AMS Target Research Articles

Attempts to understand potential deficiencies in chemical procedures for AMS

A major research focus of the DREsden Accelerator Mass Spectrometry (DREAMS) facility is sample preparation: in-house and at cooperating laboratories. Besides routine applications, developments are mainly driven by the users’ demands such as “new” radionuclides (e.g. short-lived 7Be), radionuclides from new matrices or in larger sample amounts and at lower concentrations. Within the chemical preparation of BeO for 7,10Be, Al2O3 for 26Al, AgCl for 36Cl, and Fe2O3 for 60Fe and actinide AMS targets, we investigated and improved mostly particular steps: For oxide preparation aging for several hours and two-times rinsing of hydroxides is recommended, especially for Be(OH)2 and Al(OH)3, to prevent losses in the final preparation steps. Rinsing of freshly precipitated hydroxides can yield to losses as high as 31%. For Al(OH)3 rinsing with H2O (pH5) instead of pH8-9 is recommended for further reducing redissolving. For 60Fe and coprecipitated actinides (the latter tested by U6+ and Er3+), rinsing of overnight-aged Fe(OH)3, yield to 2.6–3.5% losses. The depletion of the isobar 7Li for (7)BeO is easily gained by hydroxide precipitation and rinsing. The quality of BeO and Al2O3 AMS targets can be monitored and improved (e.g. by using larger ion exchanger columns) by using the so-called “quality factor”, which is the current of the stable nuclide (9Be or 27Al), normalised to the current of the standard of the same batch. Finally, there is a high potential for ion exchange as a pre-enrichment tool for Cl for large ice samples containing low natCl concentrations and low absolute amounts of natCl.

Preliminary study of 10Be/7Be in rainwater from Xi’an by Accelerator Mass Spectrometry**Supported by National Natural Science Foundation of China (11205161) and CAS Key Technology Talent Program

The 10Be/7Be ratio is a sensitive tracer for the study of atmospheric transport, particularly with regard to stratosphere-troposphere exchange. Measurements with high accuracy and efficiency are crucial to 7Be and 10Be tracer studies. This article describes sample preparation procedures and analytical benchmarks for 7Be and 10Be measurements at the Xi’an Accelerator Mass Spectrometry (Xi’an-AMS) laboratory for the study of rainwater samples. We describe a sample preparation procedure to fabricate beryllium oxide (BeO) AMS targets that includes co-precipitation, anion exchange column separation and purification. We then provide details for the AMS measurement of 7Be and 10Be following the sequence BeO−→Be2+→Be4+ in the Xi’an- AMS. The 10Be/7Be ratio of rainwater collected in Xi’an is shown to be about 1.3 at the time of rainfall. The virtue of the method described here is that both 7Be and 10Be are measured in the same sample, and it is suitable for routine analysis of large numbers of rainwater samples by AMS.

Contamination on AMS Sample Targets by Modern Carbon is Inevitable

AbstractAccelerator mass spectrometry (AMS) measurements of the radiocarbon content in very old samples are often challenging and carry large relative uncertainties due to possible contaminations acquired during the preparation and storage steps. In case of such old samples, the natural surrounding levels of 14C from gases in the atmosphere, which may well be the source of contamination among others, are 2–3 orders of magnitude higher than the samples themselves. Hence, serious efforts are taken during the preparation steps to have the samples pristine until measurements are performed. As samples often have to be temporarily stored until AMS measurements can be performed, storage conditions also become extremely crucial. Here we describe an assessment of this process of contamination in background AMS samples. Samples, both as pressed graphite (on AMS targets) and graphite powder, were stored in various storage conditions (CO2-spiked air) to investigate the extent of contamination. The experiments clearly show that the pressed targets are more vulnerable to contamination than the unpressed graphite. Experiments conducted with enriched CO2-spiked laboratory air also reveal that the contaminating carbon is not only limited to the target surface but also penetrates into the matrix. A combination of measurements on understanding the chemical nature of the graphitization product, combined with long-available knowledge on “adventitious carbon” from the surface science community, brought us to the conclusion that contamination is to a certain extent inevitable. However, it can be minimized, and should be dealt with by sputter-cleaning the samples individually before the actual measurement.

Separation of Be and Al for AMS using single-step column chromatography

With the aim of simplifying AMS target preparation procedures for TCN measurements we tested a new extraction chromatography approach which couples an anion exchange resin (WBEC) to a chelating resin (Beryllium resin) to separate Be and Al from dissolved quartz samples. Results show that WBEC–Beryllium resin stacks can be used to provide high purity Be and Al separations using a combination of hydrochloric/oxalic and nitric acid elutions. 10Be and 26Al concentrations from quartz samples prepared using more standard procedures are compared with results from replicate samples prepared using the coupled WBEC–Beryllium resin approach and show good agreement. The new column procedure is performed in a single step, reducing sample preparation times relative to more traditional methods of TCN target production.

A cold finger cooling system for the efficient graphitisation of microgram-sized carbon samples

At ANSTO, we use the Bosch reaction to convert sample CO2 to graphite for production of our radiocarbon AMS targets. Key to the efficient graphitisation of ultra-small samples are the type of iron catalyst used and the effective trapping of water vapour during the reaction. Here we report a simple liquid nitrogen cooling system that enables us to rapidly adjust the temperature of the cold finger in our laser-heated microfurnace. This has led to an improvement in the graphitisation of microgram-sized carbon samples. This simple system uses modest amounts of liquid nitrogen (typically <200mL/h during graphitisation) and is compact and reliable. We have used it to produce over 120 AMS targets containing between 5 and 20μg of carbon, with conversion efficiencies for 5μg targets ranging from 80% to 100%. In addition, this cooling system has been adapted for use with our conventional graphitisation reactors and has also improved their performance.

Performance of Accelerator Mass Spectrometry for 129I using AgI–AgCl carrier-free coprecipitation

129I has been successfully applied as tracer in environmental, geological, and oceanographic research. For samples with low stable iodine concentration and ultra low level 129I, the sample preparation technique to separate iodine prior to AMS measurement has been a bottleneck, limiting the applicability of 129I. We have reported a carrier-free method, using coprecipitation, to avoid the potential introduction of 129I through the use of stable iodine carrier iodine. In this work, the detection limit and the analytical uncertainty of this method are investigated and minimum sample amount required to obtain reliable analytical results are estimated. The method is validated with a series of samples in ranges of known iodine concentrations and 129I/127I ratios. The results confirm our previous conclusion that an AMS target containing 5.0μg iodine can be used for analyzing samples with 129I/127I>10−12, and that for samples with 129I/127I<10−13 more than 25μg iodine is necessary.

On-line ion chemistry for the AMS analysis of 90Sr and 135,137Cs

The analysis of 90Sr by AMS has so far required the use of very large tandem accelerators in order to separate the isobar 90Zr by the rate-of-energy-loss method. The analysis of 135,137Cs by AMS has never been attempted as the separation of the isobars 135,137Ba by the traditional method requires even higher energies, so that this approach would become prohibitively expensive for routine analysis. Following the successful demonstration of Cl−–S− separation by the Isobar Separator, the same apparatus was used to test the separation of other pairs of isobars. Surprisingly effective results were obtained with NO2 gas in the cases of SrF3−–ZrF3− and CsF2−–BaF2− separations. Reduction factors of ∼4×10−6 for ZrF3−/SrF3− and ∼2×10−5 for BaF2−/CsF2− were measured. SrF3− and CsF2− are both super-halogen anions and are preferentially produced in the ion source rather than ZrF3− and BaF2− when using the PbF2 matrix-assisted method. Reduction factors for ion source production with such targets of ∼3×10−5 for ZrF3−–SrF3− and ∼5×10−4 for BaF2−–CsF2− were found. The combined methods would suggest a theoretical detection sensitivity for 90Sr/Sr ∼6×10−16, 135Cs/Cs ∼7×10−15 and 137Cs/Cs ∼1×10−14, assuming 10ppm Zr and Ba contamination in the AMS targets. In addition to the earlier Cl−–S− separation work, these measurements further illustrate the potential of on-line ion chemical methods for broadening the analytical scope of small AMS systems.

Development of a preparation system for the radiocarbon analysis of organic carbon in carbonaceous aerosols in China

Carbonaceous aerosols comprising a large fraction of elemental carbon (EC) and organic carbon (OC) are considered to affect both global climate and human health. Radiocarbon measurements have been proved to be a useful isotopic tracer for distinguishing contemporary and fossil emissions. An optimized system of a two-step thermal preparation system for radiocarbon ( 14C) measurement of OC/TC is firstly established in China. In this system, OC/TC are converted into carbon dioxide under a pure oxygen flow at 340 °C/650 °C and then reduced to graphite for AMS target using the method of zinc reduction. Afterwards, radiocarbon measurements of the targets performed by the NEC Compact AMS System at the Institute of Heavy Ion Physics, Peking University. The measured results for estimated reference martial including HOx I, HOx II and IAEA-C6 are consistent with internationally accepted values. The radiocarbon-based source appointment of carbonaceous aerosols in China would be much more convenient and faster with the preparation system developed in this work.

Quality of graphite target for biological/biomedical/environmental applications of 14C-accelerator mass spectrometry.

Catalytic graphitization for 14C-accelerator mass spectrometry (14C-AMS) produced various forms of elemental carbon. Our high-throughput Zn reduction method (C/Fe = 1:5, 500 °C, 3 h) produced the AMS target of graphite-coated iron powder (GCIP), a mix of nongraphitic carbon and Fe3C. Crystallinity of the AMS targets of GCIP (nongraphitic carbon) was increased to turbostratic carbon by raising the C/Fe ratio from 1:5 to 1:1 and the graphitization temperature from 500 to 585 °C. The AMS target of GCIP containing turbostratic carbon had a large isotopic fractionation and a low AMS ion current. The AMS target of GCIP containing turbostratic carbon also yielded less accurate/precise 14C-AMS measurements because of the lower graphitization yield and lower thermal conductivity that were caused by the higher C/Fe ratio of 1:1. On the other hand, the AMS target of GCIP containing nongraphitic carbon had higher graphitization yield and better thermal conductivity over the AMS target of GCIP containing turbostratic carbon due to optimal surface area provided by the iron powder. Finally, graphitization yield and thermal conductivity were stronger determinants (over graphite crystallinity) for accurate/precise/high-throughput biological, biomedical, and environmental14C-AMS applications such as absorption, distribution, metabolism, elimination (ADME), and physiologically based pharmacokinetics (PBPK) of nutrients, drugs, phytochemicals, and environmental chemicals.

Biological/Biomedical Accelerator Mass Spectrometry Targets. 2. Physical, Morphological, and Structural Characteristics

The number of biological/biomedical applications that require AMS to achieve their goals is increasing, and so is the need for a better understanding of the physical, morphological, and structural traits of high quality of AMS targets. The metrics of quality included color, hardness/texture, and appearance (photo and SEM), along with FT-IR, Raman, and powder X-ray diffraction spectra that correlate positively with reliable and intense ion currents and accuracy, precision, and sensitivity of fraction modern (Fm). Our previous method produced AMS targets of gray-colored iron−carbon materials (ICM) 20% of the time and of graphite-coated iron (GCI) 80% of the time. The ICM was hard, its FT-IR spectra lacked the sp2 bond, its Raman spectra had no detectable G′ band at 2700 cm−1, and it had more iron carbide (Fe3C) crystal than nanocrystalline graphite or graphitizable carbon (g-C). ICM produced low and variable ion current whereas the opposite was true for the graphitic GCI. Our optimized method produced AMS targets of graphite-coated iron powder (GCIP) 100% of the time. The GCIP shared some of the same properties as GCI in that both were black in color, both produced robust ion current consistently, their FT-IR spectra had the sp2 bond, their Raman spectra had matching D, G, G′, D+G, and D′′ bands, and their XRD spectra showed matching crystal size. GCIP was a powder that was easy to tamp into AMS target holders that also facilitated high throughput. We concluded that AMS targets of GCIP were a mix of graphitizable carbon and Fe3C crystal, because none of their spectra, FT-IR, Raman, or XRD, matched exactly those of the graphite standard. Nevertheless, AMS targets of GCIP consistently produced the strong, reliable, and reproducible ion currents for high-throughput AMS analysis (270 targets per skilled analyst/day) along with accurate and precise Fm values.

Relation between acid dissolution time in the vacuum test tube and time required for graphitization for AMS target preparation

Availability of an effective graphitization system is essential for the successful operation of an AMS laboratory for radiocarbon measurements. We have set up a graphitization system consisting of metal vacuum lines for cleaning CO2 sample gas which is then converted to graphite. CO2 gas from a carbonate sample is produced in vacuum in a test tube by injecting concentrated phosphoric acid. The tube is placed into a heated metal block to accelerate dissolution. However, we have observed systematic differences in the time required to convert the CO2 gas to graphite under a hydrogen atmosphere, from less than 3h to over 10h. We have conducted a series of experiments including background measurements and yield measurements to monitor secondary carbon contamination and changes in isotopic fractionation. All of the tests show that the carbon isotope ratios remain unaffected by the duration of the process. We also used a quadrupole mass spectrometer (QMS) to identify possible contaminant gases. Contaminant peaks were identified at high mass (larger than 60) only for long duration experiments. This suggests a possible reaction between the rubber cap and acid fumes producing a contaminant gas that impeded the reduction of CO2.

7Be and 10Be concentrations in recent firn and ice at Law Dome, Antarctica

Over the past three years, the Australian National Tandem for Applied Research (ANTARES) AMS facility at ANSTO has been expanding its sample preparation and measurement capability, particularly for 10Be, 26Al and 36Cl. During this time, ANSTO has continued its collaboration with the AAD and CSIRO Atmospheric Research on the measurement of cosmogenic isotopes from Law Dome, Antarctica. This research program has been supported by the construction of a dedicated geochemistry laboratory for the processing of ice and rock samples for the preparation of AMS targets. Here we present our first results for 10Be concentrations measured in ice cores from three sites at Law Dome and describe the sample processing protocol and aspects of the AMS measurement procedure. These sites are characterised by an eightfold difference in accumulation rate with a common precipitation source. In combination with an established ice chronology, this has enabled some preliminary findings concerning the relationship between the snow accumulation rate and the measured 10Be concentration for Law Dome during recent times. Additionally, we present 7Be and 10Be/ 7Be measurements made for a few surface snow samples from Law Dome and Australia.

The chemical behavior of Be, Al, Fe, Ca and Mg during AMS target preparation from terrestrial silicates modeled with chemical speciation calculations

Using the chemical speciation program MINEQL +, we have modeled the chemical speciation of Be and Al during the various steps whereby they are extracted from terrestrial quartz samples for surface exposure dating. The distribution coefficients we used we culled from the literature from reviewed sources (i.e. making our own database) rather than using the constants included in the MINEQL + package. We set-up a formalism whereby we could also apply the program to model the exchange of cations on an exchange resin. With the model we successfully calculated the species distribution of Be and Al in solution in the presence of Cl and F over the whole pH range. Next we investigated the effect of Fe, Ca and Mg on the outcome of the precipitation as well as on the column separation. The presence of such interfering cations is frequently encountered when relatively large quartz mineral separates must be dissolved (i.e. on the order of 50 g). Several points are thus revealed as important during sample preparation: because the presence of F will cause Ca to be precipitated as flourite together with Be- and Al-hydroxide, it is very important that all the SiF4 be fumed off right after total digestion. Once in the sample, Ca cannot be effectively separated from Al using cation exchange. Second, the pH of the first precipitation step is important, values that are too high or too low can cause loss of some of the Be or Al, the latter being more sensitive in this respect. High pH can also cause the inclusion of Mg as brucite. Further, excess cations can cause Be to be eluted off the column earlier than expected. By modeling the chemical speciation of Be and Al, our calculations allow the prediction of the fate and location of Be and Al during sample preparation, as well as revealing the reasons behind such behavior.

AMS sample handling in Groningen

The Groningen AMS target laboratory has a capacity of producing 2000 targets annually. Samples are combusted by a CN analyser/mass spectrometer combination. A 25-fold graphitization setup is employed. We emphasize a combination of automation and quality. Backgrounds of better than 45 ka are routinely achieved. Test measurements on contamination, backgrounds and fractionation are reported.

Study of the 14C-Contamination Potential of C-Impurities in CuO and Fe

The carbon concentration in CuO and iron was determined by isolating C. The values were in agreement with results reported in other studies. Contaminating carbon from CuO and Fe was transformed to AMS targets and measured for 14C. C-traces in CuO were shown to be the major contribution to the 14C sample processing blank. In addition, there is a significant variability in the 14C content of CuO observed between different production batches. The combined contamination potential of CuO and Fe was found to be 4.47–8.92 μg recent carbon, whereas the more realistic estimate for AMS-target preparation conditions ranged between 1.63 and 3.24 μg recent carbon, depending on the 14C level in CuO.

A Simple Technique for Converting CO2 to AMS Target Graphite

I describe a simple, rapid and inexpensive method for converting CO2 samples into AMS target graphite. The technique is applicable for both small- and large-scale production facilities. With some modification, the method is suitable for use with very small samples.

Illumination of a Black Box: Analysis of Gas Composition During Graphite Target Preparation

We conducted a study of relative gas composition changes of CO2, CO and CH4during the formation of graphite targets using different temperatures, catalysts and methods. Reduction with H2increases the reaction rate without compromising the quality of the AMS target produced. Methane is produced at virtually any temperature, and the amount produced is greater at very low temperatures. The reduction of CO to graphite is very slow when H2is not included in the reaction.