One of the more basic techniques in the microbiologist’s toolkit is gel electrophoresis. It’s used to separate long molecules, like proteins, RNA and DNA from one another. Different organisms have different DNA sequences, so electrophoresis can be used to identify organisms and for DNA fingerprinting. Chromatography is also used to separate different molecules, usually pigments. Therefore, using some filter paper, food coloring, and popsicle sticks I created a nice little chromatographic fingerprinting lab exercise using chromatography as an analogue for electrophoresis.
Using a standard set of four food colors (red, blue, green and yellow), I grabbed each students individually and had them add three drops of the colors of their choice to a test tube with 1 ml of water in it. One students went with three straight blue drops, but most picked some mixture of colors. I kept track of the color combinations they used, and labeled their test tube with a unique, random number.
When they’d all created their own “color fingerprint” in the test tubes, I handed them back out randomly, and gave them the key of names and color combinations (but no numbers). They had to find out whose test tube they had.
I was kind enough to give them a few little demonstrations of chromatography I’d been experimenting with over the last day or so. The easiest technique is simply to place a couple drops of the sample on a filter paper (we used coffee filters “requisitioned” from the teachers lounge), and chase it with a couple drops of water to help the dye spread out. This method works, but since the sample spreads out in a circle, the inverse square law means that the separation of colors can be hard to see.
While the drop method worked well for most students, one who was a bit more analytically-minded, interested in the project, and had a particularly difficult sample, tried doing it using a filter paper column. Since I wanted to show them the proper way of conducting experiments, particularly about the importance of using standards, and I wanted to check if they were able to interpret their results correctly, I also did the full set of samples myself as columns. The standards are essential, because the green food color is actually a mixture of green and blue dyes.
Our color chromatography setup is as you see at the top of this post. We used popsicle sticks to keep the filter paper strips away from the glass surface.
The experiments worked well, and for best results, let the it dry because the colors show up better. One focus with my students was on note-taking and recording results; after a few iterations that worked out well too. Another nice aspect of using the series of columns is that it looks a lot like the electrophoresis bands.
I did try some other variants of the chromatography: top down, bottom up and even taped down. The last version, where I taped the filter paper to the glass to create a restricted column, worked very well.
