Topic outline

● Description of Planer Chromatography● Description of theoretical principles of Planer

Chromatography

● Degree of retention (Rf)

● Applications

Dr. Khalid Hussain (University College of Pharmacy)

hussain_761@yahoo.com

Planer Chromatography● This chromatography is performed on plane

surface, rather than a column● It offers a unique advantage of 2-Dimentional

operation● Selective properties of 2 different solvents can be

used in developing a single chromatogram● It includes: paper chromatography (PC) and thin

layer chromatography (TLC)/High performance thin layer chromatography (HPTLC)

Theoretical principles● The principles of column chromatography also apply

here i.e.

1- Separation

is accomplished by successive equilibrations of the sample components between two phases, the mobile phase (m) and stationary phase (s).

2- Non-ideal process, similar to column chromatography, cause zone spreading on the plane

● Degree of retention in plane chromatography is expressed as retardation factor (Rf)

Rf = Distance solute moved Distance solvent movedFor substances that are very soluble in the liquid, Rf will be close to ....

For substances that are rather insoluble in the liquid, Rf will be close to ....

1

0

Hence, Rf lies between 0 and 1

Origin Line

Solvent Front Line

Distance traveledby solvent

Distance traveled by spot

Google images

Distance of the solute moved is measured from the centre of the spot

K=Cs/CM

In column chromatography, distribution coefficient is expressed by

The simple relationship between K and Rf is given as follows:

Rf = Number of moles of solute in moving phase /

Number of moles of solute in both phases

Rf= CmAm/ CmAm+CsAs

Here Am and As are the cross-sectional areas of the two phases

Dividing numerator and denominator by CM, we get

Rf =Am

Am+CsAs/Cm

= Am/Am+KAs

Since, K= Cs/CM

Factors effecting Rf

● Rf values are subject to minor influences such as

1- Variations of stationary phase

2- Method of application and development

3- Size of sample

4- Concentration of sample

Paper Chromatography● It is mainly used for qualitative and semi-

quantitative purposes● It is easy to perform● Mechanisms involved: liquid-liquid partition,

adsorption, hydrogen bonding, and ion exchange

Nature of the paper● Made up of highly purified cellulose- a polar

compound- hence has great affinity for water and polar solvents, hold them through H-bonding

● Paper may be impregnated with alumina, silica and ion exchange resin etc.

Procedure● Application of sample as a small spot● Development in a closed chamber saturated with

solvent either by

1- Ascending method:● Simplest and most popular● Solvent ascends through the paper by

capillary action● The rate of ascent is slow and decrease with the

passage of time due to gravity● Slow rate enhances possibility of achieving partition

equilibrium. Google images

2- Descending method: ● Direction of flow of solvent is downward● Paper is folded U-shape to prevent rapid

siphoning of solvent● Solvent descends through gravity and capillary

action● It is much faster than the ascending method

Google images

Detection● After the development, solute may be detected by a number of

methods briefed as:– Inherent visible colors– Derivitization– UV absorbance, fluorescence– IR absorbance– Radioactivity– Chemical tests– Bio-autography

Sinensetine 2

Applications● Qualitative (measurement of Rf value)

● Isolation● Purification● Semi-quantitative

1-Extract the compound from the paper followed my spectrophotometry

2- Densitometric determination.

Techniques● Two dimensional development● Radial development

Sample is spotted at the centre and developed, components move radially forming circles of increasing diameters. Fast separation by moving disc so solvent move by CF and CA

● Reversed-phase chromatography.

Paper is coated with hydrophobic substance, polar phase is used to elute, separation of non-polar compounds

Thin Layer Chromatography

1- Here the mobile phase is a liquid, flowing past a thin layer of powder on a solid support

2- Substances that are less attracted to the solid or more soluble in liquid move faster.

3- And so move further up the plate by the time that the process has been stopped by taking the plate out of the liquid- larger Rf

Very popular method and often used same as pc. Thin layer of finely divided adsorbent is supported on glass or plastic plates. It can be used for qualitative, quantitative and preparative purposes.

Nature of the layer● Commonly used adsorbents are silica gel, alumina,

diatomaceous earth and powdered cellulose.● Silica gel which is acidic and has high capacity, is

useful for adsorption and partition chromatography● Alumina is basic and used primarily for adsorption

chromatography● Diatomaceous earth is nearly neutral and used for

partition chromatography● Thickness of the layer varies from 0.15-2.00 mm● TLC plates can be prepared in Lab or purchased

commercially (much more convenient and more reproducible than home-made plates)

Procedure of development

● Same as paper chromatography, mostly

By ascending method or horizontal method

Isocratic

Gradient

Camag website

3. Detection:

If the spots are not colored and can’t be seen by the eye, use:• UV lamp for UV-active compounds (most aromatics are UV-active)• If compounds are not UV-active, use derivatization

Once you visualize the spots, circle them with a pencil.

4. Calculate Rf values for each spot

Rf = distance spot traveled from origin line/distance of solvent front

Rf values will help to identify an “unknown”

Make sure to use the same mobile phase as Rf’s will vary with varying mobile phases.

Other special techniques

● Modern TLC

Modern TLC● Automatic (sample application, development, detection

and quantification)● Advantages

1- Cheap and easy

2- More versatile than pc

3- Faster and more reproducible

4- Often used for complex sample separation and visualization

5- Modern TLC complementary to HPTLC

4- Many samples and standards can be applied on the same plate

Camag website

Sinensetine

R

Betulinic acid

R

Thanks

Google images