Page 121 - phytochemistry general program
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(3) Have known uniform layer thickness for both analytical and preparative uses.
(4) Require no activation before use.
(5) Hard enough, tolerable to damage.
b. Coating materials
(1) Silica gel, silicic acid, silica
The most important TLC adsorbent. The sorption characteristics of silica gel are
due to the polar surface silanol (SiOH) groups.
Used as NORMAL phase stationary phase.
These are weakly dipolar and strongly hydrogen bonding. The most common silica
gel for TLC is “Silica Gel 60” which has a pore size of 6 nm (60 A )̊ .
Note: .1
.2
When organic acids are to be separated, a small amount of an acid is added,
either included in the layer or in the developing solvent.
When used for separating bases (e.g. alkaloids) ammonia or other bases are
usually added to the developing solvent.
Why? To retain the compound in the non-ionised form.
(2) Chemically Bonded Silica Gel Layers
The reactive silanol groups on the silica surface facilitate chemical modification by
reaction with organosilane reagents containing different substituent groups. These
materials are called chemically bonded phases and are suitable for a wide range
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