A slide containing catechol SOA (before or after heating) was placed in a beaker and rinsed with 3 mL of acetonitrile and 3 mL of nanopure water. Based on a visual inspection, SOA fully dissolved off the slides shortly upon exposure to solvent. The dissolved SOA was then rotary evaporated and redissolved in 0.5 mL acetonitrile and 0.5 mL nanopure water, as 1:1 (v/v) ACN/H2O is the solvent system used for HRMS. Blanks of each clean beaker were run on HRMS and factored into the background signal.