Basics Basics of the 14
Basics Basics of the 14 C method How to use radiocarbon dating to determine the biobased carbon content By Ann-Sophie Kitzler Hans-Josef Endres Andreas Schettler all University of Applied Sciences and Arts Hanover (Institute for Bioplastics and Biocomposites) and Michael Nelles University of Rostock, Department of Waste Management and Material Flow This article is an excerpt of a longer version which is available for download at www.bioplasticsmagazine.de/201202 Carbon exists in nature in the form of three isotopes – carbon 12 ( 12 C), carbon 13 ( 13 C) and carbon 14 ( 14 C) – which are present in the atmosphere in various proportions. 12 C, at just about 99%, represents the majority, whilst 13 C at something in excess of 1% is the second largest proportion. A 14 C atom, statistically speaking, occurs only in trace amounts as about 1 part per trillion of the carbon in the atmosphere, yet is the key to radiocarbon dating [1]. 14 C occurs in the upper atmosphere (lower stratosphere and upper troposphere). Cosmic radiation impacts on the atoms in the atmosphere and via a splintering (spallation) reaction liberates neutrons. In a further process such neutrons react with nitrogen ( 14 N) and lead to a nuclear reaction. Here a proton breaks away and a 14 C atom is produced. After it has been formed, this atom, like other carbon isotopes, combines with the oxygen in the air and forms carbon dioxide. This carbon dioxide is distributed in the atmosphere and by photosynthesis finds its way into the biosphere. It is absorbed by plants and forms part of the food chain [1]. 14 C, unlike 12 C and 13 C, is very unstable and is subject to radioactive break-up which produces low level Beta radiation. This reaction is the origin of the name “radiocarbon dating” and there are various research procedures that depend on this process [1]. The half-life of 14 C, according to Willard Frank Libby, is around 5568 years (± 30) [2]. By the constant exchange of carbons from the atmosphere and the biosphere (plants and animals on the earth) we can assume a constant balance between the three isotopes, which is in line with that natural balance described above. This means that even in the so-called renewable resources the maximum possible 14 C content from the atmosphere is to be found. This content level decreases at a precise rate when the exchange of carbons due to a breakdown in biological activity occurs, i.e. when the metabolic process can no longer be followed, and the organism dies. The reduction in 14 C, always in line with the half-life given above, is no longer compensated by new 14 C formed in the atmosphere. Thus there is a change in the natural ratio of 12 C to 14 C [1, 3], within the biomass integrated in a no longer living metabolism. This means that in fossil materials such as coal, petroleum, or natural gas, 14 C is no longer contained because the material has been dead for a very long period of time. Thus the changes that have taken place in their natural condition allow conclusions to be drawn regarding the age of materials. 50 bioplastics MAGAZINE [02/12] Vol. 7
Cosmic Ray Proton Basics 14 N + n → 14 C + p 7 6 Spallation Products Thermal Neutron 14 C Oxidation 14 CO 2 Photosynthesis Disolved CO 2 Carbonates Bicarbonates Fig. 1: Graphic representation of the formation, distribution and breakdown of natural 14C, modified [2] Description of the method Before any analysis can take place the sample has to be prepared. In the case of radiocarbon dating the sample is oxidised to form carbon dioxide by heating to a very high temperature. The resultant CO 2 can now be further converted to suit the appropriate analysis method (for example into benzol), and for a method using mass spectrometry it can be reduced to pure carbon or fed directly to the Counter for analysis [3, 4]. Depending on the carbon content and the size of the sample there are two different methods specified by the standard for radiocarbon dating. The 14 C content can be determined by counting the decomposing 14 C atoms in the Counter (Liquid Scintillation-Counter (LSC)) in line with Libby or (preferred) the still available 14 C atoms (Accelerator Mass Spectrometry (AMS). In the following we briefly describe the mass spectroscopy method. About 0.5 - 0.7 grams of the test material is heated in a quartz sample vessel at 900°C for at least 2 hours. After cleaning the pure CO 2 gas this is liquefied under cryogenic conditions (i.e. by liquid nitrogen at very low temperature) and fed, in a liquid state, into an AMS sample vessel. The isotope ratios of 14 C/ 12 C and 13 C/ 12 C are calculated relative to a standard substance [4]. Here a figure of 0 pmC 14 C (pmC = percent modern carbon), means that the sample is a fossil carbon source whilst the reading of 100 pmC 14 C points to the sample being a modern carbon source [4]. All of the chemicals required to purify the combustion gas, as well as the precise method and interim steps, can be found under the applicable standard (ASTM-D6866) Examples of the application of 14 C analysis Even though radiocarbon dating has its origins in determining the age of archaeological specimens (radioactive age determination) and thus is used for dating organic articles, it has now found applications in some very different fields of research. It can be used to identify works of art and fakes, or in the testing of foodstuffs, for example to differentiate synthetic, chemically identical, but not natural foods (such as aromas and fragrances, alcohols or carbon dioxide in sparkling wines and mousses. It is also used for research and determination of the unique fingerprint of biobased raw materials in industrial products such as lubricants, cosmetics and plastics [2, 3]. Whilst bioplastics MAGAZINE [02/12] Vol. 7 51
