What is paper chromatography? How is it used? A demonstration of chromatography with spinach leaves.
Without pigmentation looking at the world would be like looking at an old 1950's re-run, in black-and white; only without the black. Pigments are chemical compounds that reflect specific wavelengths of visible light. Although aesthetically it is important that these pigments reflect such light, thus beautifying our world, it is their ability to absorb certain wavelengths that supports our ecosystem. Autotrophs, organisms that produce their own food, utilize certain pigments to carry out photosynthesis.
Due to the wide range of the visible light spectrum, several different classes of pigments are needed in order to maximize the energy taken from the sun. The most prevalent of these pigments is chlorophyll. Due to the porphyrin ring structure of chlorophyll, electrons can move easily, thus allowing chlorophyll to capture the sun's energy. There are four types of chlorophyll ranging from a-d. The most important of these is chlorophyll a, for it passes the energized electrons to the molecules that produce glucose. Seventy-five percent of chlorophyll in green plants is chlorophyll a. Chlorophyll a is green.
Chlorophyll b is also an important type of chlorophyll, but unlike chlorophyll a, it is not vital to photosynthesis. Chlorophyll b is found in green algae and certain plants, it is yellow green. Chlorophyll b absorbs light of a different wavelength than chlorophyll a, but ultimately transfers it to chlorophyll a for conversion into chemical energy. Chlorophyll c is only found in photosynthetic Chromista and Dinoflagellates.
Another class of pigments are the carotenoids. Carotenoids are typically yellow, orange, or red pigments. Carotenoids do not dissolve in water due to their two six carbon rings which are connected by a chain of carbon atoms. Carotenoids are accessory pigments, they cannot transfer the energy absorbed from the sun into photosynthetic pathways, instead they must give the energy to chlorophyll a.
The third class of pigments are the phycobilins. Phycobilins are water-soluble and are found in the stroma or cytoplasm of the chloroplast. This last class of pigments is only found in the marine organisms Cyanobacteria and Rhodophyta. Phycobilins effectively absorb energy and are also used by scientists for cancer research. Scientists often use phycobilins to track tumor cells.
Chromatography is a technique used to separate mixtures of compounds. This technique was discovered in 1906 but not widely used until the 1930's. There are many different types of Chromatography, including: paper chromatography, ion concentration chromatography, gas-liquid chromatography, thin layer chromatography, partition chromatography and gel-permeation chromatography. Chromatography consists of two phases: the stationary phase and the mobile phase. The stationary phase consists of a stationary item which allows the molecules to pass through at different times, based on their size. The mobile phase is the solvent which dissolves the substance being tested.
Paper chromatography uses sheets of paper as the absorbent stationary phase, acetone is commonly used for the mobile phase. Some of the pigments dissolve quicker than others, which causes them to move up the paper through capillary action into different areas. This separation allows the different pigments to be identified while on the paper through color identification or by measuring absorbance with a spectrophotometer.
Green spinach leaves are perfect specimens for chromatography. Spinach leaves contain chlorophyll as well as other pigments which are necessary in photosynthesis. Spinach leaves contain chlorophyll a due to it's crucial role in photosynthesis. Chlorophyll b is also present due to it's prevalence in plants. A carotenoid of some color is also present as an accessory pigment.