Improvements in Procedural Blanks at NOSAMS: Reflections of Improvements in Sample Preparation and Accelerator Operation

Abstract

ABSTRACT. 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°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°C for S 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.

Pb isotopic variability in the modern-Pleistocene Indus River system measured by ion microprobe in detrital K-feldspar grains

“…This material was prepared for AMS radiocarbon dating at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) at the Woods Hole Oceanographic Institution, USA. The methodology for AMS radiocarbon dating is presented on the NOSAMS site http://nosams.whoi.edu and discussed in McNichol et al (1995). All radiocarbon dates and their calibrated equivalents, together with the location of the dated samples with each section are presented in Table 2.…”

Direct application of compound-specific radiocarbon analysis of leaf waxes to establish lacustrine sediment chronology

“…This wood fragment was also sent to NOSAMS for 14 C dating by AMS using a more conventional AMS method (McNichol et al 1995;Pearson et al 1998).…”

Radiocarbon Dating the Last Glacial-Interglacial Transition (Ca. 14–9 14C Ka Bp) in Terrestrial and Marine Records: The Need for New Quality Assurance Protocols

“…Although increased precision may be best achieved, perhaps, through "wiggle-matching" (to calibration curves) of a series of 14 C dates (see below), there is, nevertheless, an imperative to improve analytical precision still further through improved sample preparation protocols, use of additional counters to enable longer counting times, more rigorous analysis of variability in laboratory standards, and the development of even purer blanks and targets (e.g. McNichol et al 1995;Schneider et al 1995;Bird et al 1999). …”