Degradation of the protein fraction can also occur in hot, arid conditions, without actual burning; then the degraded components can be washed away by groundwater. In this case the sample is often usable. 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.
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. If so, separating the amino acids may be necessary to allow independent testing of each one—agreement between the results of several different amino acids indicates that the dating is reliable. Removing the carbonates also destroys the calcium hydroxyapatite, and so it is usual to date bone using the remaining protein fraction after washing away the calcium hydroxyapatite and contaminating carbonates. This is then converted to lithium carbide, and then to acetylene, and finally to benzene. This results in a coating of filamentous carbon usually referred to as graphite on the powdered catalyst—typically cobalt or iron. The calculations involve several steps and include an intermediate value called the "radiocarbon age", which is the age in "radiocarbon years" of the sample: 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. In this case the sample is often usable. However, if the bone was heated under reducing conditions , it and associated organic matter may have been carbonized. The different elements of the carbon exchange reservoir vary in how much carbon they store, and in how long it takes for the 14 C generated by cosmic rays to fully mix with them. 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. Pretreatment[ edit ] Two common contaminants are humic acid, which can be removed with an alkali wash, and carbonates, which can be removed with acid. The above calculations make several assumptions, such as that the level of 14 C in the atmosphere has remained constant over time. The recrystallized calcium carbonate is generally in the form of calcite, and often has a powdery appearance; samples of a shiny appearance are preferable, and if in doubt, examination by light or electron microscope, or by X-ray diffraction and infrared spectroscopy, can determine whether recrystallization has occurred. The three major components of peat are humic acid, humins , and fulvic acid. It is preferable to sieve the soil for fragments of organic origin, and date the fragments with methods that are tolerant of small sample sizes. Calcium carbonate is very susceptible to dissolving and recrystallizing; the recrystallized material will contain carbon from the sample's environment, which may be of geological origin. Collagen is sometimes degraded, in which case it may be necessary to separate the proteins into individual amino acids and measure their respective ratios and 14 C activity. 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. These treatments can damage the structural integrity of the sample and remove significant volumes of material, so the exact treatment decided on will depend on the sample size and the amount of carbon needed for the chosen measurement technique. A rough guide follows; the weights given, in grams, are for dry samples, and assume that a visual inspection has been done to remove foreign objects. 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. The calcium carbonate was then converted back to CO 2 again, dried, and converted to carbon by passing it over heated magnesium. Radiocarbon ages are still calculated using this half-life, and are known as "Conventional Radiocarbon Age". This protein component is called collagen.
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