High throughput mass spectrometry (MS)-based metabolomics is a popular platform for small molecule metabolites analyses that are widely used for detecting biomarkers in the research field of environmental assessment.

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Multiple freeze-thaw cycles due to temperature fluctuations irreversibly damage the muscle tissue of fish, thereby reducing their edibility. The aim of this study was to determine the effects of the number of freeze-thaw (F–T) cycles on protein and lipid oxidation, microstructure, physical index, and nutritional quality of rainbow trout muscle. The results showed that F–T cycles accelerated protein carbonyl formation and thiobarbituric acid reactive substances (TBARS) generation (p < 0.05), as well as increased the loss of total sulfhydryl (SH) groups (p < 0.05). Moreover, transmission electron microscope (TEM) images illustrated that the microstructure of muscle fibers was loosed and disintegrated after the third F–T cycle, causing a reduction in water holding capacity (WHC). In addition, muscles lost the intrinsic color of fresh meat after the fifth cycle, with lightness L* and yellowness b* increasing, while redness a* declined (p < 0.05). The hardness, springiness, and chewiness of muscles decreased, and the shear force first increased and then decreased after the third cycle. Furthermore, the proximate components, essential amino acids (EAAs), and total amino acids (TAAs), decreased significantly after the third cycle (p < 0.05) due to the decrease of WHC as well as protein and lipid oxidation. The results indicated that the quality of rainbow trout muscle changed after the third cycle, deteriorated seriously after the fifth cycle, and was unacceptable after the seventh cycle. Therefore, it is necessary to reduce the temperature fluctuation to less than three times during freezing. The results provided a reference for the identification and classification of frozen aquatic products.

An efficient magnetic dummy template molecularly imprinted polymer nanocomposite was prepared using multi‐walled carbon nanotubes as a support and metolachlor deschloro as a dummy template. The obtained nanocomposites were characterized using Fourier transform infrared spectroscopy, vibrating sample magnetometry, scanning electron microscopy, and transmission electron microscopy. The adsorption performance of the obtained nanocomposites was evaluated through binding experiments, including static adsorption, kinetic adsorption, and selective recognition studies. The obtained nanocomposites were successfully applied as selective sorbents for the magnetic solid‐phase extraction of seven amide herbicides (alachlor, acetochlor, pretilachlor, butachlor, metolachlor, diethatyl ethyl, and dimethachlor) coupled with liquid chromatography‐tandem mass spectrometry from fish samples. Under the optimized conditions, the limit of detection was 0.01–0.1 μg/kg. The obtained recoveries of the amide herbicides from the fish samples were in the range of 88.0 to 102.1% with a relative standard deviation of less than 7.5%. This method, which eliminated the effect of template leakage on qualitative and quantitative analysis was found to be superior to the methods reported in the literature. The results indicated that it could be successfully applied to analyze amide herbicides in fish samples with satisfactory recoveries.