## 1 Introduction  
As an important method for the prevention and control of animal diseases, veterinary drugs play a crucial role in improving animal production efficiency and livestock product quality, and maintaining ecological balance. Animal-derived foods, such as meat, eggs, aquatic products and dairy products constitute a large category of human foods that can provide vitamins, minerals, fats, and proteins for the human body [1]. Veterinary drug residues in animal-derived foods refer to the accumulation and residue of original drugs, their metabolites and related impurities in animal tissues, milk or eggs of mammals or the ecological environment after the use of veterinary drugs (including drug additives) [2]. Veterinary drug residues not only directly endanger the human body and increase drug-resistant strains, but also pose a potential threat to the environment and human health through multiple food chains [3, 4]. China has a higher level of production, processing and consumption of livestock and poultry products. Ensuring the quality and safety of animal products contributes to the health of the people, the development of production and trade, and the sustained and healthy development of the economy in China. Therefore, it is important to take effective measures to control and reduce the occurrence of veterinary drug residues. At present, veterinary drug residues and illegal additives in food are commonly detected by the immune analysis method [5], biosensor method [6], instrument analysis method [7] and other methods. Among them, instrumental analysis is mainly performed based on large, expensive and sophisticated instruments, including liquid chromatography, gas chromatography and its tandem mass spectrometry [8, 9, 10, 11]; it is the mainstreamdetection method. As with the detection techniques specified in the national standard, the above methods can be adopted to obtain reliable results and can also be used to analyze the specificity of the test object under the interference of multiple substances. Owing to its high sensitivity, small sample size and small matrix interference, UPLC-MS/MS has become an advanced method for detecting veterinary drug residues [12; 13]. However, veterinary drug residues are characterized by low residual levels in samples and complex sample matrices, which makes it difficult to directly determine trace and trace residues in samples. In addition, it is also difficult to obtain the required precision for veterinary drug residue detection. Furthermore, detection instruments are prone to damage [14]. Therefore, sample pretreatment technology plays a crucial role in veterinary drug residue detection. Sample pretreatment is a complex process which is mainly composed of extraction, purification and concentration [15]. In terms of the analytical methods with low detection limits, such as liquid chromatography-mass spectrometry, sample preparation is more important than the detection process, and complex pretreatment procedures are needed. Sample pretreatment technology is the key link during veterinary drug residue detection, and it directly affects analytical efficiency and accuracy. Due to their cumbersome operation, poor stability and low specificity, traditional sample preparation technologies have been replaced by some new separation and purification methods, such as solid phase extraction [16], solid phase microextraction [17], matrix dispersive solid phase extraction [18], immunoaffinity chromatography, molecular imprinting technology [19; 20], supercritical fluid extraction and accelerated solvent extraction [21]. These new technologies feature a small sample size, favorable specificity and selectivity of method, high automation, simple manual operation, environmental friendliness and small dosages of organic reagents.