14C Information
The Principles of Carbon-14 Dating
Radiocarbon dating has been used across scientific and industrial fields since its development in the late 1940s to determine the age of carbon-containing materials. Because 14C decays at a known, constant rate — with a half-life of approximately 5,730 years — the technique can reliably date materials up to approximately 42,000–50,000 years old. Radiocarbon dating applies to any organic material that once exchanged carbon with the atmosphere, whether through photosynthesis, respiration, or the formation of carbonates from dissolved atmospheric CO₂.
While an organism is alive, its 14C concentration remains in equilibrium with the atmosphere through continuous carbon exchange. At death, this exchange ceases and the remaining 14C begins to decay at a known, constant rate. By measuring the 14C remaining in a sample at the time of analysis, the time elapsed since death can be calculated.
AMS-based radiocarbon analysis involves three preparation steps before measurement: pretreatment to remove potential contaminants, combustion or acid digestion to convert the sample carbon to CO₂, and reduction (graphitization) to convert the CO₂ to solid graphite for AMS analysis.
pretreatment protocols
Acid/Alkali/Acid
Also known as ABA (Acid/Base/Acid), this protocol is applied to most organic materials. A hot hydrochloric acid (HCl) wash is first applied to remove carbonates, followed by hot sodium hydroxide (NaOH) washes to eliminate secondary humic acids. A final hot HCl wash is performed before the sample is rinsed to neutral pH and dried in preparation for combustion.
Collagen Extraction
This protocol is applied to bone samples. The sample surface is first mechanically cleaned, after which the AAA process is applied to isolate the protein fraction of the bone (collagen) through dissolution of the mineral matrix (calcium hydroxyapatite). The extracted collagen is rinsed to neutral pH and dried in preparation for combustion.
Acid Only
This protocol is typically applied to organic sediments. Samples are mechanically sieved, then treated with an HCl wash to remove any carbonates present. The sample is rinsed to neutral pH and dried in preparation for combustion.
Carbonate Extraction
This protocol is typically applied to cremated bone. Heating above 600°C causes the bioapatite to recrystallize into larger, more ordered crystals that are generally resistant to post-depositional contamination. Following mechanical cleaning of the sample surface, samples are treated with sodium hypochlorite (NaClO) for 48 hours, then acetic acid (CH₃COOH) for 24 hours. The sample is subsequently rinsed to neutral pH and dried in preparation for acid digestion.
Acid Etch
This protocol is applied to most carbonate materials, including shells and corals. Samples are first rinsed with water and mechanically cleaned to remove surface debris, then exposed to a dilute HCl wash. The sample is subsequently rinsed to neutral pH and dried in preparation for acid digestion.
Dissolved Inorganic Carbon
Strontium chloride (SrCl₂) is added to approximately 250 mL of the water sample under slightly basic conditions, precipitating the dissolved inorganic carbon as strontium carbonate (SrCO₃). The precipitate is collected, dried, and carried forward to acid digestion.
Combustion
Combustion
Following pretreatment, organic samples undergo combustion to convert the sample carbon to CO₂. A measured quantity of the pretreated material is sealed in an evacuated quartz tube with excess of an oxygen source (CuO or O₂). The amount of material used is determined by availability and the estimated carbon content of the material type. The sealed tube is then heated to 800°C, driving the oxidation reaction to completion. The combustion reaction is summarized below, where CxHy represents the sample carbon:
CxHy + (2x + ) CuO
xCO2 + H2O + (2x + ) Cu(s)
CxHy + (x + ) O2
800 °C
xCO2 + H2O
Acid Digestion
Carbonate samples undergo acid digestion following pretreatment to liberate the sample carbon as CO₂. The pretreated material is first crushed, then transferred to a reaction tube that is evacuated before digestion begins. H₃PO₄ is then introduced to the sample under vacuum, reacting with the carbonate to release CO₂ gas. As with CO₂ produced via combustion, this gas is carried forward to the reduction (graphitization) step for conversion to solid graphite prior to AMS analysis. The acid digestion reaction is summarized below:
Reduction
The CO₂ produced during combustion or acid digestion is purified and transferred under vacuum. During transfer, the gas passes through a −78 °C dry ice/isopropanol slush trap to remove residual water. Cryogenic trapping using liquid nitrogen (−196 °C) is then employed to collect and transfer the CO₂ into the reduction vessel. The reduction vessel contains Co(s) as a catalyst and Mg(ClO₄)₂ as a desiccant. Hydrogen gas is introduced, and the system is heated to 550 °C to drive the two step reduction that converts CO₂ into solid graphite.
The resulting graphite is pressed into aluminum cathodes, placed into a sample wheel, and analyzed using Accelerator Mass Spectrometry (AMS) to determine pMC (percent modern carbon) and Conventional Age.
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