strontium is incorporated in place of calcium in bones and tooth enamel. In contrast to the bone Sr is no longer exchanged in the enamel after the fourth birthday. Therefore, the isotopic ratio there remains identical to the living place of the child. The strontium isotope analysis one uses for archaeological investigations of skeletal remains. Comparing the Sr isotope ratio in the bones with the molars, has a different ratio one took place after the age of four walking motion.

Thorium - uranium-lead method

→ Main article uranium -thorium dating, uranium - lead dating

In the Th- U-Pb method to determine the concentrations and isotopic ratios of the elements thorium, uranium and lead. Each of the three isotopes 238U, 235U and 232Th decays radioactively about complicated decay chains in exactly one isotope of lead:

Since it determines the isotope of three independent decay series, a three-dimensional representation of the results is theoretically possible. Mostly, however, it differs from a two-dimensional representation and uses the 207Pb/232Th-System to Fraktionierungskorrektur.

Applications

• Dating of minerals: apatite (enamel, see above), monazite or zircon
• The origin of clothing, people, animals, food can be distinguished on a global scale between different continents. Finer distinctions are at a given regional containment, for example butter from Germany, through the analysis of the water in foods possible.
• Through the study of Th- U-Pb isotopes can be 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