Thin Layer Chromatography Prashant Pandey M.Pharm (Pharmaceutics) PPT/PDF

Save (0)



Thin Layer

Prashant Pandey
M.Pharm (Pharmaceutics)


• Chromatography is a physical method of separation in which the components to

be separated are distributed between two phases, one of which is stationary
(stationary phase) while the other (the mobile phase) moves in a definite direction.

• Types of Chromatographic Techniques:

Technique Stationary Mobile Phase

Column/Adsorption Chromatography solid Liquid

Partition Chromatography Liquid Liquid

Paper Chromatography Liquid Liquid

Thin Layer Chromatography (TLC) Liquid/Solid Liquid

Gas – Liquid chromatography (GLC) Liquid gas

Gas – Solid Chromatography (GSC) Solid gas

Ion Exchange Chromatography Solid Liquid


TLC is one of the simplest, fastest, easiest and least
expensive of several chromatographic techniques used in
qualitative and quantitative analysis to separate organic
compounds and to test the purity of compounds.

TLC is a form of liquid chromatography consisting of:
A mobile phase (developing solvent) and
A stationary phase (a plate or strip coated with a
form of silica gel)
Analysis is performed on a flat surface under
atmospheric pressure and room temperature


• Thin Layer Chromatography can be defined as a method of separation or

identification of a mixture of components into individual components by
using finely divided adsorbent solid / (liquid) spread over a glass plate and
liquid as a mobile phase.

• Synonyms: Drop, strip, spread layer, surface chromatography and open
column chromatography

– Adsorption or retention or partition or both or any other
principle of a substance (s ) on the stationary phase

– Separation of the adsorbed substances by the mobile phase
– Recovery of the separated substances by a continuous flow

of the mobile phase (elution)
– Qualitative and quantitative analysis of the eluted substances


Michael Tswett is credited as being the father of liquid chromatography.
Tswett developed his ideas in the early 1900’s.

1938:- Izmailov & shraiber described basic principle and used it for
separation of plant extracts.

1944:- Consden, Gorden & Martin started using filter papers for separation of
amino acid.

1950:- Kirchner who used impregnated glass plate coated with alumina,
identified terpenes.

1958:- Ergon stahl introduced a standard equipment for preparing uniform
thin layers of known thickness


When TLC used ?



Where TLC used


Principle of TLC

It is based on the principle of adsorption chromatography or
partition chromatography or combination of both,
depending on adsorbent, its treatment and nature of solvents

The components with more affinity towards stationary phase
travels slower.

Components with less affinity towards stationary phase travels


• In TLC, a solid phase, the adsorbent, is coated onto a solid
support (thin sheet of glass, plastic, and aluminum ) as a thin
layer (about 0.25 mm thick). In many cases, a small amount of
a binder such as plaster of Paris is mixed with the absorbent to
facilitate the coating.

• The mixture (A + B) to be separated is dissolved in a solvent
and the resulting solution is spotted onto the thin layer plate
near the bottom. A solvent, or mixture of solvents, called the
eluatant, is allowed to flow up the plate by capillary action.
At all times, the solid will adsorb a certain fraction of each
component of the mixture and the remainder will be in
solution. Any one molecule will spend part of the time sitting
still on the adsorbent with the remainder moving up the plate
with the solvent. A substance that is strongly adsorbed (say, A)
will have a greater fraction of its molecules adsorbed at any
one time, and thus any one molecule of A will spend more time
sitting still and less time moving and vice versa.



• Separation of mixtures in microgram quantities by
movement of a solvent across a flat surface; components
migrate at different rates due to differences in solubility,
adsorption, size or charge; elution is halted when or before
the solvent front reaches the opposite side of the surface
and the components examined in situ or removed for
further analysis.



Separations in TLC involve distributing a mixture of two or
more substances between a stationary phase and a mobile phase

1.The stationary phase:
is a thin layer of adsorbent (usually silica gel or alumina) coated
on a plate.

2.The mobile phase:
is a developing liquid which travels up the stationary phase,
carrying the samples with it.
Components of the samples will separate on the stationary phase
according to:
how much they adsorb on the stationary phase versus
how much they dissolve in the mobile phase


Basic Theory







Factors affecting Rf value

It depends on following factors:
Nature adsorbent
Mobile phase
Thickness of layer
Dipping zone
Chromatographic techniques


Selection of Stationary Phase
The choice of the stationary phase for a given separation problem
is the most difficult decision in TLC

The chose of stationary Phase in following characters

The chemical composition of the stationary Phase and in
particular that of its surface, must be suitable for the task. To
obtain satisfactory separation efficiency, the mean particle size,
the particle size distribution and the morphology of the particle
must be considered


Stationary phases for thin-layer chromatography


The choice of mobile phase is largely empirical but general rules can be
formulated. A mixture of an organic solvent and water with the addition of
acid, base or complexing agent to optimize the solubility of the components
of a mixture can be used. For example, good separations of polar or ionic
solutes can be achieved with a mixture of water and n-butanol. Addition of
acetic acid to the mixture allows more water to be incorporated and increases
the solubility of basic materials, whilst the addition of ammonia increases the
solubility of acidic materials. If the stationary phase is hydrophobic, various
mixtures of benzene, cyclohexane and chloroform provide satisfactory mobile
phases. It should be emphasized that a large degree of trial and error is
involved in their selection. For TLC on silica gel, a mobile phase with as low a
polarity as possible should be used consistent with achieving a satisfactory
separation. Polar solvents can themselves become strongly adsorbed thereby
producing a partition system, a situation which may not be as desirable


Least Eluting Power
(alumina as adsorbent) Mobile Phase

-Petroleum ether
(hexane; pentane) • The eluting solvent should also show

-Cyclohexane a maximum of selectivity in its ability
-Carbon to dissolve or desorbs the substances

tetrachloride being separated.
– • A more important property of the

Dichloromethane solvent is its ability to be itself
-Chloroform ; – adsorbed on the adsorbent.

Ether -Ethyl acetate
• A number of common solvents in

-Acetone approximate order of increasing

(anhydrous) adsorb ability, and hence in order of
-Ethanol ; increasing eluting power.

-Methanol • Mixtures of solvents can be used
-Water ; – and, since increasing eluting power

Pyridine results (0.5 to 2% by volume)
Greatest Eluting Power

• solvents to be used in
(alumina as adsorbent) chromatography should be quite dry

– Organic acids


Selection of adsorbents

Solubility of compound e.g, hydrophilic or lipophilic

Nature of substance to be seperated i.e whether it is acidic,

basic or amphoteric

Adsorbent particle size

Adsorbent should not adhere to glass plate

Reactivity of compound with the solvent or adsorbent

Chemical reactivity of compounds with binders


Chromatographic media-coating material

That in experiments performed to solve various problems by the adsorption
method the use of various sorbents would be necessary. They tested
various substances, including aluminum oxides, aluminum silicates, calcium
carbonate,kaolin, kieselguhr, magnesium oxide, powdered sugar, silica gels,
starch and talc

The separation efficiency obtained in TLC is essentially determined by the
mean particle size and the size distribution of the sorption agent used in the
preparation of the layer. As can be seen from Fig. Below, the mean particle size
of silica gel of a quality suitable for HPTLC is 5 m, that of TLC quality ca. 11 m
and that of PSC quality over 20 m.














What Are the Uses of Precoated Layers

TLC investigations are mainly concerned wit the
determination of

• Identity
• Purity
• Assay
• or a combination of these parameters



• Glass plates or flexible plates are commonly used for
adsorbent. Size used depends on type of separation
to be carried out, the type of chromatographic tank
and spreading apparatus available.

• The standard sizes are 20 x 5 cm, 20 x 10 cm or 20 x
20 cm .

• The surface should be flat without irregularities.
• The standard film thickness is 250um


Methods for application of adsorbent.



• Pouring: The adsorbent of finely divided and
homogeneous particle size is made into slurry and is
poured on a plate and allowed to flow over it so that
it is evenly covered.

• Dipping : This technique is used for small plates by
dipping the two plates at a time, back to back in a
slurry of adsorbent in chloroform or other volatile
solvents. Exact thickness of layer is not known and
evenness of layer may not be good.


• Spraying : Slurry is diluted further for the operation
of sprayer. But this technique is not used now a days
as it is difficult to get uniform layer.

• Spreading : All the above methods fail to give thin
and uniform layers. Modern methods utilize the
spreading devices for preparation of uniform thin
layers on glass plates. Commercial spreaders are of
two types (a) Moving spreader, (b) Moving plate
It gives layer thickness from 0.2 to 2.0 mm.



• After spreading plates are allowed to dry in air
and further dried and activated by heating at
about 1000c for 30 mins.

• By removing the liquids associated with layer
completely, the adsorbent layer is activated.


Solvent Systems
The solvent system performs the following main tasks:
• To dissolve the mixture of substances,
• To transport the substances to be separated across the sorbent layer,
• To give hRf values in the medium range, or as near to this as possible,
• To provide adequate selectivity for the substance mixture to be separated.

They should also fulfill the following requirements:
• Adequate purity,
• Adequate stability,
• Low viscosity,
• Linear partition isotherm,
• A Vapor pressure that is neither very low nor very high,
• Toxicity that is as low as possible



• The choice of the mobile phase is depends upon
the following factors:-

1. Nature of the substance to be separated
2. Nature of the stationary phase used
3. Mode of chromatography ( Normal phase or

reverse phase)
4. Separation to be achieved- Analytical or



• The organic solvent mixture of low polarity is used
Highly polar solvents are avoided to minimize
adsorption of any components of the solvent
mixture. Use of water as a solvent is avoided as it
may loosen the adhesion of a layer on a glass plate.

• Solvents with an increasing degree of polarity are
used in liquid-solid or adsorption chromatography.
The solvents listed in elutropic series are selected.


Storage of solvents

Storage of solvents is unnecessary if they are used in a TLC
chamber immediately after they have been prepared. However, it
is sometimes that certain solvent systems can be stored for
several months. In this case, the best advice is to store them in a
dark bottle in a cool place. The “daily quota” of a solvent system
should also be kept cool in the summer, e.g. if laboratory
temperatures exceed 25 °C. Care must be taken to adjust the
temperature to room temperature before the development


Some type of solvents
1 n-Heptane
2 n-Hexane
3 n-Pentane
4 Cyclohexane
5 Toluene
6 Chloroform
7 Dichloromethane
8 Diisopropyl ether
9 tert-Butanol
10 Diethyl ether
11 Isobutanol
12 Acetonitrile
13 Isobutyl methyl ketone
14 2-Propanol
15 Ethyl acetate
16 1-Propanol
17 Ethylmethyl ketone


• n-Hexane
• Cyclohexene
• Toluene
• Benzene
• Diethyl ether
• Chloroform
• Dichloromethane
• 1,2 dichloroethane Increasing
• Acetone polarity
• Ethyl acetate
• Acetonitrile
• Propanol
• Methanol
• Acetic acid
• Water.



• Sample solution in a non polar solvent is applied.
• The concentration of a sample or standard solution

has to be minimum of a 1% solution of either
standard or test sample is spotted using a capillary
tube or micropipette.

• The area of application should be kept as small as
possible for sharper and greater resolution.


Sample Application (spotting)

TLC plate

 1 cm.
Process “finishing line”

A. Draw “guide lines”

lightly with pencil

B. Dissolve solid
sample in CH2Cl2

C. Use TLC capillary
to transfer and spot
dissolved sample “starting line”

 1 cm.

T-stillbene benzoic acid

9-fluorenone unknown


How to Run Thin Layer Chromatography

• Step 1: Prepare the developing container

• Step 2: Prepare the TLC plate

• Step 3: Spot the TLC plate

• Step 4: Develop the plate

• Step 5: Visualize the spots


Preparation of the developing container
• It can be a specially designed

chamber, a jar with a lid, or a beaker
with a watch glass on the top

• Pour solvent into the chamber to a
depth of just less than 0.5 cm.

• To aid in the saturation of the TLC
chamber with solvent vapors, you
can line part of the inside of the
beaker with filter paper.

• Cover the beaker with a watch
glass, swirl it gently.

• Allow it to stand while you prepare
your TLC plate.


Preparation of the TLC plate
1. Pouring, Dipping, Spraying, Spreading
2. TLC plates used are purchased as 5 cm x 20 cm sheets.
Each large sheet is cut horizontally into plates which are 5
cm tall by various widths;
3. Handle the plates carefully so that you do not disturb
the coating of adsorbent or get them dirty.Measure 0.5
cm from the bottom of the plate.
4. Using a pencil, draw a line across the plate at the 0.5
cm mark. This is theorigin: the line on which you will spot
the plate. Take care not to press so hard with the pencil
that you disturb the adsorbent.
5. Under the line, mark lightly the samples you will spot
on the plate, or mark numbers for time points. Leave
enough space between the samples so that they do not
run together; about 4 samples on a 5 cm wide plate is


Spot the TLC plate
• Prepare 1% solution of drug dissolving in volatile

solvents like hexanes, ethyl acetate, or methylene

• Dip the microcap or microcapillary into the solution
and then gently touch the end of it onto the proper
location on the TLC plate.

• Don’t allow the spot to become too large – if
necessary, you can touch it to the plate, lift it off and
blow on the spot. If you repeat these steps, the wet
area on the plate will stay small.

• This example plate has been spotted with three
different quantities of the same solution and is ready
to develop


Thin Layer Chromatography Column Development

• Place the prepared TLC plate in the developing
beaker, cover the beaker with the watch glass, and
leave it undisturbed on your bench top.

• The solvent will rise up the TLC plate by capillary
action. Make sure the solvent does not cover the

• Allow the plate to develop until the solvent is about
half a centimeter below the top of the plate.

• Remove the plate from the beaker and immediately
mark the solvent front with a pencil


Visualize the spots
• If there are any colored spots, circle them

lightly with a pencil.

• Most samples are not colored and need to
be visualized with a UV lamp.

• Hold a UV lamp over the plate and circle
any spots you see.

• Make sure you are wearing your goggles
and do not look directly into the lamp.
Protect your skin by wearing gloves


Chromogenic reagents for visualizing thin-
layer chromatograms


General Review of preparation of materials

• The thin layer chromatography plates are commercial pre-prepared
ones with a silica gel layer on a glass, plastic, or aluminum backing.
Use the wide plates for spotting several compounds on the same
plate. This allows for more precise comparison of the behavior of the

• The samples are spotted on the thin layer plates using fine capillaries
drawn from melting point capillaries. You will need to draw several

• Samples for spotting are prepared by dissolving approximately 0.1 g
(the amount on the tip of a spatula) of the compound in less than 0.5
mL of a solvent (ethyl acetate, dichloromethane, or ether work well).


General Review of preparation of materials
• When spotting samples on the TLC plates, it is a good idea to

check if enough sample has been spotted on the plate. Allow
the solvent to evaporate and then place the plate under a short
wavelength ultraviolet lamp. A purple spot on a background
of green should be clearly visible. If the spot is faint or no
spot is apparent, more sample will have to be applied to the

• The chromatograms are developed in a 150-mL beaker or jar
containing the developing solvent. The beaker is covered with
a small watch glass. A wick made from a folded strip of filter
paper is used to keep the atmosphere in the beaker saturated
with solvent vapor.


General Review of preparation of materials
• When the plates are removed from the developing solvent, the

position of the solvent front is marked, and

• the solvent is allowed to evaporate. The positions of the spots
are determined by placing the plates under a short wavelength
ultraviolet lamp.

• The silica gel is mixed with an inorganic phosphor which
green in the UV light. Where there are compounds on the
plates, the fluorescence is quenched and a dark purple spot


TLC Developing Chambers

• Ascending development,
• Descending development,
• Horizontal development.





Vertical Development

1. Solvent in Liquid-Vapour equilibrium

2. Solvent in Vapour adsorbs on the layer

3. Solvent migrating in the layer vaporizes

Effect of gravity

In pre-
Migration saturated
distance chamber

In non

Analysis time


Horizontal Development

1. HPTLC plate (layer facing down)
No effect of gravity

2. glass plate for sandwich configuration
Migration speed is constant

3. reservoir for developing solvent
Better resolutions can be

4. glass strip

5. cover plate
6. conditioning tray

Better control of the operating conditions
(saturation, evaporation)

Possibility to develop both sides of the plate
= Twice more samples


Development of Thin-Layer Chromatograms
• 1. One-dimensional development
• Single development
• – vertical
• – horizontal, in one direction
• – horizontal, in opposite directions
• – circular
• – anticircular
• Multiple development
• – separate runs over the same migration distance
• – stepwise, increasing
• – stepwise, decreasing
• – automated multiple development, stepwise with

solvent gradient


2. Two-dimensional development
• Two dimensions, one solvent system
• Two dimensions, two solvent systems
• SRS (separation in 1st dimension chemical

reaction separation in 2nd dimension)


One-Dimensional Development

Most thin-layer chromatograms are produced in
one dimension, and in fact even today it is very
difficult to obtain quantitative results from
plates developed in more than one dimension.
All present-day commercially available TLC
scanners therefore operate on the principle of a
one-dimensional chromatographic lane.



Two-Dimensional Development

• More complete separation of sample
components can be achieved by two-
dimensional development. In this process, the
plate is developed normally and following
complete drying, it is turned 90o and the
development of the plate is continued. This
second development is performed using a
different mobile phase with very different
selectivity (otherwise little further separation
would result).





Above Figure. Position of the urine amino acids on a 20 × 20 cm
TLC plate after a two-dimensional development and subsequent
derivatization with the ninhydrin reagent


Applications of TLC
• It is used for separation of all classes of natural products and is

established as an analytical tool in modern pharmacopoeias.
– E.g. Acids, alcohols, glycols, alkaloids, amines,

macromolecules like amino acids, proteins and peptides, and

– for checking the purity of samples
– as a purification process
– examination of reaction
– for identifying organic compounds

• Extensively used as an identification test and test for purity.
• As a Check on process – checking of distillation fractions and for

checking the progress of molecular distillation.


Applications of TLC
• Applications of TLC for separation of Inorganic Ions – Used

for separating cationic, anionic, purely covalent species and
also some organic derivatives of the metals.

• Separation of Amino Acids- two dimensional thin – layer

• Separation of vitamins – vitamin E, Vitamin D3, vitamin A

• Application of TLC in quantitative analysis




HPTLC is a sophisticated & automated form of TLC
Efficient separation in short time


• HPTLC is a form of thin-layer chromatography (TLC) that

provides superior separation power using optimized coating
material, novel procedures for mobile-phase feeding, layer
conditioning, and improved sample application.

• The basic difference between conventional TLC and HPTLC
is only in particle and pore size of the sorbents.

• The principle of separation is similar that of TLC adsorption.

• It is very useful in quantitative and qualitative analysis of




Advantages of HPTLC Over Other Chromatographic

1. In HPTLC, simultaneous processing of sample and

standard – better analytical accuracy & precision
2. Lower analysis time & less cost per analysis
3. HPTLC is very simple
4. In HPTLC, the sample preparation is simple


5. Solvent used in HPTLC needs no prior treatment
like filtration & degassing
6. In HPTLC, the M.P consumption for sample is
extremely low
7. HPTLC allows the use of corrosive & UV
absorbing M.P


Advantages of HPTLC
8. It promotes high separation efficiencies/ resolution of

zones due to higher number of theoretical plates.
9. Shorter developing times or analysis time
10. Lower amounts of mobile phase / solvent consumption
11. Enormous flexibility
12. Parallel separation of many samples with minimal time

13. Simplified sample preparation due to single use of the

stationary phase.
14. Efficient data acquisition and processing



1.Sample preparation
2.Selection of chromatographic layer
6.Sample application


9.Chromatographic development
10.Detection of spots
11.Scanning & documentation



HPTLC: Separation and Resolution
To which extent various components of a formulation are

separated by a given HPTLC system is the important factor
in quantitative analysis. It depends on the following factors:

• Type of stationary phase
• Type of precoated plates
• Layer thickness
• Binder in the layer
• Mobile phase
• Solvent purity


Above slide Continue

• Size of the developing chamber
• Saturation of chamber
• Sample’s volume to be spotted
• Size of the initial spot
• Solvent level in the chamber
• Gradient
• Relative humidity
• Temperature
• Flow rate of solvent
• Separation distance
• Mode of development


Validation process involved in HPTLC


Type of analytical procedures and required
validation characteristics


Basic acceptance criteria for evaluation validation experiments-
(Ferenczi-Fodor et al. 2001; Patel et al. 2010)





Sample preparation

1.For normal phase chromatography using silica
gel / alumina pre-coated plates, solvents – non
2.RP chromatography , usually polar solvents

Selection of Chromatographic layer
» Depends on the nature of material to be

Commonly used materials are Silica gel 60F,
Alumina, Cellulose etc




» to remove water vapors
» volatile impurities
Which might get trapped in the plates
To avoid this, plates are cleaned by using
methanol as solvent by ascending or
descending etc.



Plates activated by placing them in an oven at
120°C for 15 to 20 minutes.

Sample Application
Application of 1.0 – 5µl for HPTLC
Application carried out by Linomat applicator
on the plates which give uniform, safe & std.


Sample Application
Usual concentration of applied samples 0.1 to 1 µg / µl for

qualitative Analysis and quantity may vary in quantization based
on UV absorption 1 to 5 µl for spot and 10 µL for band application.

Manual with calibrated capillaries
Semi and auto-application through applicators

• Applicators use spray on or touch and deliver technique for


Manual Sample Applicator
• The Nanomat serves for easy application of

samples in the form of spots onto TLC and
HPTLC layers .

• The actual sample dosage performed with
disposable capillary pipettes , which are
precisely guided by the capillary holder.

The nanomat is suitable for
• Conventional TLC plates including self-

coated Plates up to 20 × 20cm
• HPTLC plates 10 × 10 cm and 20 × 10 cm
• TLC and HPTLC sheets up to 20 × 20 cm


Semi automatic sample applicator
• The instrument is suitable for routine use for

medium sample throughout . In contrast to the
Automatic TLC sampler , changing the sample
the Linomat requires presence of an operator.

• With the linomat , samples are sprayed onto
the chromatographic layer in the form of
narrow bands.

• During the spraying the solvent of the sample
evaporates almost entirely concentrating the
sample into a narrow band of selectable length.


Automatic Sample Applicator
• Samples are either applied as spots
through contact transfer (0.1-5 micro lit)
or as bands or rectangles (0.5->50 micro lit)
using the spray on techniques.

• Application in the form of rectangles
allow precise applications of large volume
with out damaging the layer.

• ATS allows over spotting.


Sample Application parameter on HPTLC plate


Chromatographic development

Ascending, descending, horizontal,
continuous, gradient, multidimensional…

HPTLC – migration distance of 5-6mm is
sufficient, after development, plates removed
& dried.

Common problems encountered during
chro. Development are as follows…
1. Tailing: due to the presence of traces of
impurities, this can be reduced by buffering
the M.P


2.DIFFUSION: This is seen as zones on
chromatographic plates. This may arise
due to non-uniformity of M.P


DEVELOPING CHAMBER – Twin trough chamber
• Low solvent consumption: 20 mL of solvent is

sufficient for the development of a 20x20cm plate.
This not only saves solvent , but also reduces the
waste disposal problem

• Reproducible pre –equilibrium with Solvent
vapor: For pre-equilibration, the TLC plate is placed
in the empty trough opposite the trough which
contains the pre-conditioning solvent. Equilibration
can be performed with any liquid and for any period
of time.

• Start of development : It is started only when
developing solvent is introduced into the trough with
the plate.


Automatic developing chamber (ADC)
• In the ADC this step is fully automatic

and independent of
environmental effects.

• The activity and pre-conditioning of
the layer , chamber saturation developing
distance and final drying can be pre-set
and automatically monitored by ADC.



Detection can be done by iodine vapor in
iodine chamber. Visual inspection at 254nm of
UV region in UV cabinet


Scanning & Documentation

1.HPTLC plates are scanned at selected
UV regions WL by the instrument & the
detected spots are seen on computer in
the form of peaks.

2.The scanner converts band into peaks
& peak height or area is related to the
concentration of the substance on the



Detection: Direct optical evaluation under 254-nm UV light using the TLC-Scanner II
The scans show the following pesticides from left to right:
Hexazin, Metoxuron, Monuron, Aldicarb, Azinphos methyl, Prometryn, Pyridat, Trifluralin
Sample volume: 50 nl, normal chamber without chamber saturation, solvent system:
petroleum ether (40–60 °C) + acetonitrile (70 + 30 v/v), migration distance: 7 cm.





Application of HPTLC Seperation
• Multidimensional and multimodal seperation by HPTLC in

• Stability-indicating HPTLC determination of imatinib mesylate in

bulk drug and pharmaceutical dosage
• A Quality control for authentication of herbal photochemicals
• Herbal drug quantification
• Determination of artemisinin and its derivatives in bulk

pharmaceutical dosages
• Biomedical application.


Comparison between HPTLC and TLC on the basis of parameters


Features of HPLC & HPTLC