Thin Layer Chromatography, TLC (OCR A Level Chemistry)

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Thin-Layer Chromatography Practical

  • 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
      • 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 non-polar liquid (solvent) that carries components of the compound being investigated
      • Polar solvents - water or alcohol
      • Non-polar solvents - alkanes

  • If the sample components are coloured, their spots are easily identifiable on the chromatogram
  • If the sample components are not coloured, then we can locate the spots on the chromatogram and draw around them in pencil
    • To locate the spots we can use:
      • UV light
      • Ninhydrin (carcinogenic)
      • Iodine vapour

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 solvent does not cover the spot - 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

TLC & Rf Values

Rf values

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

Rffraction numerator D i s t a n c e space t r a v e l l e d space b y space c o m p o n e n t over denominator D i s t a n c e space t r a v e l l e d space b y space s o l v e n t end fraction

  • 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

Examiner 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.

  • 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 of separating molecules in the investigated sample depends on the solubility in the mobile and stationary phases
  • Knowing the Rf values, of compounds being analysed, helps to compare the polarity of various molecules

Worked example

A mixture of amino acids is analysed by thin layer chromatography (TLC). The TLC plate produced is shown below. Identify the amino acids in the sample using the table of values for Rf.TLC Worked example a chromatogram, downloadable AS & A Level Chemistry revision notes

Answers:

Blue spot Rf value = 2.82 ÷ 4.63=  0.61 = Leucine

Green spot Rf value = 2.45 ÷ 4.63= 0.536 = Isoleucine

Red spot Rf value = 2.04  ÷ 4.63= 0.44 = Valine

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.