Stationary Phase

As mentioned above, in reversed-phase chromatography - i. e., the chromatographic approach to deriving lipophilicity descriptors - hydrophobic, nonpolar material is used as the stationary phase. In some cases [20, 21] cellulose plates were used. According to Geiss [22] this material has no reversed-phase properties. Silica-gel plates, impregnated by hand with paraffin, silicone or other oils deserve mention here; in addition, commercially available octadecyl silylated silica-gel (ODS) materials are employed. Some typical examples of stationary phases, being far from complete, are summarized in Table 1.

The major part of TLC work has been done by using silica-gel plates, impregnated by hand either with silicone oil (in particular the group of Biagi and coworkers) or with paraffin. Such plates can be moistened with pure water, which explains their common use in lipophilicity determinations by TLC.

For the sake of reproducibility and comparability between laboratories, however, the use of commercially available plates is recommended. Nevertheless, the latter material can also suffer from some drawbacks. Impregnated silica-gel 60 plates from Merck, e. g., seem to decompose, resulting in irregularities of the running of the plates. In addition, this plate material cannot be moistened with solvent mixtures containing more than 60 % water. In the case of all impregnated plates comparability of the TLC data to log Poct values is rather limited (for a detailed comment see section 8. 6.2). HPTLC plates, coated with highly C18 etherified silica-gel, can only be moistened with solvents containing at most 40 % water [23-25]. In our opinion, an extrapolation of TLC data to modifier-free conditions is mandatory (see section 8. 4); small unavoidable

144 8 Estimation of Lipophilicity by Reversed-Phase Thin-Layer Chromatography Table 8.1. Typical examples of stationary phases, used in RP-TLC

Stationary phase

References

Kieselguhr/oleyl alcohol

[51, 68]

Silanized Kieselguhr

[52]

Silica gel impregnated with liquid paraffin

[19, 23, 33, 36, 53, 54]

Silica gel impregnated with silicone oil

[14-17, 41, 55-57]

Silica gel impregnated with octanol

[55, 58]

Silica gel impregnated with tricaprylmethylammonium

[33,35]

Silanized precoated silica gel Merck

[41, 42]

RP8 HPTLC Merck

[25, 58]

RP18 HPTLC Merck

[23, 25, 58]

Whatman KC 18 F

[25, 32, 60-63]

RP2 TLC Merck

[25, 37, 64]

RP8 TLC Merck

[25, 37, 64]

RP18 TLC Merck

[25, 32, 33, 37, 64-66]

errors in estimating RM at low water contents of the solvent will result in unacceptable errors in extrapolating to RMv,

In recent years many investigators prefer the use of ODS-coated plate material. The silica-gel material of RP 18 plates suffers from the low etherification (22%) of the silica-gel OH-groups [26]. Thus, in the case of solvents with low water contents, silano-philic forces become prevalent, resulting in relatively too high Ru (Fig. 1).

Taken together, the use of RP 18 TLC plates demands a high accuracy in extrapolating to RMw, which we discuss in detail in section 8. 4.

Figure 1. RM values of 3,4-dimethoxy-benzoic acid as a function of the modifier fraction of methanol (□) or acetonitrile (o). Solid lines represent the extrapolation lines. The dashed extrapolation line corresponds to a theoretical, complete independence of i?Mw from modifier.
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