Methods
Radiocarbon dating of soil organic matter (SOM) is problematic because SOM accumulates from heterogeneous sources. Fractionation of the heterogeneous organic carbon sources limits the application and interpretation of carbon dating of SOM. To remedy the inconsistencies in previous methods of carbon-14 dating of SOM, a high-temperature, pyrolysis-combustion technique was used. A combustion system used by the Illinois State Geological Survey (ISGS), under vacuum, fractions the SOM into a volatile and residual fraction. The volatile residue contains low-molecular-weight organic compounds, whereas, the residual residue contains high- molecular-weight organic compounds.
Preceding extraction of carbon dioxide from SOM samples, pretreatment is necessary. Each sample must be pretreated with heated 2 N HCl followed by rinsing with deionized water and vacuum filtration. Drying of the sample in a furnace will reduce the accumulation of water within the system.
The combustion system utilized by the ISGS consists of an inner and an outer quartz tube. To ensure pure CO2 production, a vacuum of -25 psi must be established. During volatile pyrolysis, the inner tube is purged with Argon while the outer tube is purged with oxygen. As the oxygen is purged through the outer tube, the volatile compounds released from the sample are carried by the argon into the outer tube where they are oxidized at 800 degree Celsius to form carbon dioxide. The CO2 and other gases produced from the volatile fraction are then passed through a cupric oxide furnace and wash traps including 0.5 N AgNO3 solution and a solution of 7.3 g Na2Cr2O7 in 50% H2SO4 for purification purposes . After the filtration, the CO2 is then passed through a Dry Ice-isopropanol trap to trap the water and the CO2 is finally collected in liquid nitrogen traps. The end of the volatile fraction is marked by the disappearance of the flame in the ignition furnace. Once the purified CO2 is transferred, the residual pyrolysis begins with the purging of the inner tube with oxygen and outer tube with argon. In pyrolysis of large samples, a stainless steel chamber and a crucible furnace connected to the inner tube of the combustion system must be used.
The purified CO2 is then converted to benzene for liquid scintillation spectrometry.
Read more about this topic: Radiocarbon Dating
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