Bile acid, a group of cholesterol metabolites, plays a crucial role in human metabolism by aiding in lipid transport within bile [1,2]. Disruptions in bile acid homeostasis are related to a variety of diseases and drug-induced liver injury [[3], [4], [5]]. The involvement of abnormal bile acid metabolism in the occurrence and development of hepatobiliary tumors has been confirmed by a large number of studies. As the main component of bile, bile acid is in direct contact with gallbladder mucosa. The changes of bile concentration and composition in patients with gallbladder cancer play a key role in the occurrence and development of gallbladder cancer, serving as important diagnostic indicators. However, clinical limitations in obtaining bile hinder its use as a screening tool [6,7]. Due to the existence of bile acid “enterohepatic circulation” in the body, the level of serum bile acid has become a sensitive index of liver disease [8,9]. Monitoring serum total bile acid concentration is an experimental method for diagnosing liver disease, yet the diverse physiological and pathophysiological activities of different bile acids restrict its diagnostic utility [10,11]. Therefore, a rapid, simple, and sensitive method for monitoring various bile acids in biological fluids is essential [12].

The bile acids in biological body fluids are often determined by gas chromatography (GC) [13], high performance liquid chromatography (HPLC) [[14], [15], [16]], gas chromatography-mass spectrometry (GC–MS) [[17], [18], [19]], HPLC-MS/MS [[20], [21], [22], [23]] and so on. Due to the high boiling point and difficulty in gasifying bile acids, GC is not ideal for their routine detection. Since most bile acids differ mainly in the number and position of hydroxyl groups, leading to the formation of structurally similar molecules, chromatography is essential for their separation. Therefore, liquid chromatography-mass spectrometry (LC-MS) or tandem mass spectrometry (LC-MS/MS) has emerged as the predominant method for analyzing bile acids in complex samples in recent years.

However, biological samples possess a high level of complexity, necessitating preliminary treatment prior to analysis. Liquid-liquid extraction (LLE) and solid-phase extraction (SPE) are commonly used sample pretreatment techniques in chemical analysis, which are used to separate and enrich target analytes from complex matrices [24].

Liquid-liquid extraction is a separation technique based on different partition coefficients of different components in two immiscible solutions. Therefore, the most important component in this separation system is the extractant. Farajzadeh Co., Ltd [25] first carried out homogeneous liquid-liquid extraction of derivative analytes with acetonitrile, and then dispersed liquid-liquid microextraction with trichloroethane. Through two times extraction, analytes were enriched up to 440-fold. However, the liquid-liquid extraction method also has some disadvantages, such as tedious operation, long time, consuming a lot of organic solvents and so on.

SPE stands as a widely utilized method for sample pretreatment in various fields like chemical analysis, environmental monitoring, and food safety [26,27]. By employing selective adsorption and desorption, SPE efficiently isolates target compounds from complex samples to enhance their purity and concentration, thereby elevating analysis precision and sensitivity. Noteworthy for its user-friendly operation, reproducibility, and minimal use of organic solvents, SPE plays a pivotal role in research endeavors [[28], [29], [30]]. However, in the research process of SPE, the researchers found that the hydrophilic-lipophilic balance (HLB) of the filled extraction column has a great influence on the performance of the SPE column [31]. The HLB value can directly determine the applicable range of the prepared extraction column. Therefore, researchers can adjust the HLB value of the filling material through a variety of functional monomers to design an ideal SPE column, i.e., process, ruggedness, improvement, matrix effect, ease of use (PRiME) HLB column.

Compared with traditional SPE columns, PRiME HLB columns have obvious advantages in particle size and pore size uniformity, pH application range, adsorption capacity, reproducibility and extraction efficiency. Therefore, PRiME HLB columns are more suitable for dealing with more complex sample substrates, such as biological samples, environmental samples and food samples, which can remove impurities and extract target analytes more effectively. In 2015, yang Co., Ltd successfully applied PRiME HLB column to extract and cleanup of residual veterinary drugs in meat samples [32]. Whether it is a traditional SPE column or a PRiME HLB column, the sorbent is the core. With the continuous research, more and more new environment-friendly and easy-to-synthesize porous materials are used as sorbents for SPE column. It includes carbon nanomaterials, covalent organic framework materials, porous silica, metal-organic frameworks, molecularly imprinted materials and so on.

In 2021, a new type of economical covalent organic skeleton material was synthesized by combining the resin with covalent organic framework. This material boasted abundant internal pores, a well-organized structure, a sizable specific surface area, and commendable thermal stability. Notably, it exhibited exceptional prowess in separating phenols, achieving an impressive recovery rate of up to 101.79 % [33]. In 2022, Yang Company first applied amino modified mesoporous silica as the adsorbent of matrix solid phase dispersion system to the analysis of free fatty acids in krill. The highest recovery rate was 96.43 % [34]. COF and silica materials have outstanding performance in the field of SPE: COF materials have good hydrophobicity, while silica has high mechanical strength. But they can't avoid the disadvantage of high cost. The organic resin can reduce the production cost very well. However, its mechanical strength and hydrophobicity are not excellent. To solve these problems, this study adopts the hybrid of organic resin and Fe3O4 and the functionalization of C18 group to control the HLB value of the material, so as to solve the problem of insufficient mechanical strength and hydrophobicity.

Here, the C18-PS-DVB-GMA-Fe3O4 was synthesized by simple emulsion suspension polymerization. The C18-PS-DVB-GMA-Fe3O4 are used as an adsorbent in the PRiME pass-through cleanup process for the extraction of 15 bile acids in human plasma. Therefore, an analytical method combining PRiME C18-PS-DVB-GMA-Fe3O4 pass-through cleanup strategy with LC-MS/MS was proposed. The established method has good sensitivity and selectivity for 15 kinds of bile acids and can be used for the analysis of trace bile acids in clinical plasma.