*2.4. Preparation of Standards*

A stock solution of 4-n-NP (1.00 × 103 <sup>μ</sup>g/mL) was prepared by dissolving 0.01 g of 4-n-NP using 10.0 mL of methanol. A working solution (100 μg/mL) was prepared with a methanol dilution 10 times higher than the stock solution.

An internal standard 4-n-NP RING-13C6 (10.0 μg/mL in methanol) was prepared by diluting 1.00 × <sup>10</sup><sup>2</sup> <sup>μ</sup>g/mL in nonane using methanol. Regarding the standard curve of the low-fat matrix, standard solutions of 4-n-NP were spiked at concentrations of 1.00 × 101, 2.00 × 101, 1.00 × 102, 5.00 × 102, and 1.00 × 103 ng/mL; for the standard curve of the high-fat matrix, standard solutions of 4-n-NP were spiked at concentrations of 1.00 × 102, 2.00 × <sup>10</sup>2, 5.00 × 102, 1.00 × <sup>10</sup>3, and 2.00 × <sup>10</sup><sup>3</sup> ng/mL.
