## 2 Materials and Methods
### Overview of Sulfonated Magnetic Polystyrene Microspheres  
The magnetic polymer microspheres can be prepared by suspension polymerization based on the following principles. In the presence of magnetic powder, suspension stabilizer and surfactant, one or more monomers are homopolymerized or copolymerized on the surface of magnetic particles under the action of initiators, and the magnetic particles are wrapped in the polymerization products. Suspension polymerization is restricted by the wide distribution of particle sizes. Micro-suspension polymerization is a reduced suspension polymerization. In this reaction, the oil phase is dispersed into small droplets and polymerized at a certain temperature by a high-speed shear device in a system composed of magnetic powder, water, monomer, oil-soluble initiator, dispersant and surfactant. The particle size of suspension polymerization products is 0.01-5 mm, with the most ranging from 0.05 mm to 2 mm. The particle size of emulsion polymerization products is about 0.05-1 \(\upmu\)m, which is much smaller than that of suspension polymerization products. In this paper, micro-suspension polymerization was used to prepare magnetic polymer microspheres with a particle size between both. [PERSON] [26] prepared some magnetic polystyrene microspheres with a particle size ranging from 0.5 \(\upmu\)m to 2 \(\upmu\)m using micro-suspension polymerization. [PERSON] [27] synthesized magnetic polystyrene microspheres with a particle size distribution of about 20 \(\upmu\)m using micro-suspension polymerization. Based on previous efforts, magnetic polystyrene microspheres with monodisperse, uniform particle size distribution and good  
Figure 1: Flow chart of sample pretreatment magnetic solid phase extraction technology. magnetic response were prepared using an improved micro-suspension method. Based on the work of [PERSON] [28] et al. we obtained satisfactory magnetic composite microspheres by optimizing the ratio of initiator and magnetic microspheres, the reaction time and the pH of the reaction system as a prerequisite for the experiments conducted in this paper. The preparation technology of magnetic polystyrene microspheres (sulfonated magnetic polystyrene microspheres, Fe\({}_{3}\)O\({}_{4}\)@SPS) functionalized by strong acids has been improved specifically by adding concentrated sulfuric acid or other sulfates to modify polystyrene (PS) coated on the surface of these magnetic microspheres, mainly using the sulfuric acid method [29], sulfur trioxide method [30] and chlorosulfonic acid method [31]. The preparation of Fe\({}_{3}\)O\({}_{4}\)@SPS with the chlorosulfonic acid as a sulfonating agent has some advantages, including strong reaction ability, low temperature, short time, small amount of feed, high sulfonation degree and easy discharge of generated HCl [32]. Hence, it is an ideal sulfonation method. [PERSON] [33] et al. explored the sulfonation kinetics of chlorosulfonic acid polystyrene-butadiene rubber (PSBR) with different initial concentrations. A prediction model was established to predict the sulfonation degree at different initial concentrations of acid. [PERSON] [34] et al. investigated the effects of reaction temperature, time, dosage of sulfonating agents, solvent system and solvent dosage on the preparation of Fe\({}_{3}\)O\({}_{4}\)@SPS with the chlorosulfonic acid as a sulfonating agent. [PERSON] [31] et al. prepared sulfonated porous polystyrene microspheres as heterogeneous catalysts for biodiesel using a simple two-step synthesis method. These sulfonated magnetic polystyrene microspheres had high acid density (2.611 mmol/g), good thermal stability (up to about 200 \({}^{\circ}\)C) and a suitable specific surface area. In this paper, the preparation of Fe\({}_{3}\)O\({}_{4}\)@SPS with the chlorosulfonic acid as a sulfonating agent was modified and optimized.