Treatments of carbon samples for ¹⁴C measurement with AMS

Pretreatment is a multistep process of isolating the required carbon fraction from a carbonaceous sample for radiocarbon dating. In most cases, the fractions desired for ¹⁴C measurement are those from molecules indigenous to the organism when it was alive, and isolating this native carbon means removing any contaminating organic material that may have been introduced during sample preservation after its death.

 

Once a sample is submitted to the AMS ¹⁴C dating laboratory, the first step is physical selection and cleaning of the sample. The sample is checked under a microscope for any obvious extraneous elements, such as glue or ink that might be attached to the sample surface. The sample is rinsed with appropriate solvents and any contaminants are completely removed from the sample surface. Some kinds of samples are sieved to select the fraction of an appropriate size. Finally, the sample is crushed or ground to reduce the size and increase the surface area. The second step is chemical cleaning. The chemical pretreatment removes any contaminants in the sample that can be made soluble in a series of heating with relevant solvents. The temperature and duration of heating and solvents used are determined carefully according to the species, condition and size of the sample.

 

General examples of chemical pretreatment

Wood, charcoal or plant material:

Samples are treated with an ultrasonic cleaner in distilled water to clean the sample surface, and with an organic solvent (acetone) to remove adhesives, oils and fats. Acid-alkali-acid (AAA) treatments with 1.2 mol/L HCl and 1.0 mol/L NaOH are normally followed to remove impurities. Finally the samples are rinsed with distilled water and dried.

 

Wood - cellulose extraction:

After the AAA treatment, the wood sample is bleached with sodium chlorite (NaClO2) in 1.2 mol/L HCl at 70 oC – 80 oC for one night. Then the sample is rinsed with distilled water and treated with a 17.5% NaOH solution for half an hour. The solution is neutralized by using 1.2 mol/L HCl and completely rinsed with distilled water and dried.

 

Textiles:

Textile samples submitted to the laboratory may vary in degree of preservation and condition. Old wool, for example, may be quite degraded and is readily hydrolyzed and dissolved in alkaline solution. The cellulose of cotton will usually sustain a more vigorous treatment and large samples of hemp or other cellulosic fibers may even be subject to cellulose extraction as described for wood samples. In general, the purpose of pretreatment for textiles is to remove dirt, grease, fats, oils, and dyeing agents.

 

In summary, depending upon the preservation and condition of the fibers, textile samples are treated through a sequence of physical cleaning, and then are shredded into small fibers. The sample is sequentially rinsed, first by heating in hexane, filtering and drying, second by heating in isopropanol, filtering and drying, and finally heating in acetone, filtering and drying. The residue is then treated with a series of acid-alkali-acid cleaning with 1.2 mol/L HCl and 0.1 mol/L NaOH, rinsed thoroughly with deionized H2O, and dried.

 

Bone, ivory and tooth:

After surface examination and physical cleaning of bone, ivory and tooth samples, as well as treatment with organic solvent (acetone), they are ground down into powder. Certain amounts of the powder are decalcified with 0.5 mol/L HCl. The acid insoluble part of the decalcified bone sample is treated with 0.1 mol/L NaOH solution to remove secondary organic acids, rinsed to neutral, and converted to gelatin at 90 oC for 10 hours in 0.01 mol/L HCl (pH=3). The solution is centrifuged, the gelatin fraction in the solution part is separated by filtration from the solid fraction, and the gelatin is collected in a solid form by freeze-drying. Part of the gelatin fraction is separated at this time for analysis of stable isotope ratios of carbon and nitrogen, and for amino acid analysis, in addition to the ¹⁴C AMS measurement.

 

Sample combustion to produce CO2

After chemical pretreatments, most of the carbonaceous samples are combusted at 900 oC for 3 hours in evacuated and sealed quartz tubes, with cupric oxide and silver wire. The cupric oxide provides oxygen for the combustion and the silver reacts with sulfur and halogens and absorbs them. After combustion, the produced CO2 is cryogenically purified by passing it through a temperature trap of an ethanol and dry ice mixture kept at around -100 oC to separate water. The purified CO2 is collected separately in two glass vessels, one for the graphitization and the other for stable carbon-isotope ratio measurement with a mass spectrometer.

 

Graphitization of the produced CO2

The final step of sample preparation is graphitization to convert the CO2 sample to elemental carbon graphite. The CO2 sample is mixed with an appropriate amount of hydrogen gas inside a glass reaction vessel. The reaction vessel contains a small amount of iron powder that works as a catalyst for CO2 reduction. The iron powder portion at the end of the reaction vessel is heated in an electric furnace at 640 oC for about 8 hours, and the CO2 is reduced to graphite. Water produced in the CO2 reduction is trapped out at the other end of the reaction vessel.

 

Stable isotope measurements

The d13C (13C/12C ratio) values are normally measured for all samples, using the AMS system. The cost of the d13C measurement is included in the overall cost of the ¹⁴C AMS measurement. The d13C value is measured on the prepared graphite along with the ¹⁴C abundance and is used for correcting the isotopic fractionation of carbon isotopes for evaluating accurate ¹⁴C concentration of the sample.