Nh4 2so4 salts

All consumables were double acid washed (10 and 1 % HNO₃m/m prepared from concentrated HNO₃ (p.a., Merck, Darmstadt, Germany), diluted with laboratory water type I (0.055 μS cm⁻¹; TKA-GenPure, Niederelbert, Germany), and rinsed with laboratory water type I. Laboratory water type I and nitric acid were further purified by using a sub-boiling distillation system (Milestone Inc., Shelton, CT, USA) and were used for dilution of standards and preparation of reagents. (NH₄)₂SO₄ salts (AnalaR, VWR, Leuven, Belgium, further named as “V”; p.a., Merck, further named as “M”) were used for method development and optimization of method parameters (e.g., anion exchange time, tuning of instruments). NaHCO₃ was used for regeneration of anion exchange membranes. Isotope certified reference materials (CRMs) IAEA-S-1, silver sulfide and IAEA-S-2, silver sulfide (both IAEA, Vienna, Austria) were used for calibration and validation of the MC ICP-MS measurement. The solid CRMs were dissolved by microwave-assisted acid digestion (Multiwave 3000, Anton-Paar, Graz, Austria): 6 mL sub-boiled HNO₃ was added to 75 mg of a CRM. The digested material was diluted with sub-boiled water to obtain a 3.1 mmol L⁻¹ S stock solution.