The principle of solid-phase extraction (SPE) or liquid-solid extraction (LSE) is similar to that of liquid-liquid extraction, involving a partitioning of compounds between two phases . In SPE, the compounds to be extracted are partitioned between a solid and a liquid.The interactions responsible for the separation between liquid and solid phase are noncovalent (ionic, Van der Waals, hydrophobic), and those interactions can be modulated by the physical properties of the eluent (liquid phase) and the adsorbent (solid phase).
In principle, there are no major differences between solid-phase extraction and liquid—liquid extraction, but SPE can avoid or reduce some disadvantages of liquid-liquid extraction. SPE can handle small samples and very dilute solutions, it overcomes the formation of emulsions, and it can be easily automated. Furthermore, the sorbents which are commonly used are available commercially as cartridges. These sorbents are alumina, silica gel, reversed-phase silica gel, and various ion-exchange resins. It is also possible to pack different adsorbents in layers inside the same cartridge to give a "sandwich type" extraction column.
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