Required Practical 12

• how do you analyse medicine samples?

  1. Crush an aspirin tablet with pestle and mortar.

  2. Transfer to a weighing boat or bottle.

  3. Dissolve approx 0.1 g of the powdered tablet in 0.5 cm of ethanol.

  4. Repeat with other painkiller tablets. (caffeine/anadin tablets should be dissolved in 7.0cm ethanol)
• Describe how you use thin layer chromatography

  1) Wearing gloves carefully use a pencil to draw a faint line 1 cm above the bottom of a TLC plate and mark five spots, equally spaced, along this line.

  2) Use a capillary tube to apply a tiny drop of each solution to a different origin spot and allow the plate to air dry. If required repeat this process to achieve small but concentrated spots

  3) Add approximately 10 cm3 of solvent to a development chamber (or suitable container with a lid)

  4) Place the TLC plate into the development chamber, making sure that the level of the solvent is below the spotting line. Replace the lid and make sure it is a tight seal.

  5) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent front with a pencil. Allow the plate to dry in the fume cupboard.

  6) Place the plate under a UV lamp in order to visualise the spots. Draw around them lightly in pencil.

  7) Calculate the Rf values of the observed spots
• why use a tiny drop of each solution?

  Too big a drop will cause different spots to merge
• why use a small solvent depth?

  If the solvent is too deep it will dissolve the sample spots from the plate.
• why use a lid?

  To prevent evaporation of toxic solvent and so that the inside of the tank is saturated with the solvent vapour.
• why allow the solvent line to run towards the top and dry in a fume cupboard?

  ● Will get more accurate results if the solvent is allowed to rise to near the top of the plate but the Rf value can be calculated if the solvent front does not reach the top of the plate

  ● Dry in a fume cupboard as the solvent is toxic
• why use in a UV lamp?

  UV lamp used if the spots are colourless and not visible.
• why use a mm ruler?

  Higher resolution - more precise Rf value calculated
• Why must any lines drawn on the TLC plate be in pencil?

  Because if drawn in ink, this will dissolve in the solvent and run up the plate with the other components, contaminating the plate.
• What happens if your sample is to concentrated?

  If your sample is too concentrated then the spots overlap