Sorbents and Modes of Interaction

F igure 1.7. Construction of the Empore extraction disk cartridge. (Published with permission of3M.) 

Another type of disk, SPEC made by Ansys, Inc., uses a glass-fiber matrix rather than Teflon to hold the sorbent particles. This disk has a somewhat more rapid flow rate and is more rigid and thicker than the Teflon disk. There is another disk called the Speedisk made by J. T. Baker that consists of lO-pm packing material that is sandwiched between two glass-fiber filters without any type of Teflon binder. The various types of disks are discussed in detail in Chapter 11. 

The sorbents used for SPE are similar to those used in liquid chromatography, 

the mechanisms of interaction include hydrogen bonding and dipole-dipole forces (polar interactions), van der Waal s forces (nonpolar or hydrophobic interactions), size exclusion, and cation and anion exchange. Some sorbents combine several interactions for greater selectivity. The extensive line of sorbent chemical structures facilitates one of the most powerful aspects of SPE, that is, selectivity. Selectivity is the degree to which an extraction technique can separate the analyte from interferences in the original sample. The number of possible interactions between the analyte and the solid phase facilitates this selectivity. 

For normal-phase SPE, cyanopropyl (CN), aminopropyl (NH2), and diol functional groups are chemically bonded to the silica gel. The loading on the cyano, amino, and diol colunms are sufficiently large ( 6-10% as carbon) that they may sometimes be used for reversed-phase applications, especially for the removal of hydrophobic solutes from water or other polar solvents. These hydrophobic solutes would otherwise sorb too strongly to a more hydrophobic C-8 or C-18 sorbent and would be difficult to elute. Straight silica gel also is 

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