A total of 23 hazardous and/or physicochemically diverse acidic substances (see Table for details and physicochemical properties) were purchased from Sigma-Aldrich, Accustandard (New Haven, CT, USA), Supelco (Bellefonte, PA, USA) and Manchester Organics (Runcorn, UK). A Suwannee River Fulvic Acid Standard (SRFAS) was obtained from the International Humic Substances Society (St. Paul, USA). For confirmation of suspect screening candidates, 15 commercially available reference materials were purchased from a variety of suppliers (see Table ).