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The allure of Rƒ: How does this mysterious number affect the motion of molecules on paper?
Paper chromatography is an analytical method used to separate colored chemicals or other substances. Although this technique is currently used primarily as a teaching tool, in the laboratory it has been replaced by other chromatographic methods such as thin layer chromatography. This analytical method has three components: mobile phase, stationary phase and support medium. The mobile phase is a solution that rises by capillary action on the stationary phase, which is usually composed of a non-polar organic solvent and the stationary phase is a polar inorganic solvent, water. Here, the paper acts as a support for the stationary phase, with the water molecules being held in the voids in the fiber network of the paper.
The key difference between thin layer chromatography and paper chromatography is that in thin layer chromatography the stationary phase is an adsorbent layer (usually silica gel or aluminum oxide) whereas in paper chromatography a less adsorbent paper is used.
In a variation of paper chromatography, bi-directional chromatography, two solvents are used and the paper is rotated 90 degrees in between, which is very useful for separating complex mixtures of similar polarity, such as amino acids.
Rƒ Values, Solutes and Solvents
The Rƒ value (Retention factor) is used to define the ratio between the distance moved by the solute and the distance moved by the solvent. This value is used in chromatography to quantify the degree of sample retention in the stationary phase as opposed to the mobile phase. Typically, Rƒ values are expressed as decimals. If a solution has an Rƒ value of zero, it means that the solute remains in the stationary phase and does not move. When the Rƒ value is 1, it means that the solute has no affinity for the stationary phase and moves with the solvent front.
For example, if a compound moves 9.9 cm and the solvent front moves 12.7 cm, the Rƒ value can be expressed as Rƒ = 9.9/12.7 = 0.779 or 0.78. The Rƒ value will vary between different solvents, depending on the temperature in the experiment and the solvent used.
Pigments and Polarity
Paper chromatography can be used to test the purity of compounds and identify substances because the technique is relatively quick and requires only small amounts of material. This separation process is based on the principle of distribution. The material will partition between the stationary phase, which is the water held in the paper fibers, and the mobile phase, which is the developing solution that flows over the stationary phase.
When a colorful chemical sample is placed on filter paper, the colors separate as the solvent moves through it, dissolving the molecules according to their polarity.
If a sample contains multiple colors, this means that the sample must contain multiple different molecules. Since the chemical structure of each molecule is different, there is also a high chance that each molecule will have a different polarity, and therefore their solubility in the solvent will be different. This uneven solubility causes molecules of different colors to separate at different locations on the paper, and the higher the solubility, the higher the solid molecules rise.
Types of Paper Chromatography
Downward elution method
In this method, the solvent flows down the paper. The mobile phase is placed in a continuously dripping liquid, the sample spot is kept on the top of the paper, and the solvent flows down from above.
Upward elution method
In this method, the solvent moves from the bottom to the top, and both the sample and the solvent move upward. This method is very effective for separating organic and inorganic substances.
Hybrid Method
This technique is a hybrid of the two methods above. At a critical point, the upward-rising section of paper can be folded back to switch to a downward-rinsing mode.
Circular chromatography
In circular chromatography, a circular filter paper with the sample in the middle is placed in a petri dish containing the solvent. Once the solvent begins to rise, the molecules form concentric circles on the paper.
Two-dimensional chromatography
Two-dimensional chromatography uses square or rectangular paper, the sample is applied to one corner and then turned 90 degrees for a second development.
History of Paper Chromatography
In 1943, Martin and Synge discovered paper chromatography, which for the first time provided a tool for investigating, separating and identifying plant components. Thereafter, the field developed rapidly after 1945, promoting most of the research work under this technology.
The rise of this technology has changed the scientific community's understanding of chemical separation and analysis methods and has promoted the progress of many related researches.
In today's era of ever-deeper exploration of molecules and chemistry, do we fully understand the potential of Rƒ for other analytical techniques?
