**1. Introduction**

Nonylphenols (NPs) are persistent and hazardous endocrine-disrupting chemicals (EDCs), which are found in a variety of environmental matrices, including foods, human tissue samples, and blood [1]. EDCs can disrupt hormone biosynthesis, metabolism, or function by causing a disruption in normal homeostasis or reproduction [2]. These compounds interfere with the endocrine system by imitating or suppressing endogenous hormones, resulting in hormonal dysfunction and deleterious effects on living organisms. By competing with natural estrogen for estrogen receptor bindings, NP especially hinders the estrogenic balance in organisms [3]. NPs are a complex mixture of compounds composed of isomers with differently branched carbon side and linear chains, and there can, theoretically, be more than 200 constitutional isomers [4]. Linear 4-n-nonylphenol (4-n-NP) is the least active of the individual isomers, and its relative potency is known to increase with higher levels of nonyl-branching side chains [5]. Different degrading behaviors in these isomer forms of NP have been observed in soil and aquatic environments, with linear NPs degrading more rapidly than branched NPs [6].

**Citation:** Lee, S.M.; Cheong, D.; Kim, M.; Kim, Y.-S. Analysis of Endocrine Disrupting Nonylphenols in Foods by Gas Chromatography-Mass Spectrometry. *Foods* **2023**, *12*, 269. https://doi.org/10.3390/ foods12020269

Academic Editors: Dapeng Peng and Yongzhong Qian

Received: 7 December 2022 Revised: 30 December 2022 Accepted: 3 January 2023 Published: 6 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Nonylphenol polyethoxylates (NPEOs) are nonionic surfactants that are widely used for detergent, pesticide, and emulsifier production. They are biodegraded into shorter chain compounds and NPs, which do not have ethoxyl groups, under certain conditions [7]. Effluents from sewage treatment plants, industrial discharges, and municipal waste streams are the main sources of these compounds in aquatic ecosystems. During sewage treatment, NPEOs are biodegraded by hydrolytic cleavage of ethoxylate groups into short-chain ethoxylates, and finally to alkylphenols, resulting in the formation of progressively more lipophilic and persistent metabolites. De-ethoxylated NP is more resistant to biodegradation than compounds with long-chain ethoxylates. NPs and NPEOs are persistent in aquatic environments, moderately bioaccumulative, and severely toxic to aquatic organisms, which means that their commercial manufacturing, processing, and distribution pose potential ecological threats.

Humans are exposed to NPs primarily through the intake of contaminated food and water. NPs are particularly known to be present in fish and shellfish due to their exposure to aquatic environments. NPs can also be introduced to soil used for producing crops and grazing livestock when sewage sludge is applied to agricultural land. Through these diverse pathways, NPs remain in the environment, enter the food chain, and finally accumulate in lipid-rich matrices including fish and edible oil [8]. NPs can accumulate in the internal organs of fish, reaching concentrations 10 to 100 times higher than those observed in the environment, due to their strong stability and solubility in lipids. They can then be readily transferred to humans through the food chain. Notably, NP bioaccumulation has been reported in humans, with accumulation values of 57 and 37 ng/g in the adipose tissue of humans and cadaver samples, respectively [9]. NPs were also found in every urine sample examined in a recent investigation conducted in Taiwan, with concentrations ranging from 0.65 to 6.69 ng/mL, demonstrating the human exposure to these types of EDCs [10].

Because of their low cost and high availability, traditional sample treatment techniques with long analysis times and waste production have been used to determine alkylphenols in samples. Conventional procedures, such as Soxhlet extraction, require analysis times from 10 to 48 h [11]. Alkylphenols have also been extracted from food and environmental samples using steam distillation. Despite steam distillation being known to be more simple than Soxhlet extraction, it is also laborious and time-consuming. To develop a simple and efficient method for analyzing NPs, particularly in high-fat food samples, solid-phase extraction (SPE) was combined with liquid–liquid extraction to reduce the isolation time for analytes. Due to its versatility, SPE has been widely used to separate and enrich NPEOs and their degradation products, NPs, from food and environmental matrices. Many SPE procedures have been devised using various sorbents with many variables, such as SPE sorbent and eluent types, potentially influencing the recovery of target compounds [12].

Since there was no literature on monitoring NP contents in foods from Korean markets, it was necessary to determine the concentrations of NPs in various food groups based on national consumption. Previous studies on NPs in foods are also scarce, although one of the major exposure routes to humans was oral food ingestion. The objective of the present study was to develop, optimize, and validate a simple, efficient, and sensitive integrated analysis method to determine NPs in high-fat and low-fat matrices. The established method was further applied to analyze the concentrations of NPs in 1185 food samples categorized as aquatic, livestock, and agricultural products.
