Objective Thiourea is a commonly used treatment agent for footwear materials, but it is classified as a 2B carcinogen. The thiourea in footwear materials is prone to migration during use, and long-term exposure to it can pose a threat to human health. Currently, the existing domestic standards do not set specific control limits for thiourea substances in footwear foaming materials. The footwear industry will face the dual risks of "foreign recalls + domestic standard absence". This research aims to establish an ultrasonic extraction combined with high-performance liquid chromatography method to achieve accurate qualitative and quantitative determination of thiourea in footwear foaming materials.

Methods In this paper, a single-factor experiment was conducted to determine the optimal analytical conditions for the determination of thiourea in footwear foaming materials by ultrasonic extraction combined with high-performance liquid chromatography. Firstly, the optimal detection wavelength was determined through the full-wavelength scanning of ultraviolet spectroscopy. Secondly, the chromatographic conditions were optimized by setting different variables to identify the best mobile phase composition and ratio. Then, the optimal pre-treatment conditions for the extractant and ultrasonic temperature were further determined. Finally, the method performance evaluation experiments were conducted under the above optimal conditions to verify the method's precision and accuracy, and to apply it to actual sample detection to prove the method's applicability.

Results The 238 nm wavelength with a higher absorption peak of thiourea was determined as the detection wavelength. Based on the designed single-factor experiments, the optimal conditions were obtained. Under the condition of using methanol-water as the mobile phase, the peak shape of thiourea was better and the impurity peaks were fewer. In addition, the peak area increased first and then decreased with the increase of the water proportion in the mobile phase, and it reached the maximum value under the condition of 50% methanol-50% water. During the process of optimizing the pre-treatment conditions, it was found that methanol had the strongest extraction ability for thiourea in footwear foaming materials. The analysis results of thiourea under different ultrasonic temperatures showed that the peak area of thiourea was the largest at 60 ℃. Finally, the method validation results were as follows: the detection limit was 0.3 mg/kg; the quantitative limit was 1.0 mg/kg, and the recovery rate and relative standard deviation were 105.0%-108.8% and 1.2%~2.0%, respectively.

Conclusions This paper established a method for quantitative analysis of thiourea in footwear foaming materials by using ultrasonic extraction combined with high-performance liquid chromatography through a single-factor experiment. Methanol was used as the extraction agent, and the ultrasonication was carried out under the conditions of 40 Hz, 400 W, and 60 ℃ for 60 min. The optimal conditions for high-performance liquid chromatography detection were as follows: the mobile phase was methanol-water with a ratio of 50:50; the column temperature was 40 ℃, and the injection volume was 1.0 μL. The methodological evaluation results show that this method has good repeatability and accuracy. The experimental results of actual samples verify that this method is applicable for the routine detection and quality control of thiourea in footwear foaming materials.