Show Your True Colors

The second lab of the semester in Introduction to Organic Chemistry was an introduction to three types chromatography applied to the separation of plant pigments. Plant pigments are a great chromatography study because a variety of different plant sources can be used and compared. There are typically at least four or five pigments in all green plants that can be visibly separated on TLC. In this lab students were assigned one of four plants: spinach, cucumber peel, broccoli, or red-leaf lettuce. They ground them up with sand and magnesium sulfate. Part of the resulting powder was loaded on the reversed-phase solid phase extraction column. The SPE column was eluted with methanol, methanol/acetone, and finally acetone. These SPE column are very robust and can be reused a few times. Students performed the traditional silica gel TLC with acetone extract of the ground up plant. They also observed what happens when the plant material is partitioned between heptane and acetonitrile. An explanation of chromatographic theory allowed them to relate chromatographic separations to what they have been learning about solubility and hydrogen bonding. To simplify things, I just had the students use the ground up powder straight from the mortar each time. There was no filtration and concentration steps to separate extracted plant pigments from the insoluble solids.

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