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Introduction
Carbon is the basis of life and is present in all living things.
Radiocarbon, or carbon-14 (also known as 14C), is an unstable and slightly radioactive isotope of carbon, present in all living things in minute quantities. Because it is radioactive, the amount of radiocarbon gradually decreases through decay until it is completely exhausted. Radiocarbon dating uses carbon-14 to determine the time of death of something or someone.
Formation and evolution of the 14C
The production of new 14C atoms in the upper atmosphere balances the loss of 14C atoms due to radioactive decay. During its production and decay cycle, radiocarbon is oxidized to form 14CO2, and then partly dissolves in water and partly is converted into organic carbon by plants through photosynthesis.
Radiocarbon enters the food chain through plants, through primary (herbivorous animals) and secondary (omnivorous animals) consumption. Secondary and tertiary (carnivores) consumers, in turn, feed on primary (or secondary, or tertiary) consumers, thus spreading 14C throughout the food chain.
Dissolving in water, 14C also permeates the oceans, and is therefore an isotope present in all living beings according to a precise ratio, called natural isotopic abundance.
The natural abundance of 14C does not remain constant over time but is altered by variations in cosmic radiation and the Earth’s magnetic field and, more recently, by anthropogenic activities; in any case, all living beings continuously exchange radiocarbon, remaining in constant equilibrium with the natural abundance until the moment of their death.
The consequence of this last statement is that radiocarbon is a perfect marker of life: living beings continuously absorb 14C, which begins to decay only at the moment of death. Therefore, by measuring the residual concentration of 14C, and knowing the half-life, it is possible to trace the moment of death or establish the biogenic fraction (containing 14C) and the fossil fraction (derived from petroleum, not containing 14C) in any substance.
The natural abundance of 14C in the atmosphere has not always remained constant over time, but apart from significant events of natural origin (such as intense solar activity), the cause of a strong variation in the concentration of radiocarbon present in nature is attributable to man.
In fact, starting from the period of the industrial revolution, due to the extraction and use of fossil fuels such as coal and, subsequently, oil, man has introduced fossil carbon into the atmosphere, disturbing the natural carbon cycle.
The bomb peak
The so-called bomb peak is the period between 1944 and 1985, the era of nuclear testing characterized by the detonations of thermonuclear bombs, which doubled the 14C content in the troposphere. The peak radiocarbon concentration was reached in 1963, shortly before the signing of the Nuclear Weapons Test Ban Treaty between the Soviet Union, Great Britain, and the United States.
The last period is the one starting in 1980, where the contribution of fossil CO2 produced by man is the one that has the greatest impact on the seasonal cycles of 14CO2 in the Northern Hemisphere. Nowadays the concentration level has returned to the levels preceding the atomic tests, but it is certain that if man continues to release fossil CO2 into the atmosphere, this level will be destined to decrease significantly and in proportion to the emissions.
Key message
Radiocarbon dating allows us to measure the biobased content of products because they are
a combination of materials from recently lived organisms and fossil materials.
Materials derived from recently living organisms (the biobased component) contain carbon-14, while fossil materials (derived from petroleum) no longer contain this weakly radioactive isotope. Therefore, all of the carbon-14 in the product comes from the biobased component.
For example:
measuring the radiocarbon content of a fuel (or plastic) allows to determine if it is produced from fossil oil or from renewable biogenic source material.
Technologies available
The detection of 14C is really challenging , due to its extremely low concentration in nature: the required sensitivity is of 1 part in 10^15
Until a few years ago, the only two detection techniques capable of achieving the required sensitivity were:
❖ Accelerator Mass Spectrometry (AMS)
❖ Liquid Scintillation Counting (LSC)
The Liquid Scintillation Counting (LSC)
LSC is a technique for measuring radiocarbon concentration, developed in the 1960s.
Liquid scintillation is based on the phenomenon of scintillation, i.e. the emission of light by a molecule (scintillator) after it has been excited by a charged particle or a high-energy photon. In the case of radiocarbon, the beta decay of 14C consists in the emission of a high-energy electron that can excite the molecules of the scintillator.
Preparation occurs by transforming the sample into CO2 through combustion or acidification; the carbon dioxide thus produced is converted into hydrocarbons through a series of chemical reactions, or the sample can be converted into 14C-labeled benzene.
In any case, the sample is mixed with a liquid scintillator, the solution contains fluorescent molecules (primary scintillators) that emit light when excited by the particles emitted by the radioactive sample.
The Accelerator Mass Spectrometry (AMS)
AMS is a complex instrument for mass spectrometry on a very wide variety of elements with very high levels of precision. It combines traditional mass spectrometry (MS) with a particle accelerator, commonly of the tandem type, that is, it consists of two accelerator stages in series, one acting on negative ions, the other acting on positive ions obtained by extracting electrons from the negative ions accelerated by the first stage.
AMS analysis of 14C requires the samples being analysed to be converted to graphite. This process involves an initial oxidation of the carbon contained in the sample to CO2 by combustion, followed by a subsequent catalytic reduction to elemental carbon.
The graphitized sample is introduced into the AMS, where an ion source ionizes it. The next stage is the pre-acceleration phase, where the ions produced by the sample ionization are extracted from the source and accelerated to a relatively low energy.
The next stage is the passage of the ions into the first stage of the main accelerator, where an oscillating electric field is generated by a high voltage generator, which provides the energy of the order of a few MeV necessary to accelerate the negative ions.
14C SCAR – The new technology
The 14C SCAR (Saturated-absorption CAvity Ring-down) spectrometer is a new and innovative spectrometer for the 14C measure.
Developed by a team of the Italian National Research Counsil, It is able to measure the mole fraction of radiocarbon in any sample using the SCAR technique, which improves the CRD (cavity ring down) limits by more than an order of magnitude.
Comparison
Accelerator Mass Spectroscopy (AMS) detects isotope ratio of 12C,13C,14C ions. The use has increased, but its economic and energy costs make it difficult to manage.
Liquid Scintillation Counting (LSC) uses β-decay counting, but with insufficient precision for monitoring applications, thus requiring large sample amounts, high isotope doses and difficult measure automation.
14C SCAR detects absorbed IR photons. It is demonstrated and sold for 14C quantification for many applications. Qualification for spread use require fewer sample needs, better modularity and automation.
Potential applications
AMS and LSC techniques are unable to meet the ever-increasing demand for analysis. Consequently, over the years of 14C SCAR development, many organizations have become interested in this measuring instrument, fascinated by the possibility of performing radiocarbon concentration measurements with relative simplicity.
Environmental tracking: radiocarbon allows qualitative and quantitative assessments of the origin of emission sources through direct analysis of air or atmospheric particulate matter
Medical research: radiocarbon is used in some medical imaging applications, such as positron emission tomography (PET). It is also used in some pharmacokinetic studies, where the pharmacokinetic properties of new drugs are carefully tested. Radiocarbon is used as a marker to study absorption, distribution, metabolism, and excretion in pharmacokinetics.
Fuel industry: it is possible to analyse any type of fuel in order to quantify the biogenic fraction and the fossil fraction. In particular, there is a strong interest in biofuel certification, and 14C SCAR presents itself as one of the possible simple ways to certify biofuels, verifying that the declared specifications (% biofuel) are actually respected. Furthermore, fuel producing companies can benefit from tax breaks by inserting a certain percentage (which varies from country to country) of biofuel for every certain quantity of fossil fuel.
Fashion industry: this is a sector that is increasingly interested in biobased materials for its products, and in reducing the use of fossil-based materials (derived from petroleum) throughout the production chain. In this case too, 14C SCAR presents itself as an easy-to-use tool for certifying the origin of a raw material, or of the finished product.
Nuclear monitoring: radiation from nuclear fusion significantly increases the concentration of radiocarbon in the materials it affects. Consequently, it is possible to monitor radioactive levels in the nuclear industry by measuring the concentration of 14C.
Potential configurations
Based on the application, NC Technologies can provide the best solution for the 14C measurements.
In addition to the elemental analyser, mod. ECS 8024 – C14 and the concentrator mod. 8070 AIR CO2, we can supply an IRMS detector, mod. COMPACT IDmicro, for the analysis of stable carbon isotope.
Indeed, the measurement of stable isotopes (carbon-13/carbon-12) corrects the radiocarbon age by correcting for the effect of isotopic fractionation, which can occur during biological processes.
This analysis is crucial for unidentified plant materials and bone samples to improve dating accuracy and determine sample quality.
Reservoir effect correction is another necessary correction for marine samples, to compensate for the dilution of carbon-14 in seawater compared to the global average.
