Towards more precise 10Be and 36Cl data from measurements at the 10-14 level: Influence of sample preparation

The spreading application of accelerator mass spectrometry (AMS) to the geosciences will require measurement of increasing numbers of samples with low Be-10/Be-9 and Cl-36/Cl isotopic ratios. To distinguish radionuclide concentrations in samples from corresponding processing blanks, samples must be prepared using stable isotope carriers with low intrinsic radionuclide concentrations. Measurements at different AMS facilities have shown that commercially available Be-9 carriers rarely meet these requirements and use of Be-9 carriers prepared from Be-containing minerals such as Be2SiO4 is advisable. For precise determinations at the low 10(-14) level, samples need to produce the highest Be-9 currents possible. Measurements performed at the two AIMS facilities in France show that mixing BeO with Nb powder generally yields higher Be-9 currents than mixing with Ag powder. Contamination of BeO by other elements such as Ti or Al will reduce the current by a simple dilution effect, thus limiting the statistical precision with which Be-10 can be determined. In the Case Of Cl-36 analyses, repeated water-leaching of calcite samples sufficiently removes all atmospheric Cl-36 contamination, allowing determination of terrestrial cosmogenic in situ produced Cl-36 concentrations for surface exposure dating. (C) 2008 Elsevier B.V. All rights reserved.