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LIQUID PHASE EXTRACTION: SOLID PHASE EXTRACTION

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p127-128

2026-07-04

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LIQUID PHASE EXTRACTION: SOLID PHASE EXTRACTION

There are times when a liquid phase extraction is not suitable. A relatively large amount of material, at least a few grams, is normally required for an effective liquid phase extraction. Because of similarities in pH or polarity, it may not be feasible to separate two substances by liquid phase extraction. In such cases, there is another technique that can overcome these limitations. It is called solid phase extraction. Solid phase extraction does not involve a partitioning mechanism. Instead, it relies on adsorption, a process whereby a solid, liquid, or gaseous analyte is attracted to the surface of a solid adsorbing material. The analyte molecules adhere to its surface. If just one component of a mixture is adsorbed onto the surface of the adsorbent, it can be separated using this method. In some cases, two or more components of an analyte mixture can be separated this way if they have different affinities for the adsorbent. A number of chemical processes affect the tendency and tenacity of the adsorption including polarity. One example of solid phase adsorption is the trapping of a hydrocarbon accelerant that was used to start a fire, on the surface of a plastic strip coated with finely divided charcoal, a potent adsorbing material. The strip is immersed in a can containing the residue from a fire suspected to contain the accelerant. As the can is heated, the accelerant molecules evaporate into the air space above the debris (called the headspace) and adsorb on to the surface of the coated strip. When it is removed from the can, the strip can be heated or added to a solvent to elute (remove) the adsorbed accelerant. Another example of a solid phase extraction is the “cleanup” process of a biological sample containing a drug. A column of adsorbing material is inserted into a large tube and a blood or urine sample is then added. It flows through the tube under a vacuum and the drug is adsorbed on to the surface of the adsorbing material whereas the rest of the blood or urine passes through. The drug can then be eluted from the adsorbent with a compatible solvent. There are a variety of solid phase adsorbing materials available for different types of drugs and other substances. In the section on chromatography in this chapter, the pioneering chromatography experiment, separating plant pigments on a solid column is described. It is an example of solid phase extraction.

FIGURE 1 : The solid phase microextraction (SPME) apparatus. The fiber at the end of the wand can be coated with a particular polymer or other coating so that it will efficiently adsorb certain types of analytes. For example, there are SPME fibers that will adsorb molecules of flammable liquids mixed in with debris from a fire.