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These phases are made from a variety of adsorbent materials: silica or alumina deactivated by mineral salts, molecular sieves 5 Å (0.5 nm), porous glass or polymers, or graphite (e.g. Chromosorb^® 100, Porapak^®). Capillary columns made by deposition of these materials in the form of a fine porous uniform layer are called PLOT. They are employed to separate gaseous or highly volatile samples. Columns containing graphitized carbon black have been developed for the separation of N₂, CO, CO₂, and very light hydrocarbons. The efficiency of these columns is very high (Figure 2.11).

Figure 2.10 Chiral stationary phase in GC. Among the three chiral vectors encountered in GC (1 and 2, β‐cyclodextrins, 3, crown ethers, 4, diamides), cyclodextrins are by far the most commonly used. They include three types of sites: A, axial hydroxyl, B, equatorial hydroxyl and C, hydroxymethyl. Their reactivities are sufficiently different to enable selective reactions and thus obtain some 50 phases (e.g. 2, ungrafted cycloSil‐B phase from Chromoptic). Partial chromatograms of natural extracts, demonstrating the separation of optical isomers of carvone.

Figure 2.11 Gas analyses. Left, one of the earliest ever chromatograms, obtained point by point and representing a mixture of air, ethylene and acetylene separated on silica gel (E. Cremer and F. Prior, Z. Elektrochem. 1951, 55, 66). Right, an analysis of gas on a modern PLOT column (reproduced courtesy of Supelco).

Historically, silica gel, a thermostable material that is insensitive to oxygen, was one of the first compounds to serve as a stationary phase for GC columns (Figure 2.11). Today, solid phases have become much more elaborate.