Sample preparation and analysis at NSRL
NSRL is able to prepare a wide variety of solid and gas sample types.
Automated Pretreatment
System Figure 1. Charcoal and macrofossil samples are
pretreated with acid/base/acid to remove contaminating
carbon, using our automated pretreatment system. Our system
(pictured) is a modification of that described by Bradley
and Stafford (1994).
Oxidation and Hydrolysis of Solid Samples
The pretreated sample is converted to carbon dioxide gas by one of two methods. Organic materials are transferred to a quartz tube containing precleaned copper oxide and reduced silver wire; the tube is evacuated, sealed and combusted for 4 hours at 800°C to produce CO2 gas. Carbonate samples are individually hydrolyzed with dry phosphoric acid, off-line at 60°C, in glass side-arm reactors.
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Figure 2 (left). Sample carbon dioxide produced by hydrolysis (carbonate samples) or combustion (organic carbon samples) is cryogenicly purified and is divided into aliquots for delta 13C measurement and graphitisation. Figure 3 (right). Carbonate samples are hydrolysed offline in glass sidearm reactors. |
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Carbon dioxide produced by either combustion or hydrolysis is cryogenically purified and split into fractions for 14C dating and 13C analysis. Any remaining gas is archived in sealed pyrex tubes. An in-house 13C measurement (at INSTAAR's Stable Isotope Lab, Dr. J. White, Director) is performed on a small aliquot of CO2 gas.
Extraction of CO2 From Whole Air
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Figure 4. CO2 is extracted from whole air samples by a simple cryogenic purification procedure. A collection flask (or flasks) is fitted to the evacuated line and flask air flowed through it at a rate set by a mass flow controller. Water is first separated from the air gas mixture by freezing in a water trap at -80°C. CO2 is then frozen into the CO2 trap with liquid nitrogen at -196°C and remaining non-condensable components are removed by the vacuum pump. Once all of the air in the flask has been flowed through the system, the CO2 trap is isolated and the CO2 is allowed to sublime. The total amount collected is measured and sealed into a pyrex tube for graphitization. |
Reduction to Graphite
Approximately 80 µm of CO2 is reduced to graphite over iron in a hydrogen atmosphere (Vogel et al. 1987; McNichol et al. 1992).
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Figure 5 (left). Carbon dioxide gas is reduced to graphite with hydrogen over an iron catalyst. Optimal reaction conditions are obtained by heating the catalyst to 625°C, while water produced by the reaction is frozen out in an ethanol slush bath to ensure complete reaction. Figure 6 (right). Two graphitisation lines with 4 ports each allow reduction of 8 samples at a time. Reactions typically take about 5 hours to completion. Each reaction is monitored for quality control with small pressure transducers combined with labVIEW software. The graph shows the progress of a typical set of 8 reactions. Pressure decreases as CO2 and H2 gases are converted to graphite and water. |
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Figure 7. The reduced graphite and iron catalyst mixture is pressed into an aluminium cartridge for introduction into the accelerator. |
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AMS Measurement and Analysis
Standards and process blanks are produced at NSRL and are analyzed concurrently with unknowns. Standardization is via: NBS Oxalic Acid I (NIST-SRM-4990) and Oxalic Acid II (NIST-SRM-4990c), with the 14C activity ratio of Oxalic Acid II (∂13C =-17.3 ‰) to Oxalic Acid I (∂13C =-19.0 ‰) taken to be 1.293. Two additional Oxalic Acid (I or II) standards are analyzed as unknowns concurrently with each batch, and the result used for quality control.
Fraction Modern (Fm) is a measurement of the deviation of the 14C/C ratio of the sample from "modern". Modern is defined as 95% of the radiocarbon concentration (in AD 1950) of NBS Oxalic Acid I normalized to ∂13CPDB of -19 ‰ (Olsson, 1970). AMS results are calculated using the internationally accepted modern value of 1.176 ± 0.010 x 10-12 (Karlen et al 1968).
Results are corrected for both background contamination and fractionation. IAEA C-1 Carrarra marble is either hydrolyzed or combusted and measured concurrently with the samples to provide inorganic and organic process blank measurements. Fm is then normalized to a ∂13C value of -25 ‰. Where insufficient material is available for ∂13C measurement, an estimated value is used. Measured or estimated ∂13C is reported (±0.3 ‰ at one sigma unless otherwise noted).
Reporting of ages follows the convention of Stuiver and Polach (1977) and Stuiver (1980). Radiocarbon ages are calculated using the Libby half-life of 5568 years and are reported without reservoir corrections or calibration to calendar years. Reported errors are the larger of the internal and external measurement errors. The internal error is calculated using the number of counts measured from each target, and the external error is calculated from the reproducibility of individual analyses for each target. Errors are reported at 1 sigma.
D14C and/or d14C are reported for geochemical samples, following the convention of Stuiver and Polach.
When reporting AMS results from NSRL, the CURL number should be reported with the results. We recommend presentation of the lab reported radiocarbon ages along with any subsequent corrections. We ask that published results acknowledge NSF grant ATM-9809285 to the University of Colorado INSTAAR - Laboratory for AMS Radiocarbon Preparation and Research, and would appreciate receiving reprints or preprints of papers referencing AMS analyses made at our facility.
References
