Reversed Phase Plates

The number of available stationary phases in TLC is not very large. Silica is still widely used and some separations are better carried out on alumina, cellulose or polyamide. In the above discussion, data were compiled from experiments made with silica on which the contact angle 6' is very large and thus cos 6'= —1. Alkyl-bonded silica plates are readily available and the number of published separations on these supports is rapidly increasing. These bonded silicas are hydrophobic and some velocity constants with pure organic solvents are given in Table 4.

These data were gathered from plates with average practicle size 6.5 |im and an assumed P0 of 7.9 x 10~3. Apolar solvents completely wet the layer but polar solvents exhibit values of cos 6' much lower than 1. These TLC data are consistent with those published by Riedo et al. from capillary rise in Pyrex tubes treated with octadecyltrimethyl silane. There is no evident correlation between cos 6' and a physical property such as dielectric constant or ability to form hydrogen bonds.

With such plates water-organic solvent mixtures are used and a plot of the cosine of the contact angle versus the water concentration in a water/ethanol eluent is shown in Figure 6.

Figure 6 Plot of the cosine of the contact angle versus the water concentration of a water/ethanol mixture. •, home made plates; O, Merck plates (no spacer); □, Merck plates (with 1.5 mm spacer). (Reprinted from Guiochon G, Korosi G and Siouffi A (1979) Journal of Chromatographic Science 18: 324-329.)

Figure 6 Plot of the cosine of the contact angle versus the water concentration of a water/ethanol mixture. •, home made plates; O, Merck plates (no spacer); □, Merck plates (with 1.5 mm spacer). (Reprinted from Guiochon G, Korosi G and Siouffi A (1979) Journal of Chromatographic Science 18: 324-329.)

Figure 7 Plot of time necessary to ascend 4 cm versus percentage of water in mixtures of methanol/water and acetonit-rile/water. acetonitrile; O, methanol. Plate LiChrosorb RP-18 (7 |im).

According to Giddings, we can use the definition of the local plate height Hl as the proportionality constant between the differential increase in the variance of a Gaussian profile spot and the differential length of migration:

Hi"

The average plate height may be defined as:

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