Editing Radiocarbon dating (section)

==Samples==
{{Main|Radiocarbon dating samples}}
Samples for dating need to be converted into a form suitable for measuring the {{chem|14|C}} content; this can mean conversion to gaseous, liquid, or solid form, depending on the measurement technique to be used. Before this can be done, the sample must be treated to remove any contamination and any unwanted constituents.<ref name=Bowman_27>Bowman (1995), pp.&nbsp;27–30.</ref> This includes removing visible contaminants, such as rootlets that may have penetrated the sample since its burial.<ref name=Bowman_27/> Alkali and acid washes can be used to remove [[Humic substance|humic acid]] and carbonate contamination, but care has to be taken to avoid removing the part of the sample that contains the carbon to be tested.<ref name=AitkenWashing>Aitken (1990), pp. 86–89.</ref>

===Material considerations===
* It is common to reduce a wood sample to just the cellulose component before testing, but since this can reduce the volume of the sample to 20% of its original size, testing of the whole wood is often performed as well. Charcoal is often tested but is likely to need treatment to remove contaminants.<ref name=Bowman_27/><ref name=AitkenWashing/>
* Unburnt bone can be tested; it is usual to date it using [[collagen]], the protein fraction that remains after washing away the bone's structural material. [[Hydroxyproline]], one of the constituent amino acids in bone, was once thought to be a reliable indicator as it was not known to occur except in bone, but it has since been detected in groundwater.<ref name=Bowman_27/>
* For burnt bone, testability depends on the conditions under which the bone was burnt. If the bone was heated under [[reducing conditions]], it (and associated organic matter) may have been carbonized. In this case, the sample is often usable.<ref name=Bowman_27/>
* Shells from both marine and land organisms consist almost entirely of calcium carbonate, either as [[aragonite]] or as [[calcite]], or some mixture of the two. 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. If testing recrystallized shell is unavoidable, it is sometimes possible to identify the original shell material from a sequence of tests.<ref>Šilar (2004), p. 166.</ref> It is also possible to test [[conchiolin]], an organic protein found in shell, but it constitutes only 1–2% of shell material.<ref name=AitkenWashing/>
* The three major components of peat are [[humic acid]], [[humins]], and [[fulvic acid]]. Of these, humins give the most reliable date as they are insoluble in alkali and less likely to contain contaminants from the sample's environment.<ref name=AitkenWashing/> A particular difficulty with dried peat is the removal of rootlets, which are likely to be hard to distinguish from the sample material.<ref name=Bowman_27/>
* Soil contains organic material, but because of the likelihood of contamination by humic acid of more recent origin, it is very difficult to get satisfactory radiocarbon dates. 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.<ref name=AitkenWashing/>
* Other materials that have been successfully dated include ivory, paper, textiles, individual seeds and grains, straw from within mud bricks, and charred food remains found in pottery.<ref name=AitkenWashing/>

===Preparation and size===
Particularly for older samples, it may be useful to enrich the amount of {{chem|14|C}} in the sample before testing. This can be done with a [[thermal diffusion]] column. The process takes about a month and requires a sample about ten times as large as would be needed otherwise, but it allows more precise measurement of the {{chem|14|C}}/{{chem|12|C}} ratio in old material and extends the maximum age that can be reliably reported.<ref>Bowman (1995), pp. 37–42.</ref>

Once contamination has been removed, samples must be converted to a form suitable for the measuring technology to be used.<ref name=BowmanMeasure>Bowman (1995), pp. 31–37.</ref> Where gas is required, {{Chem|CO|2}} is widely used.<ref name=BowmanMeasure/><ref name=Aitken_76>Aitken (1990), pp. 76–78.</ref> For samples to be used in [[Liquid scintillation counting|liquid scintillation counters]], the carbon must be in liquid form; the sample is typically converted to [[benzene]]. For [[accelerator mass spectrometry]], solid graphite targets are the most common, although gaseous {{Chem|CO|2}} can also be used.<ref name=BowmanMeasure/><ref name=Trumbore96>Trumbore (1996), p. 318.</ref>

The quantity of material needed for testing depends on the sample type and the technology being used. There are two types of testing technology: detectors that record radioactivity, known as beta counters, and accelerator mass spectrometers. For beta counters, a sample weighing at least {{convert|10|g|abbr=off}} is typically required.<ref name=BowmanMeasure/> Accelerator mass spectrometry is much more sensitive, and samples containing as little as 0.5 milligrams of carbon can be used.<ref>Taylor & Bar-Yosef (2014), pp. 103–104.</ref>