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Sample Preparation for Accelerator-basedRadiocarbon DatingJ. 0. Wand, R. Gillespie and R. E. M. Hedges

Although several efficient sample preparation methods have been devised for con-ventional radiocarbon dating, the advent of the accelerator mass spectrometer (AMS)dating technique with its much smaller sample requirement and different countingmethod has meant that new preparation routes have had to be developed. Techniquesused for the Oxford AMS systemare described n this paper.Kr?~words: RADIOCARBON DATING, ACCELERATOR MASS SPECTRO-METER, SAMPLE PREPARATION.

IntroductionIn conventional radiocarbon dating, only the small fraction of 14C atoms decayingduring the counting period are measured, whereas with the AMS method the majorityof 14C atoms present in a sample can be detected without waiting for radioactivedecays to take place. Consequently, much smaller amounts of carbon are required,typica lly l-5 mg rather than lL5g. The negligible background of the AMS systemmeans that the available age range extends beyond 50,000 years. Thus the samplepreparation technique must be capable of efficiently handling milligramme sized sampleswithout introducing carbon contamination or allowing cross-contamination betweensamples.

The essence of the AMS method is the production of carbon ion beams whichare analysed by means of magnetic and electric fields into separate beams for each of thethree carbon isotopes (12C, r3C and r4C) before measurement. For the production oflarge, stable beams of negative carbon ions, a caesium beam is directed at a carbontarget in a sputter source. The best performance of the sputter source is obtained withgraphite samples, which yield good carbon ion beams with minimal cross-contaminationbetween different samples mounted in the source. General experience with carbon ionbeams in AMS systems is described in Radiocarbon (1980) and a detaileddescription of the approach used at Oxford is given in Hedges ct al. (1980). Forconvenience, the graphite should be confined to a small area and be easy to handle andstore after preparation. Thus, all samples, after decontamination and pretreatment as

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16 0 J. 0. WAND ET AL.

described in Gillesp ie or al. (1984). are converted to graphite on tantalum wire.using a combination of conventional and novel techniques :(1) carbon dioxide preparation-organic materials arc burned in oxygen, or invacuum with copper (II) oxide as catalyst and a source of oxygen; carbonates are

treated with acid;(2) acetylene preparation-carbon dioxide is reacted with molten lithium metal toform lithium carbide, this reacts with water to form acetylene;(3) graphite preparation --acetylene is pyrolized by a hot tantalum wire. graphitebeing deposited as a bead on the wire.

- Tantalum wre

lV ,30A

S a m p l etcuo

-H,O trap

CzH, trap u Hz0 trap

t igure 1. Ciraphitc preparation system.

Sample preparationThe methods for preparation of carbon dioxide follow standard practice . Organicsamples are burned in a slow flow of pure oxygen. complete combustion beingassured by an afterburner of supported platinum at 450 C or copper (II) oxide at750 C. Combustion is most satisfactory for the majority of samples with a flow ofoxygen rather than a static system, but the pressure must be kept below 1Ombar in theliquid-nitrogen-cooled carbon dioxide traps to avoid some oxygen being trapped. Wehave observed impurities in the oxygen cylinders which could be serious contaminantswith very old samples; these impurities include carbon dioxide, carbon monoxide andhydrocarbons. For this reason, elimination of oxygen may be preferred. In this case.samples are mixed with wire-form copper (II) oxide in a silica tube, evacuated andheated to about 800 C for 5-10min. For carbonates, carbon dioxide is released fromthe sample by treatment with acid; 88,6 phosphoric acid is convenient because of thelow vapour pressure. Water is removed from the carbon dioxide produced in all systemsby dry-ice-cooled traps.

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SAMPLE PREPARATION IN AMS DATING 101The acetylene preparation is similar to that described by Tamers (1975) but scaleddown for use with I--5mg samples. An all stainless steel system is used, with PTFEferrules in compression fittings for case of demounting and cleaning. Because of thesmall volumes involved, it has been found convenient to freeze carbon dioxidedirectly into the lithium reactor and heat with about 50mg of lithium (a large excess).

This also keeps the pressure higher than the 50mbar which is apparently essential forhigh yields of carbide. The reasons for this, and a proposed reaction mechanism, arcgiven in Wand ( 1980). The overall reaction may be written :1OLi + 2C02 -3 4Li,O -4 Li,C,

which proceeds at a temperature of 60&700 C. Afterheating to 900 C advocated byPolach et ul. (1972) has not been found to increase the yield, and the radon removaleffected by this step is not required for the AMS. Water is slowly added to thecooled reactor, the reactions giving off acetylene. which is collected in liquid-nitrogen-cooled traps, and hydrogen (from the excess lithium), which is pumped away. Acetyleneis routinely produced in better than 900/, yields.Pressures of carbon dioxide and acetylene are measured with stainless steel diaphragmtransducers. The vacuum conditions are monitored with Pirani and Penning gauges.and we have found a residual gas analyser to be helpful in the development of thusystem (for leak detection and gas purity analysis). .A cryopump is used to minimizecontamination from vacuum pump oils.The final stage of graphite. achieved by pyrolysis of acetylene. is new to radiocarbondating. A tantalum wire 0.5 mm in diameter is resist ively heated between stain lesssteel electrodes. a current of about 30A at 1.5 V AC being used to raise thetemperature of the wire to 2000 C. Maximum yield of about 607 graphite deposited onthe wire is obtained after 15-20 min heating, using an acetylene pressure of 20-50 mbar.Yie lds are independent of the metal used : tungsten. molybdenum and hafnium all worksatisfactorily, but tantalum is superior since it does not display embrittlemcnt underthe conditions employed. The yield is also independent ofsurface area, so short wires canbe prepared which are wel l suited to the sputter source and are eas ily handled and stored.No deterioration has been observed in the condition of the graphite after storage for 2years, any surface contamination present being removed by the ion source sputtering inless than 1 min.This apparently simple cracking of acetylene in fact proceeds by a series of gas-phase reactions forming polyacetylenic and polycycl ic species which are deposited on thewire and there dehydrogenated (Homan & Wagner, 1967; Tanzawa & Gardincr, 1978;Lahaye & Prudo. 1978). Side reactions lead to the formation of unreactive polycycl icspecies which are deposited on the walls of the cracking vessel. Rigorous clean-upprocedures are essential after each cracking reaction -we find that rinsing withchloroform followed by ultrasonic agitation in 1M HCI, distilled water and methanol.then drying at 100 C is satisfactory. If a build-up of insoluble deposit is observed, sandblasting is an effective final solution.It is also possible to thermally crack benzene to graphite. We have used petroleum-derived benzene as a background standard and the Harwell benzene standards (Otlet(I al., 1980) as known age samples for calibration. The benzene can be introducedinto the cracking vesse l either by syringe through a septum or via a micro needlevalve. To obtain a full system background, benzene may be burned on hot copper (II)oxide and processed to graphite normally. Using these techniques. the directly crackedpetroleum benzene yields graphite with a C/12C ratio

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162 J. 0 . WAN D /T ALacetylene to graphite, a ratio about 0.4% of the modern standard is obtained. This isequivalent to a maximum measurable age of about 45,000 years, which we consider to besatisfactory for our initial work.Tracer experiments were used to determine whether memory or cross-contaminationeffects were present in the various stages of the reaction sequence. Carbon highlyenriched in the mass 13 isotope was burned and converted to graphite according tothe scheme described above, and samples of carbon dioxide ,after combustion andfrom burning the prepared graphite were analysed for i3C/12C ratio on a stable isotopemass spectrometer. A memory effect was observed when using the platinum combustioncatalyst, equivalent to about 0.5% of the enriched sample carbon retained on thecatalyst and which was released as carbon dioxide during the next combustion ofnormal carbon. This was not observed when using the copper (II) oxide catalyst,which was sulTicient reason for our choice of the latter method. The lithium reactionalso showed a memory effect, confirming the work of Radnell & Muller (1980). Up to1.5% of the enriched carbon was retained in the lithium reactor, and was notcompletely removed by four subsequent reactions with normal carbon. This problemcan be reduced by using disposable crucibles inside the lithium reactor, but even inthis situation a memory effect is possible since lithium can vaporize onto the outervessel walls during the outgassing procedure. The solution adopted is to use disposablelithium reactors; as these are only 75 mm long by 12.5 mm diameter and can be madefrom an easily machined stainless steel the cost is not prohibitive. Auto-productionof acetylene from lithium, suggested by Geyh (1969), has also been observed. Theamount produced from our standard reaction was always less than 0.2% of the sampleacetylene, which we do not consider to be signi ficant at the current leve l ofaccuracy. Acetylene was also produced, at about 0.5% of sample size, from usedlithium reactors, even when no new lithium was added. Results from the AMS on0.1 mg of graphite from such blank acetylene gave a 14C/12C ratio about 70% of themodern standard. This is an indication of a memory effect averaged over manyreactions. Further experiments using mater ials of known age indicated that as much as3% contamination could be introduced from the lithium reaction, obviously a seriousproblem for routine dating of very old samples. These results all reinforce thedecision to use disposable lithium reactors.Since the yield of graphite from acetylene using the pyrolysis method as describedabove is never greater than 60%, the possib ility exists for isotopic fractionation tooccur. Measurements of the i3C/i2C ratio on carbon dioxide from the combustion ofsamples and from the combustion of the prepared graphite indicate that fractionationdoes occur at the level of -0.8 + 0.1%. This is constant for all sample materials, andwhile it is a significant amount it is probably less than that taking place in the AMSsystem. During a dating run both i4C/12C and i3C/12C ratios are measured, so thatthese fractionation effects should be correctable by the on-line computing, White (1981).

AcknowledgementsThe authors would like to thank R. Burleigh of the British Museum ResearchLaboratory for his help with stable isotope measurements on samples for this work.A. Bowles and M. Humm provided technical assistance.

ReferencesGeyh, M . A. (1969). Problems in radiocarbon dating of small sam ples by means of acetylene.

ethane or benzene. Intrrnational Journal of Applkd Radiation and Isotopc~ 20, 46Z-466.

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SAMPLE PREPARATION IN AMS DATING 163Gillespie, R.. Hedg es, R. E. M. & Wan d, J. 0. (1984). Radiocarbon dating of bone byaccelerator m ass spcctro metry . JournalqfArr~hac~ological Science 11, 165170.Hedges. R. E. M., Wand, J. 0. & White, N. R. (1980). The production of C - beams forradiocarbon dating with accelerators. Nuclear Instrum ents and Method s 173,40%421.Homann, K. H. & Wagner, H. G. (1967). Some new aspects of the mechanism of carbon

formation in premixed flames . Proceedings qf 11th Symposium on Combust ion, pp. 371-379.Lahaye, J. & Prado. G. (1978). In (P. L. Walker & P. A. Thrower, Eds) The Chem istry andPhys ics of Carbon 14, 167.Otlet, R . L., Wa lker. A. J., Hew son, A. D. & Burleigh, R. (1980). 14C interlaboratory comparisonin the UK : Experiment design, preparation and preliminary results. Radiocarbon 22, 936946.Polach, H. A., Cower, J. & Fraser, 1. (1972). Synthes is of high purity benzene for radiocarbon

dating. Proceedings qf 8th International Conference on Radiocarbon Dating, Wellington. B36 49.Radnell, C. J. & Muller, A. B. (1980). M emo ry effec ts in the production of benzene forradiocarbon dating. Radiocarbon 22,479-486.Rudiocurhon ( 1980). 22Tam ers, M. A. (1975). Chem ical yield optimization of the benzene synthesis for radiocarbondating. International Journal of Applied Radiation and Isotopes 26,67&682.Tanza wa, T. & Gardiner, W . C. (1978). Thermal decompo sition of acetylene. Proceedings qfl7thSymposium on Combust ion, 563-573.Wa nd, J. 0. (1980). Microsam ple preparation for radiocarbon dating. Unpublished D.Phil thesis.Universi ty of Oxford.

Wh ite. N. R. (1981). Isotopic fractionation in accelerator based radiocarbon dating. P roceedings of2nd Sym posium on Accelerator Mu ss Spec trometr.v, Argonne. 359-372.