Roberts Mark L.

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Mark L.

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  • Preprint
    Progress with a gas-accepting ion source for Accelerator Mass Spectrometry
    ( 2010-08) Roberts, Mark L. ; von Reden, Karl F. ; McIntyre, Cameron P. ; Burton, Joshua R.
    The National Ocean Sciences AMS (NOSAMS) facility at Woods Hole Oceanographic Institution has developed a novel, gas-accepting microwave-plasma ion-source. The source is a key component of a compact Accelerator Mass Spectrometry (AMS) system built for the analysis of 14C in a continuously flowing gas stream. The gas source produces carbon currents from a stream of CO2 with currents typical of a traditional graphite source. Details of the gas source, including ion current achieved, optimal flow rate, efficiency, and memory are presented. Additionally, data obtained from coupling a gas chromatograph to the source to will be shown.
  • Preprint
    Accelerator mass spectrometry 14C determination in CO2 produced from laser decomposition of aragonite
    ( 2008-08-29) Rosenheim, Brad E. ; Thorrold, Simon R. ; Roberts, Mark L.
    Determination of 14C in aragonite (CaCO3) decomposed thermally to CO2 using an yttrium‐aluminum‐garnet doped neodymium laser is reported. Laser decomposition accelerator mass spectrometer (LD‐AMS) measurements reproduce AMS determinations of 14C from conventional reaction of aragonite with concentrated phosphoric acid. The lack of significant differences between these sets of measurements indicate that LD‐AMS radiocarbon dating can overcome the significant fractionation that has been observed during stable isotope (C and O) laser decomposition analysis of different carbonate minerals. The laser regularly converted nearly 30% of material removed to CO2 despite being optimized for ablation, where laser energy breaks material apart rather than chemically altering it. These results illustrate promise for using laser decomposition on the front‐end of AMS systems that directly measure CO2 gas. The feasibility of such measurements depends on 1. the improvement of material removal and/or CO2 generation efficiency of the laser decomposition system and 2. the ionization efficiency of AMS systems measuring continuously flowing CO2.
  • Preprint
    Rapid, high-resolution C-14 chronology of ooids
    ( 2015-03) Beaupre, Steven R. ; Roberts, Mark L. ; Burton, Joshua R. ; Summons, Roger E.
    Ooids are small, spherical to ellipsoidal grains composed of concentric layers of CaCO3 that could potentially serve as biogeochemical records of the environments in which they grew. Such records, however, must be placed in the proper temporal context. Therefore, we developed a novel acidification system and employed an accelerator mass spectrometer (AMS) with a gas accepting ion source to obtain radiocarbon (14C) chronologies extending radially through ooids within one 8-hour workday. The method was applied to ooids from Highborne Cay, Bahamas and Shark Bay, Australia, yielding reproducible 14C chronologies, as well as constraints on the rates and durations of ooid growth and independent estimates of local 14C reservoir ages.
  • Article
    Single Step Production of Graphite from Organic Samples [Poster]
    ( 2017) Elder, Kathryn L. ; Roberts, Mark L. ; Lardie Gaylord, Mary C.
    We present a low-cost, high-throughput method for converting many types of organic samples into graphite. The method combines sample combustion and graphitization in a single process. Using a modified sealed graphitization method, samples are placed in a Pyrex tube containing zinc, titanium hydride and iron catalyst. The tube is evacuated, flamed sealed, and placed in a muffle furnace for 7 hours. Graphite forms on the iron and is then analyzed for 14C content using either of NOSAMS’s two AMS systems. This method has been shown to work on a variety of organic samples including pure compounds, wood, peat, collagen and humics. This simplified procedure could be especially useful in reconnaissance studies in which it is desired to rapidly measure a large number of samples (untreated or pretreated), at low-cost with analytical precision and accuracy approaching that of traditional hydrogen reduction methods.
  • Preprint
    Design and reality : continuous-flow accelerator mass spectrometry (CFAMS)
    ( 2010-09) von Reden, Karl F. ; Roberts, Mark L. ; McIntyre, Cameron P. ; Burton, Joshua R.
    In 2007 we published the design of a novel accelerator mass spectrometry (AMS) system capable of analyzing gaseous samples injected continuously into a microwave plasma gas ion source. Obvious advantages of such a system are drastically reduced processing times and avoidance of potentially contaminating chemical preparation steps. Another paper in these proceedings will present the progress with the development of the microwave gas ion source that has since been built and tested at the National Ocean Sciences AMS Facility in Woods Hole. In this paper we will review the original design and present updates, reflecting our recent encouraging experience with the system. A simple summary: large acceptance ion beam optics design is beneficial to accelerator mass spectrometry in general, but essential to AMS with plasma gas ion sources.
  • Article
    Fire history of a giant African baobab evinced by radiocarbon dating
    (Dept. of Geosciences, University of Arizona, 2010-08) Patrut, Adrian ; Mayne, Diana H. ; von Reden, Karl F. ; Lowy, Daniel A. ; Van Pelt, Robert ; McNichol, Ann P. ; Roberts, Mark L. ; Margineanu, Dragos
    The article reports the first radiocarbon dating of a live African baobab (Adansonia digitata L.), by investigating wood samples collected from 2 inner cavities of the very large 2-stemmed Platland tree of South Africa. Some 16 segments extracted from determined positions of the samples, which correspond to a depth of up to 15–20 cm in the wood, were processed and analyzed by accelerator mass spectrometry (AMS). Calibrated ages of segments are not correlated with their positions in the stems of the tree. Dating results indicate that the segments originate from new growth layers, with a thickness of several centimeters, which cover the original old wood. Four new growth layers were dated before the reference year AD 1950 and 2 layers were dated post-AD 1950, in the post-bomb period. Formation of these layers was triggered by major damage inside the cavities. Fire episodes are the only possible explanation for such successive major wounds over large areas or over the entire area of the inner cavities of the Platland tree, able to trigger regrowth.
  • Preprint
    Optimizing a microwave gas ion source for continuous-flow accelerator mass spectrometry
    ( 2011-09) von Reden, Karl F. ; Roberts, Mark L. ; Burton, Joshua R. ; Beaupre, Steven R.
    A 2.45 GHz microwave ion source coupled with a magnesium charge exchange canal (CxC) has been successfully adapted to a large acceptance radiocarbon accelerator mass spectrometry system at the National Ocean Sciences AMS Facility (NOSAMS), Woods Hole Oceanographic Institution. CO2 samples from various preparation sources are injected into the source through a glass capillary at 370 µl/min. Routine system parameters are about 120 - 140 µA of negative 12C current after the CxC, leading to about 400 14C counts per second for a modern sample and implying a system efficiency of 0.2%. While these parameters already allow us to perform high quality AMS analyses on large samples, we are working on ways to improve the output of the ion source regarding emittance and efficiency. Modeling calculations suggest modifications in the extraction triode geometry, shape and size of the plasma chamber could improve emittance and hence ion transport efficiency. Results of experimental tests of these modifications are presented.
  • Article
    Movement of deep-sea coral populations on climatic timescales
    (John Wiley & Sons, 2013-05-30) Thiagarajan, Nivedita ; Gerlach, Dana ; Roberts, Mark L. ; Burke, Andrea ; McNichol, Ann P. ; Jenkins, William J. ; Subhas, Adam V. ; Thresher, Ronald E. ; Adkins, Jess F.
    During the past 40,000 years, global climate has moved into and out of a full glacial period, with the deglaciation marked by several millennial-scale rapid climate change events. Here we investigate the ecological response of deep-sea coral communities to both glaciation and these rapid climate change events. We find that the deep-sea coral populations of Desmophyllum dianthus in both the North Atlantic and the Tasmanian seamounts expand at times of rapid climate change. However, during the more stable Last Glacial Maximum, the coral population globally retreats to a more restricted depth range. Holocene populations show regional patterns that provide some insight into what causes these dramatic changes in population structure. The most important factors are likely responses to climatically driven changes in productivity, [O2] and [CO32–].
  • Preprint
    Carbonate as sputter target material for rapid 14C AMS
    ( 2012-04-17) Longworth, Brett E. ; Robinson, Laura F. ; Roberts, Mark L. ; Beaupre, Steven R. ; Burke, Andrea ; Jenkins, William J.
    This paper describes a technique for measuring the 14C content of carbonate samples by producing C-ions directly in the negative ion sputter source of an accelerator mass spectrometer (AMS) system. This direct analysis of carbonate material eliminates the time and expense of graphite preparation. Powdered carbonate is mixed with titanium powder, loaded into a target cartridge, and compressed. Beam currents for optimally sized carbonate targets (0.09-0.15 mg C) are typically 10-20% of those produced by optimally-sized graphite targets (0.5-1 mg C). Modern (>0.8 Fm) samples run by this method have standard deviations of 0.009 Fm or less, and near-modern samples run as unknowns agree with values from traditional hydrolysis/graphite to better than 2%. Targets with as little as 0.06 mg carbonate produce useable ion currents and results, albeit with increased error and larger blank. In its current state, direct sputtering is best applied to problems where a large number of analyses with lower precision are required. These applications could include age surveys of deep-sea corals for determination of historic population dynamics, to identify samples that would benefit from high precision analysis, and for growth rate studies of organisms forming carbonate skeletons.
  • Article
    A preliminary determination of the absolute 14C/12C ratio of OX-I
    (Dept. of Geosciences, University of Arizona, 2007-11) Roberts, Mark L. ; Southon, John R.
    A preliminary determination of the absolute 14C/12C ratio of the oxalic acid I standard (NBS SRM 4990B) has been made. Using an accelerator mass spectrometry (AMS) system, a known number of radiocarbon ions were implanted in a thin copper foil. The foil was then combusted with the addition of 14C-free carrier material. This resulting gas (which had a known 14C/12C ratio) was graphitized and compared to OX-I material. This comparison yielded an absolute 14C/12C ratio of OX-I. The absolute 14C/12C ratio of OX-I, coupled with knowledge of the specific activity of OX-1, provides an alternative determination of the 14C half-life.
  • Preprint
    Software development for continuous-gas-flow AMS
    ( 2007-09) von Reden, Karl F. ; Roberts, Mark L. ; Jenkins, William J. ; Rosenheim, Brad E. ; McNichol, Ann P. ; Schneider, Robert J.
    The National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility at Woods Hole Oceanographic Institution is presently completing installation of a novel continuous-flow AMS system. A multi-year development of an AMS microwave gas ion source in collaboration with Atomic Energy Canada Limited (AECL), Chalk River, has preceded this final step of an implementation that is expected to add a new dimension to 14C AMS. National Instruments, NIM, and CAMAC modules have been programmed with LabVIEW on a Windows XP platform to form the basis for data acquisition. In this paper we discuss possible applications and include simulations of expected data acquisition scenarios like real-time AMS analysis of chromatograms. Particular attention is given to issues of synchronization between rapidly changing input amplitudes and signal processing cycles in hardware and software.
  • Article
    A continuous-flow gas chromatography 14C accelerator mass spectrometry system
    (Dept. of Geosciences, University of Arizona, 2010-08) McIntyre, Cameron P. ; Galutschek, Ernst ; Roberts, Mark L. ; von Reden, Karl F. ; McNichol, Ann P. ; Jenkins, William J.
    Gas-accepting ion sources for radiocarbon accelerator mass spectrometry (AMS) have permitted the direct analysis of CO2 gas, eliminating the need to graphitize samples. As a result, a variety of analytical instruments can be interfaced to an AMS system, processing time is decreased, and smaller samples can be analyzed (albeit with lower precision). We have coupled a gas chromatograph to a compact 14C AMS system fitted with a microwave ion source for real-time compoundspecific 14C analysis. As an initial test of the system, we have analyzed a sample of fatty acid methyl esters and biodiesel. Peak shape and memory was better then existing systems fitted with a hybrid ion source while precision was comparable. 14C/12C ratios of individual components at natural abundance levels were consistent with those determined by conventional methods. Continuing refinements to the ion source are expected to improve the performance and scope of the instrument.
  • Preprint
    Progress on a gas-accepting ion source for continuous-flow accelerator mass spectrometry
    ( 2007-02) Roberts, Mark L. ; Schneider, Robert J. ; von Reden, Karl F. ; Wills, J. S. C. ; Han, Baoxi ; Hayes, John M. ; Rosenheim, Brad E. ; Jenkins, William J.
    A gas-accepting microwave-plasma ion source is being developed for continuous-flow Accelerator Mass Spectrometry (AMS). Characteristics of the ion source will be presented. Schemes for connecting a gas or liquid chromatograph to the ion source will also be discussed.
  • Article
    Radiocarbon in dissolved organic carbon by UV oxidation: an update of procedures and blank characterization at NOSAMS
    (Cambridge University Press, 2022-02-11) Xu, Li ; Roberts, Mark L. ; Elder, Kathryn L. ; Hansman, Roberta L. ; Gagnon, Alan R. ; Kurz, Mark D.
    This note describes improvements of UV oxidation method that is used to measure carbon isotopes of dissolved organic carbon (DOC) at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS). The procedural blank is reduced to 2.6 ± 0.6 μg C, with Fm of 0.42 ± 0.10 and δ13C of –28.43 ± 1.19‰. The throughput is improved from one sample per day to two samples per day.
  • Article
    Advances in Sample Preparation at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS): Investigation of Carbonate Secondary Standards
    ( 2017) Cruz, Anne J. ; Childress, Laurel B. ; Gagnon, Alan R. ; McNichol, Ann P. ; Burton, Joshua R. ; Elder, Kathryn L. ; Lardie Gaylord, Mary C. ; Gospodinova, Kalina D. ; Hlavenka, Joshua ; Kurz, Mark D. ; Longworth, Brett E. ; Roberts, Mark L. ; Trowbridge, Nan Y. ; Walther, Tess ; Xu, Li
    The development of robust sample preparation techniques for ocean science research has been a hallmark of NOSAMS since its inception. Improvements to our standard methods include reducing the minimum size of the samples we can analyze, building modular graphite reactors of different sizes that we can swap in and out depending on our sample stream, and modifying our carbonate acidification methods to improve handling of the smaller samples we now receive. A relatively new instrument, the Ramped PyrOx, which allows the separation of organic matter into thermal fractions, has attracted much interest as a research and development tool. We will also discuss our progress on incorporating a Picarro isotope analyzer into our sample preparation options.
  • Article
    Age and growth rate dynamics of an old African baobab determined by radiocarbon dating
    (Dept. of Geosciences, University of Arizona, 2010-08) Patrut, Adrian ; Mayne, Diana H. ; von Reden, Karl F. ; Lowy, Daniel A. ; Venter, Sarah ; McNichol, Ann P. ; Roberts, Mark L. ; Margineanu, Dragos
    In 2008, a large African baobab (Adansonia digitata L.) from Makulu Makete, South Africa, split vertically into 2 sections, revealing a large enclosed cavity. Several wood samples collected from the cavity were processed and radiocarbon dated by accelerator mass spectrometry (AMS) for determining the age and growth rate dynamics of the tree. The 14C date of the oldest sample was found to be of 1016 ± 22 BP, which corresponds to a calibrated age of 1000 ± 15 yr. Thus, the Makulu Makete tree, which eventually collapsed to the ground and died, becomes the second oldest African baobab dated accurately to at least 1000 yr. The conventional growth rate of the trunk, estimated by the radial increase, declined gradually over its life cycle. However, the growth rate expressed more adequately by the cross-sectional area increase and by the volume increase accelerated up to the age of 650 yr and remained almost constant over the past 450 yr.
  • Article
    Ultra-small graphitization reactors for ultra-microscale 14C analysis at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility
    (University of Arizona Libraries, 2015) Shah Walter, Sunita R. ; Gagnon, Alan R. ; Roberts, Mark L. ; McNichol, Ann P. ; Lardie Gaylord, Mary C. ; Klein, Elizabeth
    In response to the increasing demand for 14C analysis of samples containing less than 25 µg C, ultra-small graphitization reactors with an internal volume of ~0.8 mL were developed at NOSAMS. For samples containing 6 to 25 µg C, these reactors convert CO2 to graphitic carbon in approximately 30 min. Although we continue to refine reaction conditions to improve yield, the reactors produce graphite targets that are successfully measured by AMS. Graphite targets produced with the ultra-small reactors are measured by using the Cs sputter source on the CFAMS instrument at NOSAMS where beam current was proportional to sample mass. We investigated the contribution of blank carbon from the ultra-small reactors and estimate it to be 0.3 ± 0.1 µg C with an Fm value of 0.43 ± 0.3. We also describe equations for blank correction and propagation of error associated with this correction. With a few exceptions for samples in the range of 6 to 7 µg C, we show that corrected Fm values agree with expected Fm values within uncertainty for samples containing 6–100 µg C.
  • Preprint
    Comparative AMS radiocarbon dating of pretreated versus non-pretreated tropical wood samples
    ( 2009-10) Patrut, Adrian ; von Reden, Karl F. ; Lowy, Daniel A. ; Mayne, Diana H. ; Elder, Kathryn L. ; Roberts, Mark L. ; McNichol, Ann P.
    Several wood samples collected from Dorslandboom, a large African baobab (Adansonia digitata L.) from Namibia, were investigated by AMS radiocarbon dating subsequent to pretreatment and, alternatively, without pretreatment. The comparative statistical evaluation of results showed that there were no significant differences between fraction modern values and radiocarbon dates of the samples analyzed after pretreatment and without pretreatment, respectively. The radiocarbon date of the oldest sample was 993 ± 20 BP. Dating results also revealed that Dorslandboom is a multi-generation tree, with several stems showing different ages.
  • Article
    A high-performance 14C accelerator mass spectrometry system
    (Dept. of Geosciences, University of Arizona, 2010-08) Roberts, Mark L. ; Burton, Joshua R. ; Elder, Kathryn L. ; Longworth, Brett E. ; McIntyre, Cameron P. ; von Reden, Karl F. ; Han, Baoxi ; Rosenheim, Brad E. ; Jenkins, William J. ; Galutschek, Ernst ; McNichol, Ann P.
    A new and unique radiocarbon accelerator mass spectrometry (AMS) facility has been constructed at the Woods Hole Oceanographic Institution. The defining characteristic of the new system is its large-gap optical elements that provide a larger-than-standard beam acceptance. Such a system is ideally suited for high-throughput, high-precision measurements of 14C. Details and performance of the new system are presented.
  • Article
    C-14 Blank Corrections for 25-100 mu G samples at the National Ocean Sciences AMS Laboratory
    (Cambridge University Press, 2019-07-22) Roberts, Mark L. ; Elder, Kathryn L. ; Jenkins, William J. ; Gagnon, Alan R. ; Xu, Li ; Hlavenka, Joshua ; Longworth, Brett E.
    Replicate radiocarbon (14C) measurements of organic and inorganic control samples, with known Fraction Modern values in the range Fm = 0–1.5 and mass range 6 μg–2 mg carbon, are used to determine both the mass and radiocarbon content of the blank carbon introduced during sample processing and measurement in our laboratory. These data are used to model, separately for organic and inorganic samples, the blank contribution and subsequently “blank correct” measured unknowns in the mass range 25–100 μg. Data, formulas, and an assessment of the precision and accuracy of the blank correction are presented.