CIE A Level Chemistry

Revision Notes

Syllabus Edition

First teaching 2020

Last exams 2024

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8.1.1 Thin-Layer Chromatography

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Thin Layer Chromatography: Basics

  • Thin Layer Chromatography (TLC) is a technique used to analyse small samples via separation
    • For example, we could separate a dye out to determine the mixture of dyes in a forensic sample

  • There are 2 phases involved in TLC - stationary phase and mobile phase
  • Stationary phase
    • This phase is commonly thin metal sheet coated in alumina (Al2O3) or silica (SiO2)
    • The solute molecules adsorb onto the surface
    • Depending on the strength of interactions with the stationary phase, the separated components will travel particular distances through the plate
    • The more they interact with the stationary phase, the more they will 'stick' to it

  • Mobile phase
    • Flows over the stationary phase
    • It is a polar or nonpolar liquid (solvent) or gas that carries components of the compound being investigated
    • Polar solvents - water or alcohol
    • Non-polar solvents - alkanes

  • If the sample components are coloured, they are easily identifiable
  • We can examine the plate under UV light using ninhydrin to identify uncoloured components

Conducting a TLC analysis

  • Step 1:

    Prepare a beaker with a small quantity of solvent

  • Step 2:

    On a TLC plate, draw a horizontal line at the bottom edge (in pencil)

    This is called the baseline
  • Step 3:

    Place a spot of pure reference compound on the left of this line, then a spot of the sample to be analysed to the right of the baseline and allow to air dry

    The reference compounds will allow identification of the mixture of compounds in the sample

  • Step 4:

    Place the TLC plate inside the beaker with solvent - making sure that the pencil baseline is above the level of the solvent - and place a lid to cover the beaker

    The solvent will begin to travel up the plate, dissolving the compounds as it does
  • Step 5:

    As solvent reaches the top, remove the plate and draw another pencil line where the solvent has reached, indicating the solvent front

    The sample’s components will have separated and travelled up towards this solvent front

tlc-apparatus-new

A dot of the sample is placed on the baseline and allowed to separate as the mobile phase flows through the stationary phase; The reference compound/s will also move with the solvent

Rf values

  • A TLC plate can be used to calculate Rf values for compounds

Thin Layer Chromatography Basics equation

  • These values can be used alongside other analytical data to deduce composition of mixtures

Analytical Techniques - Calculating Rf Values, downloadable AS & A Level Chemistry revision notes

Rf values can be calculated by taking 2 measurements from the TLC plate

Exam Tip

The baseline on a TLC plate must be drawn in pencil. Any other medium would interact with the sample component and solvents used in the analysis process.

Interpreting & Explaining Rf Values in TLC

  • The less polar components travel further up the TLC plate
    • Their Rf values are higher than those closer to the baseline
    • They are more soluble in the mobile phase and get carried forwards with the solvent
  • More polar components do not travel far up the plate
    • They are more attracted to the polar stationary phase
  • The extent to which the components within the investigated sample are separated depends on:
    • The solubility of each component in the mobile phase
    • The interaction between each component and the stationary phase
  • Knowing the Rf values, of compounds being analysed, helps to compare the polarity of various molecules

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Francesca

Author: Francesca

Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.