Mixed mode solid phase extraction cartridges

AFB₁-lysine adduct standard was synthesized and purified as previously described by Sabbioni et al. [39 (link)]. Albumin determination reagent (bromocreosol purple), and normal human serum were purchased from Sigma Aldrich Chemical Co. (St. Louis, MO). Pronase (25 kU, Nuclease-free) was purchased from Calbiochem (La Jolla, CA). Protein assay dye reagent concentrate and protein standards were purchased from Bio-Rad Laboratories Inc. (Hercules, CA). Boric acid, o-phthaldialdehyde (OPA), 2-mercaptoethanol, FB₁ from F. verticilioides (~ 98% purity, TLC), 10× phosphate buffered saline (PBS), ammonium hydroxide, ammonium acetate, sodium chloride, sodium phosphate monobasic, hydrochloric acid, and formic acid were purchased from Sigma-Aldrich (St. Louis, MO, USA). OPA reagents were prepared by dissolving 10 mg of OPA and 30 μl of 2-mercaptoethanol in 250 μl of methanol and mixing with 4.75 ml of 3% Boric acid buffer (pH 10.5) and stored at 4 °C avoiding light before use. Mixed mode solid phase extraction (SPE) cartridges, as well as Sep-Pak reversed phase C18 cartridges were purchased from the Waters Corp. (Milford, MA). All other chemicals and solvents were of highest grade and purity available.