Attempts to understand potential deficiencies in chemical procedures for AMS: Dissolving quartz

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 quartz for in-situ-produced ¹⁰Be and ²⁶Al. The goal is to dissolve quartz only, minimising other troublesome elements such as Al, Be, and Ti from other coexisting mineral phases. Obviously, low stable Al guarantees higher ²⁶Al/²⁷Al, thus, better ²⁶Al-statistics, however, it also helps, with low Ti, to have fewer problems in chemistry, thus, also better ¹⁰Be-statistics. For correct calculation of ages and erosion rates, it is also advisable to have “pure quartz”, i.e. similar target elements as the calibration site used for production rates and no natural ⁹Be. One of the earliest established quartz cleaning methods has been routinely used at DREAMS: H₂SiF₆/HCl on a shaker table at room temperature [1]. In batches of seven samples we find up to 1.8% residue of the original “quartz” mass with a mean maximum value of 0.6% for the latest 17 batches from six different projects. Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (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 have treated 3.5 g of these residues by microwave (MW) digestion resulting in dissolving 31% of the original residue. SEM-EDX analyses of the MW-residue showed mainly pristine kyanite and heavily-attacked sillimanite only. Inductively coupled plasma mass spectrometry of the MW-solution validated the dissolution of Al, Ti, Cr, Fe, and Zr. For a typical 50 g “quartz sample” the microwave 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. Ref.: [1] Brown et al., GCA 55 (1991) 2269.