# Contamination with Pharmaceuticals in Aquatic Environment: Focus on Analytical Methodologies
## 3 Determination of Pharmaceuticals in Environmental Samples
### Sample Preparation Techniques
#### 3.1.1 Solid-Phase Extraction (SPE)  
SPE was developed in the 1970s [51] and is the most used extraction technique for the isolation of pharmaceuticals from environmental samples (Table 1). The mechanism employed in the extraction procedure is somewhat similar to that of liquid chromatography but occurs in several steps. The first step is to condition the sorbent by washing it to maximize analyte sorption by increasing the effective surface area. In the next step, the sample is applied to the sorbent, with the sorbent capacity depending on the sorbent type. For example, the sorbent capacity based on SiO\({}_{2}\) is 5 to 10% of its weight. Higher sorbent loading can lead to inefficient extraction. The third step involves washing out interferets, substances that did not bind to the sorbent or have significantly lower interactions with the sorbent than the analyte-sorbent interactions. The wash solution is a solvent that selectively removes unwanted components while not disrupting the analyte-sorbent bond. Several reagents or their mixtures may be used to achieve the desired sample purity. In the final step, the analytes are eluted from the sorbent surface. The eluent has a greater elution strength than the wash solution. The result is an analyte concentration by reducing the volume relative to the sample volume applied to the SPE device [32]. By using columns with various types of sorbents and selecting suitableuents, a degree of analyte preconcentration sufficient for further determination by various detection techniques can be achieved. Depending on the analyte properties, silica-based composites, polymeric materials, organic-inorganic hybrid materials, natural sorbents, and carbon-based materials have been successfully used [7]. Some commonly used SPE sorbents include C8, aluminum oxide, C18, and silica [51]. Oasis HLB is widely used as an SPE sorbent because it can extract a wide range of pharmaceuticals from different types of water [52]. Oasis HLB columns with various particle sizes are capable of extracting both polar and non-polar compounds over a wide pH range due to their chemical compositions (a combination of lipophilic divinylbenzene and hydrophilic N-vinylpyrrolidone polymers) [53]. Strata-X is a surface-modified styrene-divinylbenzene polymer that, similar to Oasis HLB, can retain a wide range of analytes through hydrophilic and hydrophobic interactions. It is almost a universal sorbent for acidic, basic, and neutral analytes [54]. Although SPE is predominantly used offline, online coupling is also possible using column switching. Online SPE allows the automation of the analytical procedure, resulting in increased process efficiency, reduced sample handling, and prevention of cross-contamination [55]. The online SPE technique represents an automated continuous solid-phase extraction method, most often in a SPE-LC-MS/MS chromatographic system. In this type of solid-phase extraction, the same types of sorbents are used for the SPE columns in the \"offline\" system. Most often, conventional non-polar sorbents (C18 or C8 bound to a carrier, such as silica gel), whose sorption uses hydrophobic interactions, van der Waals interactions between the hydrophobic part of the analyte and the hydrophobic part of the sorbent are the most popular. The first analytes that were extracted in the online SPE system were pesticides in water, and later, the application was also developed for pharmaceuticals [24]. An essential advantage of online SPE compared to the classic \"offline\" SPE is the timesaving and elimination of manual operations from preparation to elution of the analytes since all sub-operations of SPE take place in a closed system (within high-performance liquid chromatographic (HPLC) and ultra-high-performance liquid chromatographic (UHPLC) systems), thereby eliminating losses of the target analytes. [PERSON] et al. [24] developed and validated a multi-residue method for the determination of 40 pharmaceutical substances, where online SPE preconcentration was chosen as the sample preparation technique.