Chromatography and Pigment Separation
Chromatography is the umbrella term for scientific analytical methods that separate a mixture by passing it through a solution in which the components in the mixture move at separate rates. Chromatographic techniques can be used in conservation for identifying the number of organic compounds in a small pigment sample.
Thin Layer Chromatography (TLC) in Organic Chemistry
In my Organic Chemistry I Lab course, one of our experiments was to use TLC to separate non-volatile mixtures and to identify compounds using known samples. TLC was also used to determine the purity of our experimental substances. In the experiment process, a solvent or solvent mixture (1:1 hexane:ethyl) was drawn up a silica plate through capillary action. Since different components in the mixtures being tested ascend the plate at different rates due to their components’ polarity in relation to the solvent being used in the experiment (the more polar it is, the closer it will stay close to the base of the plate), the compounds can therefore be separated. TLC can be performed on a sheet of glass, plastic or aluminum foil coated in a thin layer of absorbent material, usually silica gel, cellulose or aluminum oxide. The plate is then observed under a UV source because TLC plates contain a UV-reactive fluorescent dye. The use of UV-light in TLC is often the first visualization technique used to determine the separation of the compounds on an eluted TLC plate because it is a simple technique to carry out and it is also a non-destructive technique
The separation of the experimental compounds under UV-light. Tailing indicates the presence of carboxylic acid.
Paper Chromatography and Pigment Separation at Home
Inspired by my organic chemistry lab on TLC, I decided to further investigate the scientific analytical method of chromatography with non-toxic and hazard free materials I have at home and observe the separation of pigments. A colored medium - whether it being ink, watercolor or paint - is made up of colored molecules of different sizes. Paper chromatography is a simple method that can used to separate the components in these colored mediums by allowing a solvent like water, isopropyl alcohol, and vinegar to absorb up a paper strip that has been prepared with samples of the color mixtures you wish to test. In paper chromatography, the rate and distance at which a molecule travels up the paper plate is dependent on the molecules interaction with the water present on the cellulose fibers that make up the paper being used. As a result, the less polar a molecule is, the less it will be attracted to the water molecules present on the surface of the paper’s cellulose fibers and move faster and further up the paper plate.
The type of papers and the polarity of the solvent being used should be considered. Note that different papers with different cellulose fibers withhold different amounts of water and the molecules in solvent selected can also interact with water on the surface of the paper plate. These factors can affect the separation of the pigment samples being tested.
Solvents:
(1) 100% distilled water
(2) 50% distilled water and 50% white vinegar
Papers:
(1) Brown coffee filter
(2) Paper towel
Drawing Ink Samples:
(1) Peat brown, (2) Brilliant green, (3) Black Indian Ink, (4) A mixture of equal parts peat brown, brilliant green, and black Indian Ink.
*Product information blurred out to protect manufacturer’s privacy
Experiment Set-Up:
*The opening of the apparatus of covered with a paper board
Results:
Observations: There is a lack of separation in both the coffee filter and paper towel plates that used a mixture of 50% water and 50% white vinegar as the solvent. In the coffee filter plate that used 100% water as the solvent, sample (1) appears to be a pure substance, sample (2) appears to be a mixture of yellow and green molecules (green dominates), sample (3) appears to be a mixture of blue and black molecules (black dominates), and sample (4) is confirmed to be an impure compound. In the paper towel plate that used 100% water as the solvent, sample (1) appears to be a mixture of light yellow and brown (ochre?) molecules (brown dominates), sample (2) appears to be a mixture of yellow and green molecules (yellow dominates), sample (3) appears to be a pure substance, and sample (4) is confirmed to be an impure substance.
Conclusions: The lack of separation observed on both the coffee filter and paper towel plates that used a mixture of 50% water and 50% white vinegar as the solvent is likely due to the increase in polarity of the solvent caused by the addition of white vinegar. The increase in polarity of the solvent likely caused a stronger interaction between the white vinegar and the water molecules within the solvent itself and the water molecules present on the surfaces of the coffee filter and paper towel plates’ cellulose fibers. Thus affecting the rate and distance of separation of the components within the sample. Ultimately, though indications of pigment separation can be observed in this experiment, the results are not obvious enough to confirm definite separation. Other paper plate and solvent combinations should be considered when reapplying this method.