Attempts to understand potential deficiencies in chemical procedures for accelerator mass spectrometry (AMS)

Attempts to understand potential deficiencies in chemical procedures for accelerator mass spectrometry (AMS)

Merchel, S.; Gurlit, S.; Rugel, G.; Scharf, A.; DREAMS-Users; DREAMS-Friends

Since 2009, the DREAMS (DREsden Accelerator Mass Spectrometry) facility offers users to do their own sample preparation for producing AMS-targets. Several projects are aiming at analysing 10Be, 26Al, and 36Cl (as BeO, Al2O3, AgCl). In cooperation with other AMS-facilities, also actinides (coprecipitation as Fe2O3) and 60Fe (as Fe2O3) are investigated.
Hence, essential steps are hydroxide or AgCl precipitation. For the determination of in-situ or atmospheric 26Al in marine and terrestrial sediments, we had sometimes unaccountable low chemical yields, which might be explained by redissolving Al(OH)3 in the last washings. Thus, we investigated these potential losses by ICP-MS as a function of alteration (waiting) times. Indeed, up to 31% of the precipitated Al was 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 (equally for 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 Fe(OH)3 is washed. Even including the supernatant, total losses of 3-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.
For a single project, we explored the possibility to measure 36Cl and natCl by (isotope dilution) AMS in “dirty” 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 tested preconcentration steps like ion exchange (DOWEX 1x8, 5 ml), which look promising.
Very often DREAMS projects focus at analysing quartz for in-situ-produced 10Be and 26Al. Dissolving quartz only will minimise other troublesome elements such as Al, Be, and Ti from coexisting mineral phases. Obviously, low stable Al leads to higher 26Al/27Al, i.e. better 26Al-statistics. However, low Ti is also helpful for fewer problems in Be-chemistry, i.e. better 10Be-statistics. For correct calculation of exposure ages and erosion rates, “pure quartz”, i.e. similar target elements as the calibration site used for production rates and no natural 9Be, is needed, too. One of the earliest quartz cleaning methods is routinely used at DREAMS: H2SiF6/HCl on a shaker table at room temperature. It produces up to 1.8% residue of the original “quartz” mass with a mean maximum value of 0.6% (values from >100 samples from six different projects). SEM-EDX identified the most prominent minerals to be zircon Zr[SiO4], white sillimanite Al2[O|SiO4], transparent to blue kyanite Al2[O|SiO4], black chromite Fe2+Cr2O4, and orange rutile TiO2. For comparison, we treated 3.5 g of these residues by microwave (MW) digestion resulting in further dissolving 31% of the original residue. SEM-EDX analyses of the MW-residue showed mainly pristine kyanite and heavily-attacked sillimanite only. ICP-MS of the MW-solution validated the dissolution of Al, Ti, Cr, Fe, and Zr. For a typical 50 g-“quartz sample” the MW-method would add more than 3 mg of Ti, over 7 mg of Al and, and, worst of all, about 25 μg of Be to the sample.
Ackn.: Thanks to B. Bookhagen, A. Gärtner, T. Opel, P. Steier, F. Quinto & S. Weiss.

Keywords: AMS; sample preparation; exposure dating; erosion rate

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