2.3.2

Thin-Layer Chromatography

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Thin-Layer Chromatography

Thin-layer chromatography (TLC) is a type of chromatography used to identify and separate chemicals in a mixture.

1) Setup the apparatus

1) Setup the apparatus

  • Use an aluminium TLC plate coated in silica or alumina.
  • The substances to be separated or identified must be dissolved in organic solvent and dotted along the bottom of the TLC plate.
  • The TLC plate is placed in a closed jar with a small amount of organic solvent inside.
2) Run the experiment

2) Run the experiment

  • The jar is left and the organic solvent in the jar rises up the TLC plate and over the dots.
  • The TLC plate is removed when the solvent front (top of the rising solvent on the plate) nears the top edge of the plate.
  • We can now use this TLC plate to either identify or separate the components in the mixture.
3) Evaluate the results

3) Evaluate the results

  • The mobile phase is the organic solvent (typically apolar).
  • The stationary phase is the TLC plate (typically polar).
    • The more polar substances will interact more with the stationary phase and so will be found lower down the plate.
    • The less polar substances will interact more with the mobile phase and so will rise with the solvent more quickly and will be found higher up the plate.

Separation and Identification

TLC can be used to separate and identify substances in mixtures.

Separation

Separation

  • When separation is required, substances from the mixture can be isolated by cutting up regions with just one spot from the TLC plate.
R<sub>f</sub> values

Rf values

  • We use Rf values to identify substances. These values are calculated as:
    • Rf = distance travelled by substance ÷ distance travelled by solvent
  • We can calculate the Rf value for reference substances and the tested substances.
Identification

Identification

  • Rf values depend on the solvent that they are measured in.
  • The identification process is comparing the Rf value of the reference and tested substance.
Calculation of R<sub>f</sub>

Calculation of Rf

  • We have a mixture and we suspect that it contains a particular component e.g. propanol.
  • Since Rf values are relative to the conditions, we must only compare values measured under the same conditions.
    • The best way to do this is to add a spot of the suspected component, e.g. propanol, to the same TLC plate as the mixture and run them together, then we can see if the Rf values align.
    • If the Rf values do align, there's significant evidence that the suspected component is in the mixture.

Colourless Chemicals

Some chemicals used in TLC can't be identified because they aren't visible on the TLC plate. The following are methods for locating these colourless chemicals:

Fluorescent dyes

Fluorescent dyes

  • Fluorescent dyes are visible when UV light is shone on them.
    • This allows us to see where the spot of the colourless chemical is and so we can calculate its Rf value.
Iodine

Iodine

  • Iodine turns the colourless spots purple.
    • This is another method that allows us to see where the spot of the colourless chemical is so that we can calculate its Rf value.
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1

Principles of Science I

1.1

Structure & Bonding

1.1.1Atomic Model1.1.2Electron Shells, Sub-Shells & Orbitals1.1.3Ionic Bonding1.1.4Representing Ionic Bonds1.1.5Covalent Bonding1.1.6Representing Covalent Bonds1.1.7Metallic Bonding1.1.8Intermolecular Forces1.1.9Intermolecular Forces 21.1.10End of Topic Test - Bonding1.1.11Relative Masses1.1.12The Mole1.1.13Molar Calculations1.1.14Molar Calculations 21.1.15Empirical & Molecular Formulae1.1.16Balanced Equations1.1.17Percentage Yield1.1.18End of Topic Test - Amount of Substance
1.2

Properties of Substances

1.2.1The Periodic Table1.2.2Ionisation Energy1.2.3Factors Affecting Ionisation Energies1.2.4Trends of Ionisation1.2.5Trends in the Periodic Table1.2.6Polarity1.2.7Metals & Non-Metals1.2.8Alkali Metals1.2.9Alkaline Earth Metals1.2.10Reactivity of Alkaline Earth Metals1.2.11Redox1.2.12Transition Metals1.2.13Redox Reactions of Transition Metals
1.3

Cell Structure & Function

1.3.1Introduction to Cells1.3.2Prokaryotes1.3.3Organelles of Eukaryotic Cells1.3.4Organelles of Eukaryotic Cells 21.3.5Organelles in Plant Cells1.3.6Microscopy1.3.7End of Topic Test - Cell Structure
1.4

Cell Specialisation

1.4.1Cell Specialisation1.4.2Blood Cells1.4.3Plant Cells1.4.4Gametes
1.5

Tissue Structure & Function

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1.7

Waves in Communication

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2

Practical Scientific Procedures and Techniques

2.1

Titration

2.1.1pH Curves & Titrations2.1.2pH Curves & Titrations 22.1.3Buffer Solutions
2.2

Colorimetry

2.2.1Measuring Rates2.2.2Beer-Lambert Law
2.3

Chromatography

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3

Science Investigation Skills

3.1

Scientific Processes

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Data Handling & Analysis

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Enzymes in Action

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Plants & Their Environment

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Electrical Circuits

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4

Principles of Science II

4.1

Extracting Elements

4.1.1Atom Economy
4.2

Relating Properties to use of Substances

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4.3

Organic Chemistry

4.3.1Naming Compounds4.3.2Functional Groups4.3.3Isomerism4.3.4Hydrocarbons4.3.5Structure of Alkanes4.3.6Boiling Points of Alkanes4.3.7Alkenes4.3.8Curly Arrows & Mechanisms4.3.9Cracking4.3.10Combustion4.3.11Free Radical Substitution of Alkanes4.3.12Electrophilic Addition of Alkenes
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Energy Changes in Industry

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The Circulatory System

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Ventilation & Gas Exchange

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4.7

Urinary System

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Cell Transport

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Thermal Physics

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Materials

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Fluids

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Analysing Scientific Information

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Practice questions on Thin-Layer Chromatography

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    When do you stop the TLC?Multiple choice
  2. 2
    Nature of phases in TLC:Fill in the list
  3. 3
    A component has a great affinity for the mobile phase. <br>Where will it be found on the TLC plate?Multiple choice
  4. 4
    What does TLC stand for?Multiple choice
  5. 5
    What are R<sub>f</sub> values dependent on?Multiple choice
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Overview of Chromatography

Column Chromatography