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 ¹⁰Be/⁹Be and ³⁶Cl/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 ⁹Be carriers rarely meet these requirements and use of ⁹Be 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 ⁹Be currents possible. Measurements performed at the two AMS facilities in France show that mixing BeO with Nb powder generally yields higher ⁹Be 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 ¹⁰Be can be determined. In the case of ³⁶Cl analyses, repeated water-leaching of calcite samples sufficiently removes all atmospheric ³⁶Cl contamination, allowing determination of terrestrial cosmogenic in situ produced ³⁶Cl concentrations for surface exposure dating.