Procedure of 14C dating in the Poznan Radiocarbon Laboratory, consists of a few stages:
- chemical pretreatment
- production of CO2 and graphitisation
- AMS 14C measurement
- calculation of 14C age and calibration of 14C age
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1. Chemical pretreatment
Methods of chemical pre-treatment generally follow those used in the Oxford Radiocarbon Accelerator Unit, as described by Brock et al. (2010, Radiocarbon, 52, 102-112).
Samples of charcoal, wood, or other plant remains (after mechanical removal of macroscopic contamination visible under binocular) are treated with 1M HCl (80°C, 20+ min), 0.025-0.2M NaOH (room temperature for fragile plant remains, 80°C for wood) and then 0.25M HCl (80°C, 1 hour). After treatment with each reagent, the samples are rinsed with deionised water (Millipore) until pH=7. For the first HCl treatment, longer time (20+) is applied, if emanation of gas bubbles from sample is still visible. The step of NaOH treatment is repeated a few times, generally until no more coloration of the NaOH solution appears (coloration of solution is caused by humic acids dissolved in NaOH), but the NaOH treatment is interrupted if there is a danger of complete dissolution of the sample.
Samples of sediments are usually treated with 1M HCl (room temperature overnight and then 80°C, 1+ hour), 0.2M NaOH (80°C, 10 min) and then 0.25M HCl (80°C, 1 hour). After treatment with each reagent, the samples are rinsed with deionised water (Millipore) until pH=7. For the first HCl treatment, longer time (1+) is applied, if emanation of gas bubbles from a sample is still visible. The step of NaOH treatment is repeated a few times, generally until no more coloration of the NaOH solution appears (coloration of solution is caused by humic acids dissolved in NaOH). In case where total organic carbon (TOC) is to be analysed, the procedure is limited to the first HCl treatment only.
In case of bones, extraction of collagen is performed using the procedures originally described by Longin (1971, Nature, 230, 241-242), with further modifications (e.g. Piotrowska N., Goslar T., 2002, Isotopes in Environmental and Health Studies, 38, 1-9). Before extraction, degree of collagen degradation is checked by measuring content of nitrogen and carbon in bone, using analyser Flash EA 1112 Series (ThermoScientific). The samples are regarded suitable for collagen dating, if nitrogen content in bone is not lower than 0.6%, and ratio C/N is not higher than 5. Suitable bones are crushed mechanically to granulation <0.3 mm, the bone powder is then treated with 2M HCl (room temp., 20 min), and 0.1M NaOH (room temp., 1 h). After each step of treatment, the sample is centrifuged and the residuum is collected. Extraction of collagen is processed in HCl (pH=3, 80°C, 10h), and after centrifugation, residuum is removed. The extracted collagen is then ultrafiltered on pre-cleaned Vivaspin 15 MWCO 30 kD filters (Bronk Ramsey et al. 2004, Radiocarbon, 46, 155-163). Quality of the collagen is ultimately assessed basing on C/N atomic ratio (interval of acceptance: 2.7-3.5) and collagen extraction yield (acceptance threshold: 0.5%). On demand, carbon and nitrogen stable isotopic composition of the collagen can be determined.
In cremated bones, where collagen is too degraded and not suitable for 14C dating, and geological situation at the site of sample collection excludes precipitation of secondary carbonates, a fraction of structural carbonates is forwarded for dating, while organic fraction is removed by treating bones with 2% NaClO for 48h and then in 8% CH3COOH for 48h (acc. to Lanting J.N., Van der Plicht J., 2001. Radiocarbon, 43, 249-254). Then the outer layer of carbonate grains is removed in 2% HCl (1h) and additionally by quick rinsing with 36% HCl.
Samples of shells (and other carbonate features) are checked and mechanically cleaned under binocular. The organic coating, if visible, is removed with H2O2 (15-30%) in an ultrasonic bath. Then the outer carbonate layer (ca. 30%) is removed in 0.5M HCl (if the sample is large enough), the remaining material is treated in 15% H2O2 again (for 10 min in a ultrasonic bath) and the remaining carbonate is leached with concentrated H3PO4 in a vacuum line.
2. Production of CO2 and graphitisation
In case of organic samples, CO2 is produced by combusting the sample. Combustion of organic samples is performed in closed (sealed under vacuum) quartz tubes, together with CuO and Ag wool, in 900°C over 10 hours. CO2 from carbonate samples is leached by treating with concentrated orto-phosphoric acid (H3PO4) in a vacuum line.
The obtained gas (CO2 + water vapour) is then dried in a vacuum line, and reduced with hydrogen (H2), using 2 mg of Fe powder as a catalyst. The obtained mixture of carbon and iron is then pressed into special aluminium holder, according to the description provided by Czernik J., Goslar T., 2001, Radiocarbon, 43, 283-291.
In the same way we prepare the standard samples, i.e. samples not containing 14C (coal or IAEA C1 Carrara Marble) and samples international modern 14C standard (Oxalic Acid II).
3. AMS 14C measurement
Measurements described in this point, are performed in the AMS 14C Laboratory of the A. Mickiewicz University in Poznań. Cooperation between the Poznań Radiocarbon Laboratory and the AMS 14C Laboratory is regulated by the Agreement between Foundation of the A. Mickiewicz University and the A. Mickiewicz University.
Content of 14C in a sample of carbon is measured using the spectrometer “Compact Carbon AMS” (produced by: National Electrostatics Corporation, USA) described in the paper: Goslar T., Czernik J., Goslar E., 2004, Nuclear Instruments and Methods B, 223-224, 5-11). The measurement is performed by comparing intensities of ionic beams of 14C, 13C and 12C measured for each sample and for standard samples (modern standard: “Oxalic Acid II” and standard of 14C-free carbon: “background”). In each AMS run, 30-33 samples of unknown age are measured, alternated with measurements of 3-4 samples of modern standard and 1-2 samples of background. In case, where organic samples are dated, the background is represented by coal, while in case of carbonate samples, the background is represented by the sample IAEA C1.
4. Calculation of 14C age and calibration of 14C age
Conventional 14C age is calculated using correction for isotopic fractionation (according to Stuiver, Polach 1977, Radiocarbon 19, 355), basing on ratio 13C/12C measured in the AMS spectrometer simultaneously with the ratio 14C/12C (note: the measured values of d13C depend on isotopic fractionation during CO2 reduction and isotopic fractionation inside the AMS spectrometer, and as such, they cannot be compared with values of d13C determined with conventional mass spectrometers on gas samples). Uncertainty of calculated 14C age is determined using uncertainty implied from counting statistics, and also spread (standard deviation) of partial 14C/12C results, whichever is bigger. Uncertainties of 14C/12C ratios measured on standard samples are additionally taken into account. The 1-sigma uncertainty of conventional 14C age given in our reports, is the best estimate of the total uncertainty of measurement.
Calibration of 14C age is performed using the program OxCal ver. 4.2 (2014), the ground of which is described by Bronk Ramsey C., 2001, Radiocarbon, 43, 355-363, while the recent version – by Bronk Ramsey C., 2009, Radiocarbon, 51, 337-360 and Bronk Ramsey C. and Lee S., 2013, Radiocarbon, 55, 720-730. Calibration is performed against the newest version of 14C calibration curve i.e. INTCAL13 (Reimer P. J., et al. 2013, Radiocarbon, 55(4), 1869-1887).