*3.2. Preparation of Aflatoxin Standards*

Standard solutions of each of the four representative AFs AFB1, AFB2, AFG1, and AFG2 (Sigma) were prepared in acetonitrile at a final concentration of 10 µg/mL (part per million; ppm) according to the Association of Official Analytical Chemists (AOAC) method [30]. To prepare 10 µg/mL individual AF stock standard solutions, 10 mg of each AF was weighed into a separate 100 mL volumetric flask. Acetonitrile (50 mL) was added to each flask, mixed, and further added to the mark and mixed again. Then, 10 mL of this solution was transferred into another 100 mL volumetric flask and diluted to the mark with acetonitrile. Working solutions (individual or mixture) were prepared in acetonitrile and stored at −20 ◦C in amber glass vials for the study period (up to 3 months). The AF standard solutions used for the HPLC calibration curve were prepared by further dilution of the working solutions with the mobile phase.
