Thin-layer chromatography or TLC is a well-established, widely used, technique that is an inexpensive and rapid method to separate and identify different compounds present in a sample or reaction.  This qualitative technique, like HPLC and other chromatographic methods, is available with both normal and reversed-phase adsorbents.  These adsorbents can be alumina, silica gel, bonded-phase silica gel, and many others.  The focus of this paper will be to elucidate the need to test more than normal phase silica gel as an adsorbent when using TLC.

In most cases, normal phase refers to unbonded silica gel and reversed-phase is silica gel bonded with a C18 alkyl chain.  These are both good starting points for most separations are very different in how they separate and elute compound.  However, there are other stationary phases that can provide better separations or alternatives including (in order of decreasing polarity): Silica > Diol > NH2 > CN > C2 > C18-50 > C18-W > C18-100.  Utilizing these different phases, based on the compound of choice, can yield better separation and provide guidance for a larger or smaller scale method.

It is known that currently only 10% of TLC users are using an alternative to normal phase silica TLC. Why? This has become a standard practice, so why deviate?  Well, most chemists use what is readily available in the lab, or, use the same method that a chemist previously referenced. However, there are other options that not only provide a different elution order, but may provide a better separation of the compounds based on some of the differences in molecular interactions.  By using the table below as a guide for your compound as well as the known solubility of the compound of choice, one can begin to see there are alternatives to silica gel for normal phase and C18 for reversed-phase.

Sorbent Material Chromatographic Principle Typical Applications
Silica Gel for Normal Phase
Silica HD, Silica G, Slicia XG
Normal Phase
Most frequent application of all TLC layers.
Nano silica 60, impregnated with caffeine for PAH determination
Charge transfer complexes
For PAH analysis based on charge-transfer complexes
Cyano-modified layer CN
Moderately polar, normal and reversed-phase
Hydrophilic or charged samples, as well as steroids, hormones, phenols, and preservatives.
Diol-modified layer - OH
Moderately polar, normal and reversed-phase
Steroids, pesticides, and plant constituents, less sensitive than silica to the water environment.
Amino-modified layer NH2
Moderately Polar, normal and reversed-phase
Use to separate hydrophilic or charged samples, Vitamins, sugars, steroids, purine derivatives, xanthine, phenols, nucleotides, and pesticides
Bonded Silica Gel for Reversed Phase
C2
Reversed-phase with up to 80% Water, and Normal phase
Polar and Nonpolar substances (lipids, aromatics), lower retention than C18 plates
C18-100, Silanized C18
Reversed-Phase with up to 20% Water
Polar Substances such as: alkaloids, amino acids, barbiturates, polycyclic aromatic hydrocarbons (PAH), drugs, peptides, flavonoids, phenols, indole derivatives, steroids.
C18-50, Silanized C18
Reversed-phase with up to 60% water
Polar Substances such as: alkaloids, amino acids, barbiturates, polycyclic aromatic hydrocarbons (PAH), drugs, peptides, flavonoids, phenols, indole derivatives, steroids.
C18-W, C18 Wettable
Normal and reversed-phase, much faster than silanized C18 counterparts
Polar and non polar substances such as: Aminophenols, barbiturates, preservatives, nucleobases, PAH, steroids, tetracyclines, phthalates