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

      McNichol, A. P.; Gagnon, A. R.; Jones, G. A.; Osborne, E. A. (Department of Geosciences, The University of Arizona, 1992-01-01)
      We conducted a study of relative gas composition changes of CO2, CO and CH4 during the formation of graphite targets using different temperatures, catalysts and methods. Reduction with H2 increases 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 H2 is not included in the reaction.
    • Improvements in Procedural Blanks at NOSAMS: Reflections of Improvements in Sample Preparation and Accelerator Operation

      McNichol, A. P.; Gagnon, A. R.; Osborne, E. A.; Hutton, D. L.; von Reden, K. F.; Schneid, R. J. (Department of Geosciences, The University of Arizona, 1995-01-01)
      During the four years the Sample Preparation Laboratory (SPL) at the National Ocean Sciences Accelerator Mass Spectrometer (NOSAMS) Facilty has been in operation we have accumulated much data from which we can assess our progress. We evaluate our procedural blanks here and describe modifications in our procedures that have improved our analyses of older samples. In the SPL, we convert three distinct types of samples—seawater, CaCO3 and organic carbon—to CO2 prior to preparing graphite for the accelerator and have distinct procedural blanks for each procedure. Dissolved inorganic carbon (ECO2) is extracted from acidified seawater samples by sparging with a nitrogen carrier gas. We routinely analyze "line blanks" by processing CO2 from a 14C-dead source through the entire stripping procedure. Our hydrolysis blank, IAEA C-1, is prepared by acidifying in vacuo with 100% H3PO4 at 60 degrees C overnight, identical to our sample preparation. We use a dead graphite, NBS-21, or a commercially available carbon powder for our organic combustion blank; organic samples are combusted at 850 degrees C for 5 h using CuO to provide the oxidant. Analysis of our water stripping data suggests that one step in the procedure contributes the major portion of the line blank. At present, the contribution from the line blank has no effect on our seawater analyses (fraction modern (fm) between 0.7 and 1.2). Our hydrolysis blanks can have an fm value as low as 0.0006, but are more routinely between 0.0020 and 0.0025. The fm of our best organic combustion blanks is higher than those routinely achieved in other laboratories and we are currently altering our methods to reduce it.
    • The Rapid Preparation of Seawater Sigma-CO2 for Radiocarbon Analysis at the National Ocean Sciences AMS Facility

      McNichol, A. P.; Jones, G. A.; Hutton, D. L.; Gagnon, A. R.; Key, R. M. (Department of Geosciences, The University of Arizona, 1994-01-01)
      We have established a laboratory for extracting Sigma-CO2 from seawater samples for AMS analysis of the radiocarbon content. The seawater samples are collected at sea, poisoned and stored until analysis in the laboratory. Each sample is acidified; the inorganic carbon is stripped out as CO2 with an inert carrier gas and then converted to graphite. We present results for Buzzards Bay surface H2O and Na2CO3 standards that demonstrate we strip >98% of inorganic carbon from seawater. Stable isotope analyses are performed to better than 0.2 per mil, and the reproducibility of 14C measurements on Buzzards Bay seawater is better than 13 per mil. Finally, we compare data from samples collected in 1991 to those collected in the 1970s and to large volume samples.