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Retention Factor in Chromatography: Definition & Formula

Retention Factor in Chromatography: Definition & Formula
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  • 0:00 Fundamentals Of Chromatography
  • 1:45 Thin Layer Chromatography
  • 2:35 Retention Factors
  • 3:50 Lesson Summary
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Lesson Transcript
Instructor: Nathan Crawford

Nathan, a PhD chemist, has taught chemistry and physical science courses.

This lesson explores chromatography in the laboratory as a way to analyze mixtures. The equation for the retention factor is defined as a means to identify the components of a mixture.

Fundamentals of Chromatography

Have you ever noticed that ink on a piece of paper will spread if water is spilled on the ink spot? Water can cause ink - even black ink - to separate into its component colors. What does this separation of colors tell you about the ink components?

Black ink separating into its components from contact with a liquid
Black ink separating into its components from contact with a liquid

The scenario described above is connected with an extremely useful analytical technique in chemistry known as chromatography. Chromatography, a word that roughly translates into 'color writing' from Greek, is a technique that separates mixtures of non-volatile substances. The technique uses a liquid that is absorbed by a solid that is known as the stationary phase. In the mobile phase, the liquid dissolves the components of the mixture and transports the different components in much the same way as the water transported the ink.

The separation of the mixture components is based on the principle of 'like dissolves like'. The liquid mobile phase acts as a solvent that dissolves the components, or solutes. The ease with which the components dissolve in the mobile phase depends on favorable interactions between the molecules of the solutes with the solvent. Solvents that are considered polar, such as water or alcohols, readily dissolve polar components like vitamin C. Non-polar solvents, such as hexane or acetone, easily dissolve non-polar solutes like oils.

As the mobile phase is drawn upwards through the stationary phase, easily dissolved components move closer to the leading edge of the rising liquid, also known as the solvent front. Components that do not dissolve as easily are left further behind the solvent front. The result is a separation of the mixture into its separate components that can be easily seen by an observer.

Thin Layer Chromatography

One form of chromatography, known as thin layer chromatography (TLC), is often seen in laboratory experiments. Thin layer chromatography uses a glass, metal, or plastic plate that is coated with the stationary phase, usually silica gel. A small drop of the mixture that is being analyzed is placed a short distance from the bottom of the TLC plate. The TLC plate is then placed into a chamber or tank with the mobile phase, like water, ethanol, acetone, or a mixture of solvents.

A TLC plate coated with a silica gel stationary phase
A TLC plate coated with a silica gel stationary phase

A TLC plate placed in the mobile phase within a TLC chamber
A TLC plate placed in the mobile phase within a TLC chamber

An example of a thin layer chromatography experiment showing the separation of black ink into different parts
An example of a thin layer chromatography experiment showing the separation of black ink into different parts

An excellent example of the use of TLC in the laboratory is the separation of pigments in plants like spinach. The experiment usually involves extracting a sample from ground spinach leaves. Once the TLC plate is placed in a mixture of non-polar solvents, such as acetone and hexane, the different pigments like chlorophyll are separated into vividly colored spots.

TLC separation of plant pigments including chlorophyll
TLC separation of plant pigments including chlorophyll

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