Procedure of 14C dating in the Poznan Radiocarbon Laboratory,
consists of a few stages:
b) production of CO2 and
AMS 14C measurement
d) calculation of 14C age
and calibration of 14C age
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%
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.
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).
c) 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.
d) 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).
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