Concept in Definition ABC

The superscript on the left (¹⁸EITHER, ²H, ¹⁵N) refers to the mass number of the atom and represents the sum total of the number of protons and neutrons. An example of isotopes are those of hydrogen, reflected by the letter H, and its isotopes are protium (¹H), deuterium (²H) and tritium (³H), illustrating that each has 1 or 2 more neutrons than the first.

Classification of isotopes

According to the stability of the nucleus of the isotopes, these are classified as stable and radioactive.

radioactive: They are also called unstable isotopes, they have the property of transforming from one isotope to another by the decay or disintegration of its nucleus, emitting

Energy in the form of radioactivity with the advance of the transformation. In the case of the hydrogen isotope example, its radioactive isotope is tritium. ³H, which can decay and transform into helium 3 (³He). But it is not the only radioactive isotope, there are many more.

stable: For their part, stable isotopes have a nucleus that does not decay to others on the geological time scale; which means that they do not transform into other isotopes. They can be found in most compounds. They have low molecular weight and relatively large mass difference.

They are very abundant elements in nature and are found in different oxidation states, forming different types of chemical bonds. Likewise, they can be classified into two types, light and heavy.

For example, the stable isotopes of hydrogen are protium (¹H) and deuterium (²H). The latter being the heavy and the former the light.

Its abundance is unequal, it depends on the process that occurs, this will determine if there are more stable light or more stable heavy isotopes, the which have one or two extra neutrons relative to protons and can be generated from radioactive decay of isotopes radioactive.

isotope fractionation

The difference in abundance between heavy and light isotopes is due to natural processes and is present in the compounds that are obtained. starting from chemical reactions, physical, biological, metabolic and geochemical processes, where they participate freely, and depend on the difference in reaction speed each.

The processes that occur and the abundance of one or the other depend on the chemical bonds and the attractive forces of the atoms, which is greater in the case of heavy isotopes, which reduces their speed of reaction because it takes more energy to break the bonds.

A given unequal distribution between a source source and its reaction products is called isotopic fractionation, and refers to the way in which isotopes are distributed between one substance and another or different phases of the same substance.

The importance of isotopic fractionation is due to the variation it gives in the ratio of stable isotopes of different groups of elements and the isotopic signal that it generates that can indicate if there is or in what magnitude a certain process took place within the cycle of an element specific.

Consequently, the products of reactions that undergo isotopic fractionation exhibit a unique isotopic composition that serves to identify the source from which it comes or the processes by which I pass.

An example of fractionation is the process of evaporation of water in the oceans, where the evaporating water carries away the light isotopes in the vapor as ¹H₂¹⁶EITHER; and leaves in the ocean water the heavy isotopes of the water as ¹H₂¹⁸or and ¹H₂H¹⁶EITHER. In this case, 18O is the heavy isotope of oxygen and 16O is the light isotope.

Now, isotopic fractionation occurs by two different processes, Balance chemical isotope and kinetic isotope equilibrium.

Chemical isotopic equilibrium

In this process the reactions that occur exchange isotope enclose the redistribution of isotopes of the same element through various species within a system that is closed and homogeneous.

Kinetic Isotope Equilibrium

In this case the process implies that the rate of reaction in both directions of a particular isotope is the same, but it does not imply that the isotopic compositions of two compounds at equilibrium are equal, it refers to the fact that the relationships that exist between two different isotopes in each compound are constant at a certain temperature.

Throughout the reactions that occur to reach equilibrium, the heaviest isotope with the highest oxidation state accumulates preferentially.

An example of isotopic equilibrium is that which occurs in the reversible physical process of condensation and water evaporation:

H₂¹⁶EITHER_(vapor) + H₂¹⁸EITHER_(liquid) ⇔H₂¹⁸EITHER_(vapor) + H₂¹⁶EITHER_(liquid)

The given differences in the final isotopic composition that is generated by isotopic fractionation can be determined using a mass spectrometer by comparing with a standard value sample and noting the difference as an enrichment or depletion of the isotope of interest and is reported using three parameters: the fractionation factor (α), isotopic difference or enrichment isotopic (ε) and discrimination isotopic (δ).

Fractionation factor (α)

The fractionation factor corresponds to the distribution of stable isotopes between two coexisting phases, one being A and the other B, and is expressed as the quotient of the amount of heavy isotope present in the liquid phase divided by the amount of heavy isotope in the gas phase, as shown in the following equation:

α ^PX = (R)_A / (R)_B. (1)

Where R is the amount of heavy isotope (^PX) divided by the amount of the light isotope (^LX), based on the phase indicated by the subscript, expressed with the following relationship:

R= ^PX / ^Lx(2)

Isotopic difference or isotopic enrichment (ε)

This is represented as the fractionation factor minus 1, in parts per thousand (‰), given by the following equation:

ε ^PX A- B = (α-1) x 1000‰ (3)

Isotopic discrimination (δ)

It is estimated by making a quotient between the amount of the heavy isotope in the sample, divided by the amount of the heavy isotope present in the standard, which is the material that is taken as a reference for the value of the heavy isotope, subtracting 1, so that the frequencies obtained from different samples are comparable.

It is expressed in parts per thousand (‰) for ease of calculation. expression of the results, as shown in the following equation:

δ ^PX_sample = {[(R)_sample / (R)_standard]-1} x 1000‰ (4)

Where R is the amount of heavy isotope (^PX) between the amount of light (^LX), both in the sample and in the standard.

It is important to clarify that the isotopic fractionation given between two phases acts based on the temperature, thus generating variations in the aforementioned relationships, especially in isotopic discrimination, which was the last to be explained.