Page 125 - phytochemistry general program

b- Stepwise development in more than one solvent.

It should be noted that after each development, the chromatoplates should be air-
dried before the next development.

(3) Two-dimensional development. It is carried out for separating mixtures
containing many components having different wide properties. The sample is
spotted at one corner of a square plate, developed in one solvent system,
removed, air-dried then rotated 90o and redeveloped usually in a second solvent

system.

(4) Continuous development, where the solvent is allowed to run off the top of the
plate over a long period of time. No solvent fronts are attained.

(5) Centrifugal (chromatotron), it is used mainly for preparative purposes. The rotor
is allowed to rotate mechanically at about 1000 rpm, where separated components

appear as concentric zones.

Visualization (Location or detection of spots) After development, plates are air- .3
dried before the detection of resolved substances on the chromatogram. .1

Visualization can be divided into two methods, universal and specific.

Universal methods are methods which can detect most organic compounds. These
can be either destructive or non-destructive. Destructive methods alter the
substances on the chromatogram irreversibly, while non-destructive methods do
not change the nature of substances to be isolated. The later is useful, particularly

in cases of some quantitative as well as preparative TLC.

Destructive methods

Universal methods, Where corrosive reagents are sprayed and the .1
chromatogram is heated in an oven. Most nonvolatile organic substances on
inorganic layers containing no organic binders give charred spots. Examples
of these reagents are: sulfuric acid, sulfuric-nitric acids, perchloric acid and

chromic acid.

Specific methods These are usually destructive and depend on specific color .2
reaction with certain spray reagents. They are usually used for qualitative
and some quantitative applications of TLC. Examples of these specific

reagents are shown:

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