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Attempts to understand potential deficiencies in chemical procedures for AMS: Hydroxides and silver chloride
Merchel, S.; Gurlit, S.; Opel, T.; Rugel, G.; Scharf, A.; Wetterich, S.
Since 2009, the DREAMS (DREsden Accelerator Mass Spectrometry) facility offers users to do their own sample preparation for AMS-targets. Several projects aimed at analysing 10Be, 26Al, and 36Cl as BeO, Al2O3, and AgCl, respectively. In cooperation with other AMS-facilities, also actinides (in Fe2O3) and 60Fe from Fe2O3 are measured. Thus, one of the essential steps for many projects is a hydroxide or silver chloride precipitation. For the determination of in-situ or atmospheric 26Al in marine and terrestrial sediments, we had sometimes unaccountable low chemical yields, which seemed to be due to redissolving aluminium hydroxide in the last washings. Hence, we investigated these potential losses by inductively coupled plasma mass spectrometry (ICP-MS) as a function of alteration (waiting) times. Indeed, up to 31% of the precipitated Al could be redissolved by immediate triple washings. After 2 h of waiting, this could be reduced to 11%. Further waiting (over-night) resulted in losses of 6% of Al (and equally Be) only. We also tested the behaviour of Fe(III), U(VI) (also as analogue for ~Pu(VI) and Np(VI)), and Er(III) (as analogue for Am(III), Cm(III), Pu(III)) when iron hydroxides are washed. Even including the supernatant, total losses of three times washing of over-night altered hydroxides are as low as 2.6-3.5%. Thus, repeated washing cycles are very advisable to reduce ions such as NH4
+ and Cl- before drying and ignition. As we were facing problems with 10Be contamination in “dirty” ground ice , we measured 36Cl and natCl by isotope dilution in permafrost ice wedge samples as heavy as 1.6 kg. The chemical yield of AgCl was only 20-35% and is a function of total natCl. Thus, we explored preconcentration steps such as ion exchange (DOWEX 1x8, 5 ml), which look promising. Ackn.: Thanks to P. Steier, F. Quinto and S. Weiss. Ref.:  Merchel et al., AMS-13.
Keywords: AMS; in-situ; actinides
14th International Conference on Accelerator Mass Spectrometry (AMS-14), 14.-18.08.2017, Ottawa, Canada