What is Solid Phase Extraction: Solid phase extraction (SPE) is a sample preparation technique used in analytical chemistry to separate and concentrate specific compounds from a mixture. It involves passing a solution containing the target analyte through a solid phase, which retains the analyte while allowing other compounds to pass through. The chemistry involved in solid phase extraction can be divided into three main steps: sorption, washing, and elution. 1. Sorption: In this step, the sample solution is passed through a solid phase that contains a sorbent material. The sorbent material is chosen to selectively retain the target analyte while allowing other compounds to pass through. The sorption process is based on various chemical interactions between the sorbent material and the analyte, such as hydrophobic interactions, hydrogen bonding, and ion exchange. 2. Washing: After the analyte has been retained on the solid phase, the solid phase is washed to remove any remaining unwanted compounds that may have adsorbed to the sorbent material. This step helps to eliminate any interference with the analysis caused by the presence of unwanted compounds. 3. Elution: The analyte is then eluted from the solid phase by passing a suitable solvent through the solid phase. The solvent should be chosen to selectively dissolve the analyte while leaving the unwanted compounds behind. The elution process is also based on various chemical interactions between the analyte and the solvent, such as solubility, pH, and ionic strength. The choice of sorbent material and elution solvent depends on the nature of the analyte and the sample matrix. For example, if the analyte is a nonpolar compound, a hydrophobic sorbent material such as C18 would be used, and the elution solvent would be a non-polar solvent such as methanol. On the other hand, if the analyte is a polar compound, a polar sorbent material such as silica gel would be used, and the elution solvent would be a polar solvent such as water. In summary, solid phase extraction is a powerful sample preparation technique that relies on various chemical interactions between the analyte, sorbent material, and elution solvent. By carefully choosing the sorbent material and elution solvent, it is possible to selectively retain and concentrate specific compounds from a mixture for further analysis.