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Figure 10. Chromatogram of a C^Cg hydrocarbon mixture (alkanes, alkenes, cy-cloalkanes, and aromatics) on a SCOT column (from the publication of Ettre, Purcell, andBilleb, 1966 [50]) 100-ft X 0.50-mm i.d. SCOT column, prepared with squalane liquid phase; phase ratio: 67. Flame ioniza­tion detector. The first five peaks: 1 = methane, 2 = ethane, 3 = propene, 4 = propane, 5 = 2-methyl-propane. Courtesy of Marcel Dekker, Inc., New York, N.Y.

Figure 11. Chromatogram of an oil of marjoram sample obtained in a GC/MS system using a SCOT column (from the investigations of Averill and Struck, 1966; see Ref­erence 49) 50-ft X 0.50-mm i.d. SCOT column, prepared with OS-138 polyphenyl ether liquid phase. Phase ratio: 50. Temperature: programmed, as given. Total ion current recording

duce the activity of the inside tube surface. Thus, conscious variation of the liquid-phase film thickness was not plausible until the advent of glass columns.

According to the theory of open-tu­bular columns, columns prepared with a thin film have a higher absolute effi­ciency. This was the primary reason why most of the glass columns were prepared with a relatively thin (about 0.2-μΐη) film. However, it had been understood for a long time that it is advantageous to select the film thick­ness according to the type of sample: This had been pointed out as early as 1961-63 by Jentzsch and Hôvermann (27,28).

In 1974 in our laboratories, Averill and March (40) illustrated the differ­ences in the separation of early peaks in complex natural samples, such as gasolines and essential oils, when

changing the film thickness from 0.25 μΐη to 0.50 μπι; in fact, columns with a 0.50-μπι film (in addition to 0.25-μΐη film columns) were commercially introduced at that time. Continuing these investigations, in 1979 Johansen (41) published his pioneering study on the characteristics of 0.27-mm i.d. col­umns coated with l-μΐη film.

The introduction of immobilized phases finally permitted the prepara­tion of open-tubular columns with film thicknesses well above 1 μιη. The first detailed reports on such columns, with film thicknesses up to 8 μπα., were published in 1983 almost simulta­neously by the Grobs (42), Sandra (43), and ourselves (44,45). Such col­umns, particularly with the simulta­neous increase of the tube diameter, represent an exciting new field in the application of open-tubular columns.

Although this discussion dealt only

1430 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985