Improvements in Procedural Blanks at Nosams

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, CaCO 3 and organic carbon—to CO 2 prior to preparing graphite for the accelerator and have distinct procedural blanks for each procedure. Dissolved inorganic carbon (∑CO 2 ) is extracted from acidified seawater samples by sparging with a nitrogen carrier gas. We routinely analyze “line blanks” by processing CO 2 from a 14 C-dead source through the entire stripping procedure. Our hydrolysis blank, IAEA C-1, is prepared by acidifying in vacuo with 100% H 3 PO 4 at 60° 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° 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

Accelerator mass spectrometry · Blank · Carbon fibers · Chromatography · Graphite · Mass spectrometry · Metallurgy · Radiochemistry · Sample Preparation · Seawater · Archaeology and ancient environmental studies · Chemistry · Geology and Paleoclimatology Research · Isotope Analysis in Ecology · Materials Science · Geology

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