Analytical Chem istry - DePauw University

326 Analytical Chemistry 2.0μL drop ofextracting solventSyringe needleFigure 7.21 Schematic of a liquid–liquid microextraction showing asyringe needle with a mL drop ofthe extracting solvent.extraction is complete, we allow the liquids to separate. The stopcock at thebottom of the separatory funnel allows us to remove the two phases.We also can carry out a liquid–liquid extraction without a separatoryfunnel by adding the extracting solvent to the sample container. Pesticidesin water, for example, are preserved in the field by extracting them into asmall volume of hexane. Liquid–liquid microextractions, in which the extractingphase is a 1-mL drop suspended from a microsyringe (Figure 7.21),also have been described. 15 Because of its importance, a more thoroughdiscussion of liquid–liquid extraction is in section 7G.So l i d Ph a s e Ex t r a c t i o n sIn a solid phase extraction of a liquid sample, we pass the sample through acartridge containing a solid adsorbent, several examples of which are shownin Figure 7.22. The choice of adsorbent is determined by the species wewish to separate. Table 7.8 provides several representative examples of solidadsorbents and their applications.As an example, let’s examine a procedure for isolating the sedativessecobarbital and phenobarbital from serum samples using a C-18 solidadsorbent. 16 Before adding the sample, the solid phase cartridge is rinsedwith 6 mL each of methanol and water. Next, a 500-mL sample of serumis pulled through the cartridge, with the sedatives and matrix interferentsretained by a liquid–solid extraction (Figure 7.23a). Washing the cartridgewith distilled water removes any interferents (Figure 7.23b). Finally, weelute the sedatives using 500 mL of acetone (Figure 7.23c). In comparisonto a liquid–liquid extraction, a solid phase extraction has the advantage ofbeing easier, faster, and requiring less solvent.15 Jeannot, M. A.; Cantwell, F. F. Anal. Chem. 1997, 69, 235–239.16 Alltech Associates Extract-Clean SPE Sample Preparation Guide, Bulletin 83.Figure 7.22 Selection of solid phase extraction cartridges forliquid samples. The solid adsorbent is the white material ineach cartridge. Source: Jeff Dahl (commons.wikipedia.org).