Chromatography is a method of separation in which the components to be separated are distributed between the two different phases, one is the stationary phase while the other one is a mobile phase that moves in a definite direction. There are different types of chromatographic techniques such as column chromatography, paper chromatography, partition chromatography, thin-layer chromatography, gas-liquid chromatography, and ion-exchange chromatography.
In this article, we will study thin layer chromatography in detail.
Thin-layer chromatography is the simple, fast, easiest, and least expensive of all the chromatographic techniques which are used in quantitative and qualitative analysis to separate organic compounds and to test the purity of compounds.
TLC is a form of liquid chromatography that consists of two phases given as follows-
A mobile phase (solvent)
A stationary phase (glass plate coated with silica gel)
[The analysis is done under atmospheric pressure and room temperature.]
Thin-layer chromatography is a method of separation or identification of a mixture of components by using finely divided adsorbent Solid/ liquid over a glass plate and liquid as a mobile phase.
Adsorption of substances on the stationary phase
Separation of adsorbed substances by the mobile phase.
Separated substances are recovered by the mobile phase through elution
Qualitative and quantitative analysis of eluted substances.
1. The distinction depends on the relative affinity between the stationary and mobile phases of the compounds. The compounds move over the surface of the stationary phase under the influence of the mobile phase (driven by capillary action). The compounds with greater affinity for the stationary phase travel slowly during this movement, while the others travel faster. The isolation of components in the mixture is thus accomplished. The individual components are visualized as spots at a different stage of travel on the plate once separation happens.
2. In a solid-phase, the adsorbent is coated onto a solid support such as a sheet of glass, aluminum, and plastic as a thin layer of about 0.25mm thick.
3. The mixture that needs to be separated is dissolved in a solvent to form a solution. Thus this solution is applied at the base of the thin-layer plate. The solution is allowed to move in an upward direction under the influence of capillary action. The solid will absorb some fraction of each component of the mixture and the remaining will be left in the solution. Anyone molecule will remain attached to the solid surface while the other components continue moving up the plate with solvent.
5. Elution is stopped when the solvent reaches the opposite side of the silica-coated glass.
A simple method of component separation.
Fewer types of equipment are used in this technique. As the components elute rapidly, the separation is achieved in a very short time.
It is possible to visualize all elements of UV light.
By this process, the non-volatile compounds can be isolated.
It is also possible to separate the microlitre volume of the sample through TLC
Easy isolation and recovery of the components of complex mixtures.
It is difficult to reproduce the findings obtained from the experiment.
Applicable for components of soluble mixtures only.
Qualitative analysis, not the analysis in quantitative terms.
Not a mechanism that's automatic.
A thin layer of chromatography operates in an open system, humidity and temperature can influence the outcomes.
As plate length is limited, the separation process takes place up to a certain length.
Identification, purity testing, and determination of active substances and preservatives in drugs and in drug preparations.
Determination of active substances and their metabolites in biological matrices, diagnosis of metabolic disorders like phenylketonuria, cystinuria, and maple syrup disease in babies.
Dye raw materials and end products, preservatives, surfactants, fatty acids, constituents of perfumes.
Groundwater analysis, determination of pollutants from soils and surface water.
Determination of metals
The retardation factor (R) in chromatography is the fraction of an analyte in a chromatographic system's mobile phase. The retardation factor is the ratio of the distance travelled by the centre of a spot to the distance travelled by the solvent front.
For the study and comparison of various substances, the retardation factor, Rf, is widely used in paper chromatography and thin-layer chromatography. It can be defined by the following ratio mathematically:
Rf= migration distance of substance/migration distance of solvent front.
Question: What are the Advantages and Disadvantages of Paper Chromatography?
Advantages of Paper Chromatography
It is much cheaper than other chromatography techniques.
It requires very less quantitative material.
Both organic and inorganic material can be identified with this technique
A large number of samples cannot be used
Cannot be used for complex mixtures
It is not much accurate technique.
Question: Why is Thin Layer Chromatography Used?
Answer: TLC is a method of chromatography used for separating non-volatile mixtures. Thin-layer chromatography can be used to monitor the progress of a reaction, to determine the purity of a substance, to classify compounds contained in a given mixture.
Question: Why is Silica Gel Used for TLC?
Answer: Silica gel is the most commonly used adsorbent by far and continues to be the dominant stationary phase for TLC. For the chromatography of important compounds, the surface of silica gel with the highest concentration of geminal and related silanols is most preferred because these silanols are less acidic.