The Principle of Solid Phase Extraction

Solid phase extraction is a physical extraction process that takes place between a liquid and a solid phase. The solid phase has a stronger affinity for the isolate than for the solvent in which the isolate is dissolved. When the sample solution passes through the bed of sorbent, the isolate becomes concentrated on the surface of the sorbent while the other constituents of the sample pass freely through the bed [20], The SPE process is illustrated in Fig. 7-3. 

Conditioning with methanol Pre-equilibration with 0.1 M phosphate buffer 

To ensure high and reproducible recovery, it is necessary to treat the column sorbent with a solvent. For cartridges, in which the sorbent is based on bonded silica gel, this procedure is necessary as the dry sorbent is pre-packed, and in this condition the active side chains are not available for interaction with the components. For non-polar and multi-bed phases, the sorbent must in some circumstances be preconditioned with several solvents, such as methanol followed by water or a buffer solution. The organic solvent used here has the function of solvating the bound functional groups and of activating the hydrocarbon chains. Also, organic residues are removed from the sorbent. Water or a buffer solution is then added to prepare the column for the aqueous sample. 

After the preconditioning of the column, the prepared sample is added and then drawn through by means of a slight vacuum. A definite low flow rate is necessary in order to concentrate on the column all the various types of drugs which can be present in a biological sample. Normally, the flow rate is 1.5 ml/min. 

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The principles of solid-phase extraction (SPE) or liquid-solid extraction (LSE) are similar to that of liquid-liquid extraction, involving a partitioning of compounds between two phases [22]. In SPE, the compounds to be extracted are partitioned between a solid and a liquid. The interactions responsible for the separation between the liquid and solid phase are non-covalent (ionic, van der Waals, hydro-phobic) and can be modulated by changing the physical properties of the eluent (liquid phase) and the adsorbent (solid phase). 

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