LAUSR

Both the structures and toxicities of polychlorinated naphthalenes (PCNs) are similar to those of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), which are persistent, bioaccumulative chemical compounds. PCNs were also listed as persistent organic pollutants (POPs) under Annexes A and C of the Stockholm Convention in 2015 (POPRC, 2017), and are no longer produced commercially. However, these compounds can still be generated along with PCDD/Fs as undesired byproducts during various high-temperature processes (Liu, Cai, & Zheng, 2014; Weidemann & Lundin, 2015; Zhao et al., 2017). There have been several studies of the mechanisms by which PCNs are formed during high-temperature combustion. One possibility is de novo synthesis from polycyclic aromatic hydrocarbons (PAHs) in the presence of copper compounds such as CuCl, CuCl2 or Cu(OH)2 (which act as catalysts) at temperatures such as 300 °C (Weber et al., 2001) or 400 °C (Iino, Imagawa, Takeuchi, & Sadakata, 1999). Kim et al. also reported the formation of PCNs via the ortho-ortho carbon coupling of phenoxy radicals during chlorophenol combustion at 550–750 °C, and demonstrated that the degree of chlorination of naphthalene declines with increasing temperature (Kim & Mulholland, 2005). PCNs can also be produced under relatively mild conditions. As an example, the chlorination reaction that is considered an important step in the formation of PCNs can take place at 200 °C (Jansson, Fick, & Marklund, 2008). The β-positions of naphthalene are more likely to be substituted by chlorine than the α-positions, possibly because of the relatively high stability of PCN congeners with β-position chlorines (Jansson, et al., 2008; Liu, Lv, Jiang, Nie, & Zheng, 2014; Zhai & Wang, 2005). However, Ryu et al. found that α-positions were favored over β-positions in PCNs produced by the CuCl2-catalyzed chlorination of naphthalene, suggesting that naphthalene chlorination is not the primary pathway for PCN formation during combustion (Ryu, Kim, & Jang, 2013). PCNs may also result from the application of high temperatures during cooking. A previous study by our group demonstrated the emission of PCNs from cooking oil containing sucralose at 245 °C (Dong, Liu, Zhang, Gao, & Zheng, 2013). Sucralose is an artificial sweetener widely used as a food additive, and dry sucralose has been found to be unstable at high temperatures because it can undergo chlorination reactions, generating potentially toxic compounds. The degradation of sucralose can proceed under relatively mild conditions (such as at 119 °C) in conjunction with the loss of both H2O and HCl (Bannach, Almeida, Lacerda, Schnitzler, & Ionashiro, 2009). The thermal decomposition of sucralose was also studied by Rahn et al., who reported levoglucosenone as the major product upon heating at 250 °C (Rahn & Yaylayan, 2010). Sucralose can also serve as a chlorine source for the formation of chloropropanols. A combination of analytical methods was recently used to study the thermal degradation of sucralose and both decomposition and the formation of chlorinated derivatives were observed after melting (Oliveira, Menezes, & Catharino, 2015). Polychlorinated aromatic hydrocarbons have been observed to form under mild conditions (Oliveira et al., 2015), while PCDD/Fs were found to be produced when sucralose was heated at high temperatures in the presence of a metal catalyst (Dong, Liu, Hu, & Zheng, 2013). Thus, decomposition, chlorination, cyclization and oxidation can all occur during the heating of sucralose. The formation of PCNs accompanied by PCDD/Fs has also been shown to result from high-temperature combustion, based on similar formation mechanisms (Iino et al., 1999; Weber et al., 2001), thus we considered that PCNs might also be produced during the heating of sucralose under certain conditions. In the present study, the effects of temperature, container type and the presence of rust (metal oxides) on the formation and distribution of PCNs during the heating of sucralose were investigated. Both residues resulting from the heating of sucralose and the smoke collected during this process were analyzed for the presence of PCNs, using isotope dilution high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS). The distributions and patterns of PCNs in these materials were determined. In addition, to identify the potential PCN formation mechanisms associated with the heating of sucralose, PAHs in the smoke samples were also monitored, using gas chromatography-mass spectrometry (GC/MS).