Chem-221 Name: Yahya Experiment# 15
Experiment: Thin Layer Chromatography
Date performed: 21/01/13
Introduction: The objective of the experiment is to find out the identity of the unknown sample provided in powder form. The goal was to check its content and find its identity by consulting the table provided in the book. There were total three possible drugs present namely caffeine, aspirin and acetaminophen. Each drug in the table has a different percentage of these substances. TLC i-e Thin layer chromatography is based on the capillary action. On the TLC paper, the sample and the substances to be compared with are placed in a minute quantity and placed vertically in a solvent. Due to the capillary action, the solvent (developing solution) or the ‘mobile phase’ moves up the TLC paper and drags the substances it encounters on the way with them in upward direction (in this scenario). All of these substances have different structures and hence different polarities so each substance will travel a different distance. The unknown substance will be corresponding with one of the substances which are known to us and with that data we can find the identity of the unknown substance. This will be performed by comparing their Rf values which are calculated by: Rf = Distance travelled by soluteDistance travelled by solvent Before we perform TLC, we need to make a solution of the substance and after performing TLC, we will use U.V light to view the spots and therefore measure the Rf values.
Experimentation: Preparing the Developing solution: The solvent provided to us and used in this experiment is ethyl acetate acetic acid 200:1. A solvent that will form a circle 1-2cm in diameter, shows well separated rings and outermost ring is 50-75% of the distance from the center to the solvent front (the line where the solvent stops/ or is stopped) is a good solvent. It is preferable to use the least polar solvent that gives good separation. Once we have the solvent, we put it in a beaker that can fit a TLC paper but isn’t too big because the solvent will create an atmosphere of its molecules inside the beaker which increases the efficiency of the process and the larger the beaker the slower it will be to fill it up. Add the developing solution i-e 7.5ml of Ethyl acetate acetic acid, 200:1 to the beaker and cover with an aluminium foil to avoid any loss of vapours. The beaker shouldn’t be kept in sunlight to avoid temperature gradients. Preparing the Unknown solution: Take 5ml of ethanol dichloroethane 1:1 in a small beaker and add the unknown powder to it, use the glass stirring rod to dissolve it completely. The solvent for the mixture is selected such that the solution is approximately 1%. Usually the solvent is a non-polar solvent and has a boiling point between 50-100°C. Once the entire solid (unknown) is dissolved in the solvent next comes the preparation of the THC paper and spotting. Preparing the THC paper: On the THC paper, use a pencil, not ink, because ink or any other substance will flow with the mobile phase and hence the whole process will be unsuccessful. Leave 1.5cm on each side of the TLC paper and 1cm from the bottom of the paper and draw a line, this will be known as the starting line. On the starting line mark the positions of the unknown substance, caffeine, aspirin and acetaminophen. Now using a micro-pipette take small and concentrated spots of each substance, larger spots spread out too much causing inaccuracy in the results. Put drops of each substance on its marked place on the TLC paper. Put the TLC paper in the developing solution such that it is immersed in it but the solvent is not crossing the starting line, otherwise the substances will dissolve in the solvent right away. Put the paper vertically in and seal the beaker again and wait for the solvent to reach nearly 1cm from the top of the paper. When this happens, mark the position of the…