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TABLE 6 Some Liquid Phases for LLC
1. Polyethylene glycol, such as
Carbowax 400 or triethylene glycol
3. Dimethyl sulfoxide
6. Water-alcohol or glycol
7. Hydrocarbons and polymers
Hydrocarbon such as pentane, isooctane, or cyclopentane; or a hydrocarbon plus a small amount of more polar liquid such as chloroform
2. Same as 1
3. Isooctane or hexane
4. Same as 3
5. Aqueous acid
6. Hexane-carbon tetrachloride
The original bonded phases were made for GC since it was anticipated that chemical bonding would prevent the stationary phase from bleeding at high temperatures. The technology for reacting organic moities with solid surfaces was already established because of the silylation reactions used to deactivate GC solid supports (see Chapter 8). The original GC bonded phases are susceptible to decomposition from trace amounts of water or oxygen13 and are therefore not widely used. Newer GC bonded phases are in use on ÎÒ columns as described in Chapter 8, and they are very stable, efficient, and popular. Similar bonded phases have been found to be excellent for LC. For a historical review, see the paper by Gilpin.14
At the beginning of this chapter it was noted that bonded phases have become the most popular form of LC, particularly those used in RPLC. The following discussion of the chemical reactions involved in producing bonded phases will be limited to these nonpolar stationary phases.
Most bonded phases are formed by reacting chlorosilanes with silica that has reactive silanol groups on its surface. (Other possibilities will be discussed later.) Figure 9.6 depicts some probable functional groups on the silica surface, and Figure 9.7 shows some typical deactivating reactions (or silanizations). For reverse phase supports, the R group, sometimes called a ligate, is an alkyl chain that can be up to 18 carbons long. If the reacting silane has only one reactive chloro group, the reaction is simple and replaces the active hydrogen with a dimethyloctadecylsilyl group. Bonded phases produced with this reaction are preferred for theoretical study because they are well-defined and regular; they are called monofunctional phases.
If the silane has more than one reactive chloro group, the reaction can be more complex, as shown. Some crosslinking can occur, resulting in an undefined polymer on the silica surface.The reaction shown in Figure 9.7 is only one of several possibilities.
Figure 9.6. Representation of some possible functional groups on the surface of silica.
Figure 9.7. Some silanizing reactions to produce bonded phases for LC.
Unreacted chloro groups can be hydrolyzed to hydroxyl groups and then be reacted again with trimethylchlorosilane to eliminate as many hydroxyl groups as possible. This last silanization step is referred to as end capping; it also removes most of the previously unreacted, residual silanol groups on the surface of the silica where the larger ODS was not able to penetrate. Supports that have been end capped are usually found to have different selectivities from those that are not as fully reacted.
A variety of measurements, such as total carbon analysis and BET surface area, have been used to determine the effectiveness of the sily-lation reactions and the coverage of the silica surface. From these measurements the percentage of carbon on the surface can be calculated and is used to designate the degree of surface coverage. Values ranging from 2 to 30% carbon have been reported.
More recently, pyrolysis GC, ESCA, and FTIR have been used to characterize the surface of bonded layers. The reason for the intense interest is the fact that there are significant differences between bonded phases manufactured by different companies, and these investigators hope to find out why. One recent study15 analyzed the decomposition products produced by the reaction of bonded reverse phases with HF, and it was able to determine the type of reaction (monofunctional or polyfunctional), the extent of end capping, and the distribution of lengths of alkyl groups. Some of the results of the study are summarized in Table 7. Such results help explain the differences between bonded phases manufactured by different companies.
TABLE 7 Analysis of Commercial ODS Phases"
Amount of Liqate
Manufacturer (particle size) Typefc ODS End Capping0
Du Pont (12 (xm) M 6.19 ND
LiChrosorb (10 (im) D 5.50 ND
Nucleosil (10 ö.ò) T --- 3.24
Alltech (5 p,m) T --- 8.91
Beckman Ultrasphere IP M 4.15 2.01