Silica Gel Bonded Phases
Reversed-Phase Bonded Layers
Reversed-phase TLC is used more now because the bonded phases eliminate previous attempts to use coated layers, which required
mobile phases saturated with the coating agent, paraffin, or silicone oils.
Similar to HPLC, alkylsiloxane-bonded silica gel 60 with dimethyl (RP-2 or C2), octyl (RP-8 or C-8), and octadecyl (RP-18
or C18) functional groups are most widely used for reversed-phase TLC of organic compounds (polar and nonpolar, homologous
compounds and aromatics), weak acids and bases after ion suppression with buffered mobile phases, and polar ionic compounds
using ion-pair reagents. Layers from different companies with the same bonded group can have different percentages of carbon
loading and give different separation results. The hydrophobic nature of the layer increases with both the chain length and
the degree of loading of the groups. Alkylsiloxane-bonded layers with a high level of surface modification are incompatible
with highly aqueous mobile phases and are used mainly for normal phase separations of low polarity compounds. Problems of
wettability and lack of migration of mobile phases on reversed-phase plates with high proportions of water have been solved
by adding 3% NaCl to the mobile phase to attain better wettability (C18 layers; Whatman/GE Healthcare) or preparing "water-wettable"
layers (for example, RP-18W; EMD/Millipore) with less exhaustive surface bonding to retain a residual number of silanol groups.
The latter layers with a low degree of surface coverage and more residual silanol groups exhibit partially hydrophilic, as
well as hydrophobic character and can be used for reversed-phase TLC and normal-phase TLC with purely organic, aqueous–organic,
and purely aqueous mobile phases.
Chemically bonded phenyl layers are also classified as reversed phase; their separation properties have been reported (23)
to be very similar to RP-2 although the aromatic diphenyl bonded groups (with their π-π interaction possibilities) would appear
to be very different than ethyl groups. RP-2 layers function with a normal-phase mechanism when developed with purely organic
mobile phases and a reversed-phase mechanism with aqueous–organic or purely aqueous mobile phases.
Reversed-Phase Bonded Layers for Enantiomer Separations
To make a chiral TLC plate it is necessary to coat or bond various chiral phases. These reagents are costly to make, and any
resulting plate would be too expensive for one use and disposal. Only one commercial layer is available under the name Chiralplate
(Macherey-Nagel) for separation of enantiomers by the mechanism of ligand exchange. These consist of a glass plate coated
with C18 bonded silica gel and impregnated with the Cu(II) complex of (2S, 4R, 2'RS)-N-(2'-hydroxydodecyl)-4-hydroxyproline as a chiral selector.
As an alternative, it is possible to do chiral separations on standard plates with the chiral selector (that is, β-cyclodextrin
or vancomycin) added to the mobile phase or impregnated into the layer. Examples of such separations and layers used are discussed
in two book chapters (26,27).
Hydrophilic Bonded Layers
Hydrophilic bonded silica gel containing cyano, amino, or diol groups bonded through short-chain nonpolar spacers (a trimethylene
chain [-(CH2)3-] in the case of NH2 and CN plates) are wetted by all solvents, including aqueous mobile phases, and exhibit multimodal mechanisms. Polarity varies
as follows: unbonded silica > diol-silica > amino-silica > cyano-silica > reversed-phase materials (24).
In the future, these layers might be of use with hydrophilic interaction chromatography (HILIC) as this mode gains favor for
analyzing polar mixtures with polar mobile phases in HPLC and TLC.