Chromatography is a process that separates the different pigments of a substance across a porous kind of paper called chromatography paper. Molecularly, this process separates molecules by the molecules’ molecular mass. In this process, the chromatography paper allows the solvent and pigments to diffuse, and the solvent and pigments are drawn up the paper. This mimics capillary action in plants, where plants pull water up from the roots to the stems and leaves of the plant. Capillary action is made possible by the forces of cohesion and adhesion of both the pigment and the solvent. When the pigment molecules have a larger molecular mass and have strong cohesive forces and adhesive forces to the chromatography paper, the molecules will stay where the molecules were placed or not move very far. In contrast, when the molecules of a certain pigment have a smaller molecular mass and adhere more to the solvent than the paper, then these molecules will move farther up the chromatography paper with the solvent. There are also many other factors that affect the rate of a pigment travelling up the chromatography paper. In addition to the pigment’s molecular mass and the the pigment’s adhesive and cohesive forces, some factors that affect the rate of the pigment during chromatography include the polarity of the solvent, the pH of the solvent, the temperature of the experiment, and many other factors.
The Rf value stands for the average distance that the pigment travelled, divided by the total distance that the solvent travelled. Dunknown signifies the average distance that the pigment travelled. This means the farthest distance that the pigment travelled, minus the closest distance that the pigment travelled, divided by two. This results in a point in the middle of the pigment range. Dsolvent signifies the total distance that the solvent travelled. This is not an average distance, but rather the specific distance that the solvent travelled up the chromatography paper. Dunknown / Dsolvent equals the Rf value. This value gives a number without units, that is always less than 1.0. This number is very useful because it can be used by scientists to get an idea of the molecular mass of the particular pigment, as well as the typical distance that the pigment will travel with a certain substance. For example, a pigment with a high Rf value will travel relatively far compared to the other pigments. This means that pigments with small molecular masses have large Rf values.
In the experiment performed in class, both green leaves and non-green leaves were tested. In the green leaf chromatogram (Figure 1), three pigments were present after the solvent separated the pigments (light orange, yellow, and green). However, in the non-green leaf chromatogram (Figure 2), there were four pigments clearly present (maroon, yellow, red, and green). This showed that although the leaves looked all one individual color, there were actually several pigments within each leaf. The non-green leaf actually contained more pigments than the green leaf. Surprisingly, the red-maroon leaf actually seemed to have about an equal concentration of green pigment as the bright green leaf, of two completely different species. This shows that while the leaves may appear to be a certain color, the leaves really contain a large array of pigments that together make up the external appearance.
After learning from my own photosynthesis Prezi presentation and those of my classmates, I learned that plants are green because the chlorophyll absorbs all other wavelengths of light, but reflects green light. However, I still wonder whether there are different wavelengths of green light that are absorbed more than others, or if all wavelengths of green are reflected. ⚘