Microwave digestion of hair samples

All hair samples were digested in 6 mL PTFE vessels with a mixture of 2 mL concentrated HNO₃ and 0.25 mL concentrated H₂O₂, using Milestone ETHOS EZ microwave acid-assistant digestion system (Milestone, Inc., Shelton, CT, USA) following the method described in Tipple et al. [15]. The digestion system was heated to 200°C at a rate of 13.3°C/min and then left at 200°C for 15 minutes. After cooling down, a 100 μL aliquot of the total ∼2 mL digest from each sample was transferred to a 15 mL tube and brought up to 10 mL with Milli-Q water for Sr concentration analysis. All dilutions were done gravimetrically. The remaining solutions were transferred into 7 mL acid-leached perfluoroalkoxy alkanes (PFA) round-interior vials (Savillex Corporation, Minnetonka, MN, USA) for Sr purification and isotopic analysis.