32
THEORY OF SORPTION AND ISOLATION
I
-Si-OH I H H H H -Si -0 H I H I H I H I I :::=si_ I _..C, I _..C, I ,....c, I _..C-H -Si -0 I C I C I C I C I OH I H I H I H I H H H H H
I
+
Derivatized Silica Surface (C-8)
Trimethyl Silane
CH 3
I·
I .
Si-CH, 'I ' H H H H -St-O CH H I H I H I H I 3 -~i -0 I_..C, I_..C, I,....C, I_..C-H I ......._si-C I C I C I C I -Si I H I H I H I H 0 H H H H
I
-0_...... \
I
CH.1-Si-CH, I . CH 3
Endcapped Silica Surface (C-8) Figure 2.6. Endcapping of free silanols with trimethylsilane for a C-8 sorbent.
Some SPE packing materials do not receive endcapping; thus, it is important to check for endcapping when choosing an SPE sorbent for a specific analyte. For example, in the case when the R group is C-18, the solute would experience reversed-phase sorption to the C-18 and in some cases sorption to the hydroxyl groups. When the interaction between the two groups is intentional, this is called a mixed-mode application of SPE. More examples of mixed-mode SPE will be shown later in this chapter. Examples of the three common types of bonded phases for SPE are shown in Figure 2.7. They include the normal-phase sorbents that are bonded with the cyano and amine functional groups, the reversed-phase sorbents (Fig. 2.7), and the ion-exchange sorbents, which are the cation (SCX, strong cation exchanger) and anion types (SAX, strong anion exchanger). The sulfonic-acid functional group is used in the sex, and the quaternary amine functional group is used in the SAX. In all cases, these examples show the monofunctional derivatization to the silica sorbent.