Selectivity in gas chromatography is influenced by the choice of stationary phase. Elution order in GLC is determined primarily by the solute’s boiling point and, to a lesser degree, by the solute’s interaction with the stationary phase. Solutes with significantly different boiling points are easily separated. On the other hand, two solutes with similar boiling points can be separated only if the stationary phase se- lectively interacts with one of the solutes. In general, nonpolar solutes are more easily separated with a nonpolar stationary phase, and polar solutes are easier to separate using a polar stationary phase.
The main criteria for selecting a stationary phase are that it should be chemi- cally inert, thermally stable, of low volatility, and of an appropriate polarity for the solutes being separated. Although hundreds of stationary phases have been devel- oped, many of which are commercially available, the majority of GLC separations are accomplished with perhaps five to ten common stationary phases. Several of these are listed in Table 12.2, in order of increasing polarity, along with their physi- cal properties and typical applications.
Many stationary phases have the general structure shown in Figure 12.18a. A stationary phase of polydimethyl siloxane, in which all the –R groups are methyl groups (–CH3), is nonpolar and often makes a good first choice for a new separa- tion. The order of elution when using polydimethyl siloxane usually follows the boiling points of the solutes, with lower boiling solutes eluting first. Replacing some of the methyl groups with other substituents increases the stationary phase’s polar- ity, providing greater selectivity. Thus, in 50% methyl-50% phenyl polysiloxane, 50% of the –R groups are phenyl groups (–C6H5), producing a slightly polar sta- tionary phase. Increasing polarity is provided by substituting trifluoropropyl (–C3H6CF3) and cyanopropyl (–C3H6CN) functional groups or using a stationary phase based on polyethylene glycol (Figure 12.18b).
An important problem with all liquid stationary phases is their tendency to “bleed” from the column. The temperature limits listed in Table 12.2 are those that minimize the loss of stationary phase. When operated above these limits, a col- umn’s useful lifetime is significantly shortened. Capillary columns with bonded or cross-linked stationary phases provide superior stability. Bonded stationary phases are attached to the capillary’s silica surface. Cross- linking, which is done after the stationary phase is placed in the capillary column, links together separate polymer chains, thereby providing greater stability.
Another important characteristic of a gas chromatographic column is the thickness of the stationary phase. As shown in equa- tion 12.25, separation efficiency improves with thinner films. The most common film thickness is 0.25 μm. Thicker films are used for highly volatile solutes, such as gases, because they have a greater capacity for retaining such solutes. Thinner films are used when separating solutes of low volatility, such as steroids.
A few GLC stationary phases rely on chemical selectivity. The most notable are stationary phases containing chiral functional groups, which can be used for sepa- rating enantiomers.6
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