*2.1. Chemicals and Reagents*

Nonylphenols (PESTANAL®, analytical standard, technical mixture, purity 96.8%) and 4-n-nonylphenol (PESTANAL®, analytical standard, purity 99.9%) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). 4-n-nonylphenol RING-13C6 100 μg/mL in nonane was purchased from Cambridge Isotope Laboratories (Tewksbury, MA, USA, purity 99.9%). Methylene chloride, acetonitrile, methanol, and hexane were acquired from J.T. Baker (Philipsburg, NJ, USA). Mega Bond Elut-C18 OH (1 g, 6 mL) was purchased from

Agilent (Santa Clara, CA, USA). Visiprep™ SPE Vacuum Manifold and disposable liners for SPE Vacuum Manifold were obtained from Supelco (Saint Louis, MO, USA).

#### *2.2. Sample Selection and Preparation*

The food samples for analysis were selected based on the "National Nutrition Statistics, Food Consumption Distribution" document published by the Korea Health Industry Development Institute (2017). Based on consumption rates and detection levels from previous studies, 79 food categories were chosen. Six major cities in Korea (Seoul, Busan, Incheon, Daegu, Daejeon, and Gwangju) were selected based on the population status data of the local government. Samples were all collected on the same day from marts and markets with high average annual sales in each city. Foods from each category were purchased from 15 different markets nationwide.

Food samples used for monitoring were analyzed in raw conditions, without having received treatments such as cooking or seasoning, and only the edible portions were separated and pretreated. Each sample was treated and homogenized using liquid nitrogen and a blender (Nutri Ninja Auto iQ, Hai Xin Technology), respectively. Each homogenized sample was placed and sealed in a separate HDPE bottle, then stored at –70◦C in a deep freezer until the analysis.
