Chromatography

Chromatography

1. Chromatography is used to identify substances in a mixture/sample

2. Gas Chromatography

   • How Does it Work?
     • 1) A gas is used to transport substances through a column packed with solid metal
       • 2) The substances (compounds) travel at different rates through the metal column and are separated out during the journey
         • 3) The substances all reach a detector at the end of the metal column at different times
           • 4) The time taken for the substances to get to the detector from entering the column is called the "retention time"
             • 5) A recorder in the detector creates a graph with the x axis measuring time. Every time a new substance reaches the detector, it creates a peak on the graph
               • 6) This graph is called a "gas chromatograph" and the number of peaks shows the number of different compounds in the sample
                 • 7) The position of the peaks on the graph shows the retention time for each substance
   • What Does it Do?
     • Gas Chromatography is used to separate out and identify a mixture of compounds when moving through a gas and identify the substances present

3. Chromatography Using Filter Paper

   • What Does it Do?
     • Chromatography using filter paper is used of commonly separating out substances in a sample, commonly artificial colourings. Eg. a food colouring might contain one dye or a mixture of dyes. Chromatography will help separate out these dyes
   • How Does it Work?
     • 1) A pencil baseline is drawn onto a strip of filter paper with a drop of food colouring (or choice of sample) placed upon this baseline
       • 2) The filter paper is then rolled up and placed in a beaker with the water of solvent of choice only just touching the filter paper. The baseline is to be kept above the water level
         • 3) The solvent will then seep up the filter paper, bypassing the baseline and taking the dyes from the baseline with it
           • 4) The mixture of dyes will then begin to separate due to the different dyes absorbing into the filter paper at different rates (heights on the filter paper)
             • 5) The filter paper is then known as a "chromatogram" and displays the different dyes on it due to the different colours at varying heights up the filter paper
               • 6) A chromatogram with 4 spots means AT LEAST 4 different dyes, not EXACTLY 4 different dyes due to there being the possibility of multiple dyes absorbing in the same place
     • How to Work Out the "Rf" Value for Each Chemical (Substance)
       • Rf= Distance Travelled By Substance (Eg. Food Colouring) ÷ Distance Travelled by Solvent (eg. Water)

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