Chromatography is a laboratory technique used for separating mixtures based on the differential movement of compounds through a mobile phase and a stationary phase. Thin Layer Chromatography (TLC), a type of planar chromatography, is widely used for analyzing mixtures and is characterized by its quick development time, simplicity, and efficiency in compound visualization and identification. It employs various techniques, with applications in purity checking, compound identification, and sample purification.

1.  Chromatography is a laboratory
technique for the separation of a mixture. The mixture
is dissolved in a fluid called the mobile phase, which
carries it through a structure holding another material
called the stationary phase. The various constituents
of the mixture travel at different speeds, causing them
to separate. The separation is based on differential
partitioning between the mobile and stationary
phases. Subtle differences in a compound's partition
coefficient result in differential retention on the
stationary phase and thus affect the separation.
Chromatography may be preparative or analytical. The
purpose of preparative chromatography is to separate the
components of a mixture for later use, and is thus a form
of purification.

2.  Chromatography Techniques
 Techniques by chromatographic bed
shape
Column chromatography
Planar chromatography
Paper chromatography
Thin layer chromatography (TLC)
 Techniques by physical state of
mobile phase
Gas chromatography
Liquid chromatography
 Affinity chromatography
Supercritical fluid chromatography

3.  Displacement chromatography
 Techniques by separation mechanism
Ion exchange chromatography
•
Size-exclusion chromatography
Expanded bed adsorption chromatographic
separation
 Special techniques
Reversed-phase chromatography
Hydrophobic interaction chromatography
Two-dimensional chromatography
Simulated moving-bed chromatography
gas chromatography
Fast protein liquid chromatography
Countercurrent chromatography
Periodic counter-current chromatography
Chiral chromatography
Aqueous normal-phase chromatography

4.  Thin layer chromatography (TLC)
 TLC is a type of planar chromatography.
 It is routinely used by researchers in the
field of phyto-chemicals, biochemistry, and
so forth, to identify the components in a
compound mixture, like alkaloids,
phospholipids, and amino acids.
 It is a semi quantitative method consisting
of analysis.
 High performance thin layer
chromatography (HPTLC) is the more
sophisticated or more precise quantitative
version.

5.  Principle
Similar to other chromatographic methods, thin layer
chromatography is also based on the principle of separation.
The separation depends on the relative affinity of compounds
towards stationary and the mobile phase.
• The compounds under the influence of the mobile phase
(driven by capillary action) travel over the surface of the
stationary phase. During this movement, the compounds
with higher affinity to stationary phase travel slowly while
the others travel faster. Thus, separation of components in
the mixture is achieved.
Once separation occurs, the individual components are
visualized as spots at a respective level of travel on the plate.
Their nature or character are identified by means of suitable
detection techniques.

6. System Components
TLC system components consists of
• TLC plates, preferably ready made with a stationary phase:
These are stable and chemically inert plates, where a thin layer of
stationary phase is applied on its whole surface layer. The
stationary phase on the plates is of uniform thickness and is in a
fine particle size.
• TLC chamber. This is used for the development of TLC
plate. The chamber maintains a uniform environment inside for
proper development of spots. It also prevents the evaporation of
solvents, and keeps the process dust free.
• Mobile phase. This comprises of a solvent or solvent
mixture The mobile phase used should be particulate-free and of
the highest purity for proper development of TLC spots. The
solvents recommended are chemically inert with the sample, a
stationary phase.

7. • A filter paper. This is moistened in the mobile phase, to be
placed inside the chamber. This helps develop a uniform rise in a
mobile phase over the length of the stationary phase.

8. Plate preparation
TLC plates are usually commercially available, with
standard particle size ranges to improve reproducibility.
They are prepared by mixing the adsorbent, such
as silica gel, with a small amount of inert binder
like calcium sulfate (gypsum) and water. This mixture is
spread as a thick slurry on an unreactive carrier sheet,
usually glass, thick aluminum foil, or plastic. The
resultant plate is dried and activated by heating in an
oven for thirty minutes at 110 °C. The thickness of the
absorbent layer is typically around 0.1 – 0.25 mm for
analytical purposes and around 0.5 – 2.0 mm for
preparative TLC.

9.  Procedure
 The stationary phase is applied onto the plate uniformly and then
allowed to dry and stabilize. These days, however, ready-made
plates are preferred.
 With a pencil, a thin mark is made at the bottom of the plate to
apply the sample spots.
 Then, samples solutions are applied on the spots marked on the
line in equal distances.
 The mobile phase is poured into the TLC chamber to a leveled
few centimeters above the chamber bottom. A moistened filter
paper in mobile phase is placed on the inner wall of the chamber
to maintain equal humidity (and also thereby avoids edge effect
this way).
 Now, the plate prepared with sample spotting is placed in TLC
chamber so that the side of the plate with the sample line is
facing the mobile phase. Then the chamber is closed with a lid.

10.  The plate is then immersed, such that the sample spots are well
above the level of mobile phase (but not immersed in the solvent
— as shown in the picture) for development.
 Allow sufficient time for the development of spots. Then remove
the plates and allow them to dry. The sample spots can now be
seen in a suitable UV light chamber, or any other methods as
recommended for the said sample.

11. Development of a TLC plate, a
purple spot separates into a
red and blue spot

12.  Analysis
• fluorescent analytes like quinine may be detected
under blacklight (366 nm)
• Often a small amount of a fluorescent compound, usually manganese-
activated zinc silicate, is added to the adsorbent that allows the
visualization of spots under UV-C light (254 nm). The adsorbent layer
will thus fluoresce light-green by itself, but spots of analyte quench this
fluorescence.
• Iodine vapors are a general unspecific color reagent
• Specific color reagents into which the TLC plate is dipped or which are
sprayed onto the plate exist.
• Potassium permanganate - oxidation
• Bromine
• In the case of lipids, the chromatogram may be transferred to
a PVDF membrane and then subjected to further analysis, for
example mass spectrometry, a technique known as Far-Eastern blotting.
• Once visible, the Rf value, or retardation factor, of each spot can be
determined by dividing the distance the product traveled by the
distance the solvent front traveled using the initial spotting site as
reference.

13. Advantages
o It is a simple process with a short development time.
o It helps with the visualization of separated compound
spots easily.
o The method helps to identify the individual
compounds.
o It helps in isolating of most of the compounds.
o The separation process is faster and the selectivity
for compounds is higher (even small differences in
chemistry is enough for clear separation).
o The purity standards of the given sample can be
assessed easily.
o It is a cheaper chromatographic technique.

14.  Applications
o To check the purity of given samples.
o Identification of compounds like acids, alcohols, proteins,
alkaloids, amines, antibiotics, and more.
o To evaluate the reaction process by assessment of
intermediates, reaction course, and so forth.
o To purify samples, i.e for the purification process.
o To keep a check on the performance of other separation
processes.
o Being a semi quantitative technique, TLC is used more for rapid
qualitative measurements than for quantitative purposes. But
due its rapidity of results, easy handling and inexpensive
procedure, it finds its application as one of the most widely used
chromatography techniques.