Normal phase cyanopropyl sorbents

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... 

Finally, sorbents such as aminopropyl, cyanopropyl, and diol can be used for both reversed-phase and normal-phase separations. Many manufacturers supply their sorbents in variety packs that may be used for methods development. Also quality assurance reports are commonly available for the various sorbents, which is a good indication of their reproducibility. Later chapters will show specific applications, such as environmental, drugs and pharmaceuticals, and food and natural products. 

The types of nonbonded phases used for normal-phase SPE are silica, alumina, and magnesium silicate (Florisil). The most popular phase is silica. Several bonded phases may also be used for normal-phase SPE, including aminopropyl, cyanopropyl, and propyldiol (Table 1.1, Fig. 2.7). Water is not used in the mobile phase in normal-phase SPE because it will sorb to the active sites of the sorbent and reduce the interaction between analyte and sorbent. Typically, normal-phase SPE is used as a clean-up procedure for organic extracts of water, soil, food, or other materials. Normal-phase SPE is also used for the isolation of analytes from organic liquids, such as oils. 

In general, basic compounds are retained more strongly on mildly acidic surfaces, such as silica or acidic alumina. Acidic compounds are retained on basic surfaces, such as basic alumina. Because both silica and alumina are hydroscopic, they adsorb water to their surface. This water greatly reduces the retention of organic solutes because it deactivates the hydrogen-bonding sites. Thus, it is important to keep the SPE sorbents dry and free from water. They may be stored in a dessicator prior to use. Very polar compounds, such as carbohydrates or amino compounds, are tightly bound to nonbonded normal-phase sorbents, such as silica and alumina. However, the use of cyanopropyl or aminopropyl phases often permit the recovery of these compounds when silica does not work. 

Figure 5.1. Hydrogen bonding normal-phase mechanism in SPE using a cyanopropyl sorbent.

There are three general types of bonded normal-phase sorbents aminopropyl, cyanopropyl, and diol (Fig. 5.3). All three are derivatized as a propyl hydrocarbon, which places them near the surface of the silica. Only the cyanopropyl sorbent is available in either endcapped or nonendcapped forms (Table 5.1). Thus, analytes have the opportunity to interact not only with the bonded phase but also with the underlying silica sorbent. Furthermore, because the hydrocarbon chain is short, only three carbons in length, the... 

The extraction of aromatic hydrocarbons (Fig. 5.5) from crude oil uses two sorbents in series, first a cyanopropyl column attached to a second silica column. In this procedure, aromatic hydrocarbons are sorbed on both the cyanopropyl sorbent and on the silica sorbent. The heteroatom hydrocarbons (containing nitrogen, oxygen, and sulfur) are trapped on the cyanopropyl sorbent and eluted as a separate fraction from the silica column. Because the major interaction of aromatic heterocyclic hydrocarbons is through hydrogen bonding to the surface of the sorbent, the cyanopropyl sorbent is easier to elute than a silica sorbent alone. For this reason, the cyanopropyl column is used before the silica column. The separation of hydrocarbons from crude oil is an example of normal-phase chromatography that has been performed for many years on silica gel prior to the introduction of SPE. 

SPE can be classified by relative polarity of the solid and mobile phases. A nonpolar solid phase that preferentially associates with nonpolar or slightly polar analytes is called reversed phase, and the solvents used as eluents are polar. Normal-phase SPE employs a polar solid phase and nonpolar solvents. Highly polar and ionizable solid phases are used in ion exchange, which can also be used as a form of analysis. The continuum of solid-phase sorbents runs from reversed-phase nonpolar such as C18, through moderately polar phases such as cyanopropyl, to strong anion and cation exchangers. 

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