thin layer chromatography

• Rf = Distance moved by component/Distance moved by solvent front

  • Units for distance is mm
  • The Rf value should be given with 2 decimal places
• Separation by adsorption:

  • Component molecules bind to the surface of the solid stationary phase
  • The stronger the adsorption to the stationary phase, the more the component molecules are slowed down
  • Has a solid stationary phase
• TLC is used to check purity / monitor the extent of a reaction.

  Phases:

  1. Stationary phase - Silica gel, SiO2 or Aluminium oxide, Al2O3
  2. Mobile phase - Solvent
• TLC stands for thin layer chromatography
• TLC is a method used for qualitative analysis
• TLC separates mixtures of components and allows identification of the components
• TLC separation occurs on a thin layer of silica on top of a solid support such as a sheet of glass or plastic called the TLC plate
• during TLC the properties of the silica and solvent are used to draw the components up the plate
• TLC: differences in the properties of the components in the mixture means each component will travel up the mixture at different rates which causes a separation
• you must handle the TLC plate only by its edges or use tweezers to ensure that the silica gel surface won't be contaminated
• a line drawn in pencil about 1cm from the bottom edge of the TLC plate is called the origin
• in TLC a capillary applicator is used to spot the plate
• TLC: you must use a clean capillary applicator for each different solution to avoid cross-contamination
• TLC: a pre-measured solvent is added to the developing chamber and then covered with a lid for a few minutes to allow the atmosphere inside the tank to become saturated with the solvent vapours
• TLC: you must always ensure that when placing the spotted plate into the solvent pool, the origin is above the surface of solvent
• TLC: the boundary between the dry and wet silica is called the solvent front
• you should remove the plate from the developing tank when the solvent front is 1cm away from the top and then mark it with a pencil
• the most common way to visualise a developed TLC plate is to place it underneath an UV light source
• interpreting spotted TLC plate under UV light:

  • if fluorescent indicator was added to silica, it will be fluorescent
  • most chemicals show up as dark spots
  • if the chemical on the plate is UV active, the spot will be a different fluorescent colour to silica
• TLC: when analysing the plate under UV you must outline all the spots with a pencil so they can be located under visible light
• making a conclusion about a compound by comparing it to different lanes of other compounds on a TLC plate is called indexing the references
• the distance travelled of a known compound is known as the standards or references
• spots on developed TLC plates can also be characterised by their Rf value
• its unlikely that Rf values of analytes will exactly match those of the standards to the second d.p. due to experimental error caused by measurement uncertainties or inconsistencies in the TLC plate
• TLC is a quick and inexpensive analytical technique
• a TLC plate is usually a plastic sheet or glass, coated with a thin layer of a solid adsorbent substance - usually silica
• In TLC, the adsorbent is the stationary phase
• TLC: The different components in the mixture have different affinities for the absorbent and bind with differing strengths to its surface
• TLC: Adsorption is the process by which the solid silica holds the different substances in the mixture to its surface
• TLC: Separation is achieved by the relative adsorptions of substances with the stationary phase
• a chromatography tank for the TLC plate can be made from a small beaker with a watch glass placed on the top, some solvent should be poured into the beaker to a depth of about 0.5 cm
• TLC: Sometimes the plate is sprayed with a chemical or a locating agent, such as iodine, to show the position of the spots that may be invisible to the naked eye.
• Rf stands for retention factor
• Each component can be identified by comparing its Rf, value with known values recorded using the same solvent system, and absorbent.