Birch bark was collected from a fallen tree in the forest of Norra Djurgården, Stockholm, Sweden. The bark was manually separated into inner and outer bark. The outer bark was dried at 70 °C for 48 h and milled to a 40-mesh using a Wiley Mill (3383 L10, Thomas Scientific, USA). The dried outer bark (DOB) was stored at room temperature in a sealed container. Ethanol (96 %) was purchased from VWR Chemicals. Deuterated chloroform (99.8 %) was purchased from Sigma-Aldrich. Heptane (>99.5 %), hydrochloric acid (37 %), and sodium hydroxide (NaOH) pellets were purchased from Fisher Chemicals. A clear PET film with a thickness of 452 ± 4 μm was cut from an electronic device's packaging.