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Laboratory contamination in radiocarbon accelerator mass spectrometry
Abstract
Factors which influence contamination during laboratory conversion of sample carbon to graphite are discussed. Careful attention to the cleaning of apparatus and the use of disposable components can reduce the extent of laboratory-induced contamination. Some examples of contamination from conservation treatments are also given.
... This procedure provided controlled characterizations of contamination from sample chemistry, which added 0.25 and 0.14 pMC respectively [18] [19]. Entry #10 in Baumgardner's Table 1 compares radiocarbon AMS with radiocarbon decay counting, showing roughly a 0.4 pMC contamination level for AMS due to sample chemistry [20]. These tests used identical materials with and without sample chemistry, not relying on assumptions that any of the materials were " radiocarbon-free, " showing that sample chemistry produces values in the range seen in the Phanerozoic biological samples. ...
... This range is essentially the same as that of Baumgardner's biological samples. The highest value of 0.7 pMC comes from an older sample chemistry procedure and may have been somewhat overestimated [20]. Baumgardner's biological sample #10 mentioned above is from this reference and is well within this sample chemistry background. ...
... These measurements yield a background of Y similar 0.03 t 0.015 pMC, equivalent to 65 ka 14C, to machine backgrounds reported for other AMS systems (e.g., Vogel et a1. 1987Beukens 1990van der Borg et al. ' 1997;Kretschmer et al. 1997;Kirner et al. 1995;Grootes et al. 1986;Gillespie and Hedges 1984). ...
... System blanks pose a problem for samples containing 0.1 mg or less. Similar observations were made in other laboratories (e.g., Vogel et al. 1987;Beukens 1990;Gillespie and Hedges 1984). ...
After routine accelerator mass spectrometry (AMS) radiocarbon dating had been established at the Leibniz-Labor with the completion of systems for CO2 production, graphitization, and target making, a systematic investigation was conducted to find the sources of 14C concentrations observed in background materials. We quantified the contributions of the AMS-system, the reduction, CO2 production from carbonate, carbonate contamination, and combustion. Carbonate contamination appears to be the dominating factor. Improvements in the pretreatment of foraminifer carbonate have led to the elimination of most of this contamination.
... Consequently, where contamination is suspected or confirmed from these sources, reliable AMS 14 C dating of museum bones could be guaranteed by dating individual amino acids and/or by selecting the regions of the bone least impregnated by conservation substances [103]. This will of course fail if animal hide or bone collagen glues were used as a preservative [106] because it would be impossible to distinguish a fossil's collagen amino acids from those in a collagen glue. In addition, other substances can also be applied in the field to consolidate or preserve bone and these materials might not have been recorded. ...
Radiocarbon ( ¹⁴ C) analysis of skeletal remains by accelerator mass spectrometry is an essential tool in multiple branches of science. However, bone ¹⁴ C dating results can be inconsistent and not comparable due to disparate laboratory pretreatment protocols that remove contamination. And, pretreatments are rarely discussed or reported by end-users, making it an ‘elephant in the room’ for Quaternary scientists. Through a questionnaire survey, I quantified consensus on the reliability of collagen pretreatments for ¹⁴ C dating across 132 experts (25 countries). I discovered that while more than 95% of the audience was wary of contamination and would avoid gelatinization alone (minimum pretreatment used by most ¹⁴ C facilities), 52% asked laboratories to choose the pretreatment method for them, and 58% could not rank the reliability of at least one pretreatment. Ultrafiltration was highly popular, and purification by XAD resins seemed restricted to American researchers. Isolating and dating the amino acid hydroxyproline was perceived as the most reliable pretreatment, but is expensive, time-consuming and not widely available. Solid evidence supports that only molecular-level dating accommodates all known bone contaminants and guarantees complete removal of humic and fulvic acids and conservation substances, with three key areas of progress: (i) innovation and more funded research is required to develop affordable analytical chemistry that can handle low-mass samples of collagen amino acids, (ii) a certification agency overseeing dating-quality control is needed to enhance methodological reproducibility and dating accuracy among laboratories, and (iii) more cross-disciplinary work with better ¹⁴ C reporting etiquette will promote the integration of ¹⁴ C dating across disciplines. Those developments could conclude long-standing debates based on low-accuracy data used to build chronologies for animal domestications, human/megafauna extirpations and migrations, archaeology, palaeoecology, palaeontology and palaeoclimate models.
... All operations after the initial sieving are carried out in the same 50-ml polypropylene centrifuge tube to minimize contamination introduced in the laboratory (Gillespie & Hedges 1984), and to allow the use of the hot hydrofluoric acid solution. The alkali solutions remove dark humic materials; several extractions are usually needed, and long centrifuge times are sometimes necessary to spin down all of the fine sediment. ...
The radiocarbon dating of volcanic ash (tephra) deposits in New Zealand has been difficult on sites remote from the eruption, which contain either little carbon or degraded and contaminated charcoal. Although many studies of contamination removal from macroscopic charcoals from tephra sequences have been made, little attention has been paid to those containing no visible charcoal, because of the difficulty of obtaining sufficient carbon for radiometric dating. We report here experiments using accelerator mass spectrometry to establish a reliable method for dating a low-carbon aeolian and peat deposit containing a tephra horizon. Results so far demonstrate that improvements to existing chemical pretreatment methods are possible, and that dates obtained on oxidized fine-grained residues can approach the maximum age determined on good quality charred wood samples.
... It is important to advise submitters and students how to better collect and pack samples, so that possible field and laboratory contamination can be avoided (Gillespie and Hedges 1984; Gupta and Polach 1985; Hedges 1992). Most facilities offer a checklist to assist submitters, which normally includes the following recommendations: (a) keep samples stored dry or frozen; (b) pack and clearly label each sample individually (preferably using Al foil and zipper bags); (c) if using vials, secure the lids; and most importantly, (d) never handle samples in or in close proximity to labs where tracer 14 ...
Over the last few decades, radiocarbon laboratories have used different procedures for measuring a broad range of carbonaceous materials. To produce reliable results, the processes employed for sample processing, graphite target production, and spectrometer measurement must be rigorous, well tested, and reproducible. Most of the procedures have been developed, improved, and published as part of the laboratories’ quality control and research programs, and can be frequently found in the literature. Nevertheless, there are suites of laboratory techniques (or “small useful skills”), products, and other resources that either have never been described in publications, or have been somewhat hidden in much larger scientific articles and reports. We feel that with the rapid rise of newer laboratories and facilities, a set of resourceful suggestions might come in handy. Here we gathered these skill sets that can be used in all aspects of 14 C sample processing, with the intention to simplify and expedite procedures, from glass-tube making to graphitization and measurements. We also included some miscellaneous items to help in laboratory setup.
... Ideally, the system background should include a 'chemistry background' calculated from measurements made on the same equipment using the same chemistry on the same sample material, which is known from other evidence to be older than the limiting measurement background. More chemical steps requiring more transfers of sample between containers means more chances for contamination to be added in the laboratory (Gillespie & Hedges, 1984). In practice, although AMS dating offers better sample selection possibilities, chemical and handling procedures for very small samples are still a limiting factor, increasing the uncertainty of measurements and reducing the maximum age limit attainable for some materials. ...
The Late Pleistocene radiocarbon record from the Willandra Lakes region is analysed to select the most reliable age estimates. Chemical pretreatment experiments addressing problems of contradictory ages on black organic samples from archaeological hearths and shell middens show that macroscopic charcoal is not present at some sites, although multiple sub-samples of well preserved charcoal have been dated at other sites. Radiocarbon measurements on properly decontaminated charcoal are as reliable as other robust single component samples such as shell, fish otoliths and emu eggshell. After calibration, 14C ages are in close agreement with luminescence based ages on suitably analysed quartz from the same sites. Regional patterns of 14C and luminescence dates are correlated across the Murray-Darling basin, implying rapid and sensitive responses to changing climate. Chemical arguments, and the likelihood of widespread sediment erosion prior to Last Glacial Maximum, suggest that probable ages for some human burials are younger than usually quoted. There is no unequivocal evidence for archaeological remains in the Willandra older than 43 ka.
... All operations after the initial sieving are carried out in the same 50-ml polypropylene centrifuge tube to minimize contamination introduced in the laboratory (Gillespie & Hedges 1984), and to allow the use of the hot hydrofluoric acid solution. The alkali solutions remove dark humic materials; several extractions are usually needed, and long centrifuge times are sometimes necessary to spin down all of the fine sediment. ...
The radiocarbon dating of volcanic ash (tephra) deposits in New Zealand has been difficult on sites remote from the eruption, which contain either little carbon or degraded and contaminated charcoal. Although many studies of contamination removal from macroscopic charcoals from tephra sequences have been made, little attention has been paid to those containing no visible charcoal, because of the difficulty of obtaining sufficient carbon for radiometric dating. We report here experiments using accelerator mass spectrometry to establish a reliable method for dating a low-carbon aeolian and peat deposit containing a tephra horizon. Results so far demonstrate that improvements to existing chemical pretreatment methods are possible, and that dates obtained on oxidized fine-grained residues can approach the maximum age determined on good quality charred wood samples.
... As described in the samples section, combusted graphite underwent considerably more manipulation than the sample placed directly in the ion source. For example, combusted graphite was finely ground, a process that Gillespie and Hedges (1984) noted appears to introduce contamination. The prebaking process might have been expected to remove these contaminants, however depending on the chemical nature and binding capacity of the contaminant, removal may have been incomplete or recontamination may have occurred during subsequent storage and/or weighing and transfer of the graphite powder to combustion tubes. ...
An assessment of the contamination contribution of various sample preparation procedures used at the Isotrace Radiocarbon Facility, University of Toronto, is described. Samples of geologic material, millions of years old, or samples derived therefrom, were tested because these would presumably contain only dead carbon. Results showed, however, that 14C contamination could be detected in several samples, complicating the contamination assess-ment. Best estimates of the contamination contribution from sample preparation were reported as: cracking: <0.17% modern, acetylene synthesis: <0.25% modern, combustion: <0.39% modern, and handling: <0.54% modern. These estimates were reported as upper lim-its because they likely represented 14C derived from two sources: sample preparation and the sample itself.
... 44 ka BP) to 0.07 pMC (ca. 58 ka) on 1-10 mg size samples (Gillespie and Hedges 1984;Vogel, Nelson and Southon 1987;Arnold et al. 1987;Schmidt, Balsley and Leach 1987;Gurfinkel 1987;Beukens 1992). These values represent the combined or "total system" (Vogel, Nelson and Southon 1987) combustion or acidification and graphitization background as defined in Table 1. ...
Vogel, Nelson and Southon (1987) determined that the total-system 14C background values in catalytically reduced graphitic carbon samples of 500 mg or less were inversely proportional to their weights. We have used wood reportedly of Pliocene age (i.e., 14C "dead") to further examine this relation. Our observations are consistent with the conclusions of Vogel, Nelson and Southon (1987). It appears that contamination can be characterized as if a constant level of modern carbon is being added. We also report a significant overall net reduction in total system background values ca. 45% for combusted samples from levels previously reported by Vogel, Nelson and Southon (1987).
... While we are not suggesting that all reported 14 C ages in excess of 40 ka are erroneous, without such caution, interpretations of the timing of geological or archaeological events may, in fact, be based on measured levels of contamination, rather than sound chronological data.'' This warning can be extended to dates with ages that exceed the fifth half-life due to the lower 14 C content, in which contamination by various physicochemical processes (Bischoff et al., 1989;Jö ris et al., 2006;Cohen-Ofri et al., 2007), even in radiocarbon laboratories (Gillespie and Hedges, 1984), is difficult to detect. ...
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ID - 646
... All of these steps were carried out in the same 50 ml polypropylene tubes, with centrifuging at 3000 rpm to separate the solid residue at each stage. This procedure reduces the number of containers used on each sample, with a consequent reduction ha laboratory contamination levels (Gillespie and Hedges, 1984). Water washes were carried out after each stage, and the final residues were oven dried prior to combustion. ...
Sediments containing small amounts of carbon are difficult to date using traditional radiocarbon techniques. This has resulted in less than satisfactory attempts to establish reliable chronologies for sedimentation and environmental change in the Australian arid zone. We describe here the first application of the AMS technique to the radiocarbon dating of salt lake core samples, using a chemical pretreatment methodology based on pollen extraction techniques. These results indicate that fine-grained charcoal and pollen have a similar source and depositional mechanism. The data from Lake Eyre imply a last glacial maximum deposition for the major buried halite layer, an Early Holocene return to lacustrine conditions, with a Late Holocene reduction of net sediment inout to Madigan Gulf as the present playa conditions were established.
p>The recent controversy over the accuracy of the higher dissolved organic carbon (DOC) concentrations reported by Sugimura and Suzuki (1988; Marine Chemistry 24, 105-131 ) obtained using high-temperature catalytic oxidation (HTCO) methods has been further critically examined during inter-calibration exercises between HTCO and ultra-violet (UV) persulphate DOC analyzers. The accuracy of these DOC methods is currently still unclear. This study and other work indicates that the HTCO method of DOC analysis may measure 5 to 60% more DOC than UV persulphate methods. An increase in DOC oxidation efficiency (6 to 8%) was achieved by adding low concentrations of Hg salts (0.004 M in the actual sample; Hg(NO3)2 and HgCl2) to the sample stream in a UV persulphate DOC analyzer. This increase in DOC oxidation efficiency implies that there is a fraction of DOC that is not measured by the UV persulphate method of DOC analysis, when used without the addition of Hg salts. The increase of DOC concentrations approaches the higher levels of DOC measured by the HTCO method.
To investigate the size fractionated distributions of organic carbon in the estuarine system a two year field study was undertaken in the Beaulieu estuary during 1990 to 1992; the Beaulieu river system is characterized by an organic rich drainage basin and is situated in the southern part of Britain. Ultrafiltration methods were developed for isolating colloidal organic carbon (COC) fractions from natural waters using the unique properties of Anodisc Anopore filters. Sources of organic carbon were differentiated using δ13C signatures and background parameters were also measured to aid interpretation.
Colloidal organic carbon represents a large fraction of carbon in the estuarine waters; up to 3.6 x `true' DOC (TDOC; i.e. the DOC concentration after filtration through Anodisc Anopore 0.02 μm filters) and up to 12 x POC concentrations.</p
Since the development of ¹⁴ C dating by W.F. Libby in the 1940s and 1950s, the method has proved to be the best absolute method for chronometric dating of organic and some carbon-containing inorganic materials. It has become an essential research tool for both archaeologists and geologists. Even broader application horizons have been opened to radiocarbon dating by use of the accelerator mass spectrometer, which allows routine analysis of submilligram samples. Art curatorship and art conservation directly benefit from this development.
There are pronounced similarities between application of radiocarbon dating in archaeology and art research but there are also major differences which make it difficult to simply take a methodology developed for archaeological research and apply it directly to radiocarbon dating of art objects.
In both fields the application of dating techniques focuses on dating an object of unknown age or cross-checking the age of an object previously dated by other methods.
Tests have been carried out on the measurement of 14 C directly from CO 2 , using a source designed by Middleton, and a Tandetron accelerator mass spectrometer system. These tests were very promising from the point of view of minimum quantity of carbon necessary (<50μg), but suffered from a background (apparently not memory effect) giving an equivalent age of ∼25,000 years. As a demonstration of the technique, a single thread from the mummy wrapping of the Egyptian king Ramses II was dated.
Collagen-based pretreatment methods for bone yield inconsistent results for those samples where protein preservation is low, as frequently found in bones from the semi-arid zones of Australia and North America. New methods for dealing with low collagen bones are needed, and this paper suggests that the non-collagenous proteins, particularly the blood proteins, may offer advantages for AMS dating because of their better preservation. Amino-acid profiles of collagen and non-collagenous proteins suggest that such differential preservation may be due to physico-chemical differences, and help to explain the poor results from dating low-collagen bones. -Author
Previous studies have determined that the total system 14C background values in catalytically- reduced graphitic carbon samples are inversely proportional to their weights. We further examine this relationship down to 40 micrograms using both assumed 14C "dead" background sample and a contemporary standard, Australian National University (ANU) sucrose. Our observations are consistent with those previously reported with respect to the inverse relationship between sample weight and 14C activity. These observations support the view that a constant addition of modern carbon contamination during the graphitization process explains the observed background14C activity in graphitic carbon samples.
Minute biological traces, with their prospect of recovering even ancient DNA, are the most attractive of archaeological materials to work with. This supplementary report on field studies of rock-art first published in ANTIQUITY further explores how these studies may in truth be carried out.
14 C dating of bone has been unreliable in comparison with more stable materials such as wood or charcoal. Attempts have been made to use various components or fractions isolated from the raw bone sample; these include dilute acid soluble, dilute acid insoluble, collagen, and gelatin, as well as alkali soluble and insoluble fractions of burned bone, and carbonate or apatite fractions of organic-poor bone. All of these fractions have yielded useful data in some cases, but no single method has proven suitable in all situations. The work reported here describes the isolation and purification of amino acids from the dilute acid insoluble fraction of bone collagen and parchment, with some preliminary experiments on amino acids from shell conchiolin.
Two of the early expectations of AMS were extension to greater 14C ages than possible with beta counting, and applications using tiny amounts of carbon. We examine each in the light of our experiences. The age limit depends on true or apparent ion counts, which may originate in sample preparation, in the accelerating system itself, or from ions other than 14C which are not adequately suppressed. 14N ions are generated spontaneously in our FN tandem, whether the ion source is open to the tandem or not, but are suppressed in the detector. There is no evidence of spontaneous 14C ion production. Thus the ion-source and tandem system alone is capable of age determinations >= 90000 yr. For presumably ``dead'' geological graphite, we obtain a maximum of 70000 yr, and about 50000 yr for ``dead'' material which has undergone chemical and physical preparation. In this respect our full AMS system is at present a mixed success. However, our applications to small fractions have been quite successful. We have applied this advantage to the study of the organic carbon components present in the Amazon River and in deep sea cores.
The joint ANSTO/ANU 36Cl program has now measured more than 700 samples from many different locations. During the course of this work, a variety of contamination problems have affected a small number of results which have provided valuable information on the effects of ion source cross-talk, sample preparation and storage procedures and sources of high-36Cl material. A sample of Weeks Island halite is processed along with every batch of field samples and the observed ratio provides a clear distinction between normal batches and those subject to contamination. Over three years, the long-term average ratio of 36Cl/Cl for normal halite samples is (1 +/- 1) × 10-15. The sample handling procedures developed during the course of this work provide a useful guide to the techniques that must be used to achieve the sensitivity limits which are potentially available using AMS.
Measurements of /14C/13C ratios were made on samples of Oxalic Acid and 14C ``dead'' materials spanning the mass range from 10 mug to ~ 1 mg. These measurements have allowed the determination of both the amount, and the 14C content, of the contaminant carbon introduced during sample processing in our laboratory. These data were used to corner measured /14C/13C ratios obtained from ANU Sucrose and `` ~ one-half-life old'' test samples for the influence of the contaminant. The test samples spanned the 10 mug to ~ 1 mg mass range and the corrections were made using three different formulae. The results obtained from these calculations allow the accuracy of these background correction formulae to be evaluated.
In this paper, we summarize the main chemical pretreatment protocols currently used for AMS radiocarbon dating at the Oxford Radiocarbon Accelerator Unit, updating the protocols last described by Hedges et al. (1989). © 2010 by the Arizona Board of Regents on behalf of the University of Arizona.
Under certain environmental conditions, post-depositional diagenetic loss of bone collagen can severely reduce the number of bones from a particular archaeological site that are suitable for stable isotopic analysis or radiocarbon dating. This study examined nearly 300 bones from 12 archaeological sites across southern England known to yield poor or variable preservation to try to identify one, or more, pre-screening technique(s) that would indicate suitable collagen preservation for radiocarbon dating. The most reliable method was shown to be the percent nitrogen (%N) of whole bone powder, which has an 84% chance of successfully predicting whether or not a bone will yield sufficient (i.e. >1% weight) collagen for dating.
To examine one component of the instrument-based background in the University of California Keck Carbon Cycle AMS spectrometer, we have obtained measurements on a set of natural diamonds pressed into sample holders. Natural diamond samples (N = 14) from different sources within rock formations with geological ages greatly in excess of 100 Ma yielded a range of currents (∼110–250 μA 12C− where filamentous graphite typically yields ∼150 μA 12C−) and apparent 14C ages (64.9 ± 0.4 ka BP [0.00031 ± 0.00002 fm] to 80.0 ± 1.1 ka BP [0.00005 ± 0.00001 fm]). Six fragments cut from a single diamond exhibited essentially identical 14C values – 69.3 ± 0.5 ka–70.6 ± 0.5 ka BP. The oldest 14C age equivalents were measured on natural diamonds which exhibited the highest current yields.
Accelerator mass spectrometry (AMS) allows radiocarbon dating to be carried out by direct counting of14C atoms, rather than the conventional counting of radioactive disintegrations. The result is that samples up to 1000 times smaller can be handled. The approach was tested in principle by 1977 and for archaeological operation by 1983. More than 2000 samples per year are now being dated worldwide. The machines can now operate to about 80 years or better. Dates older than 40,000 years have not yet been achieved, but the ability to use small samples has already had considerable impact on dating the period 10,000–30,000 years ago. Bone is an ideal material for the new technique, since amino acids can normally be isolated and purified from gram-size samples. Studies of the origins of domestication are aided by the dating of individual grains and seeds. Because small samples can be mobile in the soil, careful sample selection strategies and procedures are required. The full impact of the technique can be assessed only through the rapid and comprehensive publication of archaeological results.
Sample target preparation techniques for radiocarbon dating by accelerator mass spectrometry (AMS) are reviewed. The procedures leading to AMS targets are internally reproducible yet often result in targets of varying stability, duration and ion beam yield. Sample size requirements, factors affecting background and ion beam stability are some of the variables that are considered.
The excavations carried out in Cova Gran de Santa Linya (Southeastern PrePyrenees, Catalunya, Spain) have unearthed a new archaeological sequence attributable to the Middle Palaeoloithic/Upper Palaeolithic (MP/UP) transition. This article presents data on the stratigraphy, archaeology, and (14)C AMS dates of three Early Upper Palaeolithic and four Late Middle Palaeolithic levels excavated in Cova Gran. All these archaeological levels fall within the 34-32 ka time span, the temporal frame in which major events of Neanderthal extinction took place. The earliest Early Upper Palaeolithic (497D) and the latest Middle Palaeolithic (S1B) levels in Cova Gran are separated by a sterile gap and permit pinpointing the time period in which the Mousterian disappeared from Northeastern Spain. Technological differences between the Early Upper Palaeolithic and Late Middle Palaeolithic industries in Cova Gran support a cultural rupture between the two periods. A series of 12 (14)C AMS dates prompts reflections on the validity of reconstructions based on radiocarbon data. Thus, results from excavations in Cova Gran lead us to discuss the scenarios relating the MP/UP transition in the Iberian Peninsula, a region considered a refuge of late Neanderthal populations.
Although several efficient sample preparation methods have been devised for conventional radiocarbon dating, the advent of the accelerator mass spectrometer (AMS) dating technique with its much smaller sample requirement and different counting method has meant that new preparation routes have had to be developed. Techniques used for the Oxford AMS system are described in this paper.
- R Gillespie
- R E M Hedges
- J O Wand
R. Gillespie, R.E.M. Hedges and J.O. Wand, J. Archaeol.
Sci. 11 (1984) 165.