Carbon dioxide radiocarbon, measurement

A living plant contains about the same fraction of carbon-14 as atmospheric carbon dioxide. The observed rate of decay of carbon-14 from a living plant is 15.3 counts per minute per gram of carbon. How many counts per minute per gram of carbon will be measured from a 15,000-yr-old sample Will radiocarbon dating work well for small samples of 10 mg or less ... 

Table II. Sunnyvale Skeleton Radiocarbon Measurements by Carbon Dioxide Gas Proportional Decay Counting and...

Radiocarbon age for which the signal is equal to measured background for FOaC target prepared from carbon dioxide 40,000 years... 

Radiocarbon dating has attracted considerable attention. Carbon-14 is produced in the upper atmosphere by cosmic-ray bombardment of nitrogen-14. It is oxidised to carbon dioxide and eventually absorbed and incorporated in the tissues of plants and animals. The time taken for a carbon atom to complete such a carbon-cycle and return to the upper atmosphere is, on average, about 500 years. As the half-life of is 5568 years, the specific activity of carbon in the carbon cycle is roughly constant. But carbon removed from this life-embracing cycle by conversion to, and retention in, a solid such as wood, bone or shell loses activity at a rate determined by the decay constant for Thus the specific activity of carbon in a rock, a fossil plant or bone, or ancient artifact gives its age (Libby, 1951). Measurements are not easy because of the low specific activities but are of considerable and improving accuracy. 

The activity of radiocarbon (A 4c%o) in the atmosphere and surface ocean in response to atmospheric nuciear weapons testing. Atmospheric values are from the Carbon Dioxide Information and Analysis Center (CDIAC), Oakridge, TN. Surface ocean data are from measurement of the C activity of growth bands of corals (Druffel and Linick, 1978). 

Some of these radiocarbon atoms find their way into iiving trees and other piants in the form of carbon dioxide, as a resuit of photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stabie carbon atoms begins to faii as the radiocarbon decays. The ratio in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated. The method has been shown to give consistent results for specimens up to some 40 000 years old, though its accuracy depends upon assumptions concerning the past intensity of the cosmic radiation. The technique was developed by Willard F. Libby (1908-80) and his CO workers in 1946-47. 

After chemical pretreatment, the samples are burnt to produce carbon dioxide and nitrogen. A small amount of this gas is bled into a mass spectrometer, where the stable isotope ratios of carbon and nitrogen are measured. These ratios provide useful information on the purity of the sample and clues about the diet and climatic conditions of the living organism. The carbon isotope ratio is also used to correct for isotopic fractionation in the radiocarbon measurement. The carbon dioxide is collected in a glass ampoule or converted to graphite for radiocarbon measurement on the AMS system. 

Next, low-temperature (<150°C), low-pressure torr) oxygen plasmas oxidized organic components of the sample to CO2. Decomposition of inorganic carbon present (dolomitic limestone rock and calcite/calcium oxalate accretions) was prevented by running the plasmas at low-temperature. Carbon dioxide from the sample was flame-sealed into a glass tube cooled to liquid nitrogen temperature (-194°C), after water had been frozen out with a dry-ice/ethanol slurry (-58°C), and finally sent for radiocarbon analysis at the Center for Accelerator Mass Spectrometry at the Lawrence Livermore National Laboratory (LLNL-CAMS). It was necessary to utilize an AMS measurement due to the small sample size. 

Summary This American test method establishes the procedures, equipment, materials, and condihons to measure the carbon content of the plastic sample through radiocarbon analysis. This test method is applicable to carbon-based plastic products that can undergo combushon in the presence of oxygen to produce carbon dioxide gas (CO2). This test method can be useful for companies and US federal agencies to promote the use of biobased products in commercial, non-food products. The test method directly discriminates between 14C/12C content of plastic samples made from contemporary carbon and those plastic samples made from petroleum sources that do not have 14C. Measurement of a plastic product s 14C/12C content is determined relative to the modem carbon-based oxahc acid radiocarbon Standard Reference Material 4990c. 

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