This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. Accuracy of radiometric dating[ edit ] Thermal ionization mass spectrometer used in radiometric dating. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.
It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination.
In uranium-lead dating , the concordia diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate.
This normally involves isotope ratio mass spectrometry. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating can not be established. On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. Plants take up atmospheric 14C for as long as they live, through the process of photosynthesis.
Animals take up atmospheric 14C indirectly, by eating plants or eating other animals that eat plants. Measuring the proportion of 14C as opposed to 12C remaining in a sample then tells us how long ago the sample stopped taking up 14C — in other words, how long ago the thing died. Carbon dating has a certain margin of error, usually depending on the age and material of the sample used. Carbon has a half-life of about years, and therefore it is used to date biological samples up to about 60, years in the past.
Beyond that timespan, the amount of the original 14C remaining is so small that it cannot be reliably distinguished from 14C formed by irradiation of nitrogen by neutrons from the spontaneous fission of uranium, present in trace quantities almost everywhere. For these samples, other dating methods must be used. This can be done with a thermal diffusion column. CO 2 is widely used, but it is also possible to use other gases, including methane , ethane , ethylene and acetylene.
Libby's first measurements were made with lamp black,  but this technique is no longer in use; these methods were susceptible to problems caused by the 14 C created by nuclear testing in the s and s. To create lamp black, Libby began with acid washes if necessary to remove carbonate, and then converted the carbon in the sample to CO 2 by either combustion for organic samples or the addition of hydrochloric acid for shell material. The resulting gas was passed through hot copper oxide to convert any carbon monoxide to CO 2, and then dried to remove any water vapour.
The gas was then condensed, and converted to calcium carbonate in order to allow the removal of any radon gas and any other combustion products such as oxides of nitrogen and sulphur. The calcium carbonate was then converted back to CO 2 again, dried, and converted to carbon by passing it over heated magnesium. Hydrochloric acid was added to the resulting mixture of magnesium, magnesium oxide and carbon, and after repeated boiling, filtering, and washing with distilled water, the carbon was ground with a mortar and pestle and a half gram sample taken, weighed, and combusted.
This allowed Libby to determine how much of the sample was ash, and hence to determine the purity of the carbon sample to be tested. This is then converted to lithium carbide, and then to acetylene, and finally to benzene.
Radiocarbon dating samples
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