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Lesson 1: Chromatography Chromatography is used to determined what compounds make up an unknown sample. Samples are mixed with a gas or liquid, then passed across another substance which your sample is differentially attracted to. The compounds are sorted and separated, those with weaker attractions traveling faster and farther than those with strong attractions. These results are compared to tests of known substances to determine what the unknown sample is made of. Chromatography can be used for many tasks, including: separating plant pigments, detecting accellerants in arson investigations, determining if there is DDT in bird eggs, analyzing ink samples in detective investigations, determining the paint composition and possibly the authenticity of a painting. In this lesson, we will be using liquid chromatography to test ink samples but this will help students understand the process and its relationship to testing eggs for DDT. Time Allotment: Materials:
Tap Prior Knowledge Explain to the students that you have a mystery to solve. They are archivists working at a museum. Someone has come in with a manuscript that he claims is from a famous author who lived in the 1600's. It is your job to determine if the manuscript is really that old. Ask the students how they would do this. Write their answers on the board. Explain that for now, cost (or equipment) is no object. Discuss the pros and cons of each suggestion as well as what it might tell them about the manuscript in question. Be sure to mention when a technique might damage or destroy the object being tested (and why this is not such a good idea). Share with Neighbor Have the students group into pairs to study the sample and make a hypothesis. Is it authentic? Why do they think so? If they are having trouble, you can encourage them by explaining that even without scientific equipment, they might be able to make an educated guess using things like how old the paper looks (lined paper was not around in its current form in the 1600's) or what words are used and how they sound. Engage Students in Hands-on Activity Now even though we don't have a lot of equipment to do many of the scientific tests actual museum curators would use, we do have access to one method. We can find out what ink might have been used. Pass out the "sample inks" (i.e., Pens labeled 1-5). Each pen contains ink from a specific time period. List these on the board. Any dates will do so long as the one that will match the writing sample has the correct date: the 17th century. Have one person in the group collect the remaining supplies needed for the experiment: a shallow tray or 5 plastic cups, the strips of coffee filter, and the "sample" strip with the line of unknown ink in it. Have each group draw a line one inch from the bottom with each of the inks to be tested on the strips of coffee filter. One ink sample (pen) per strip. Each strip should then be taped to a coffee stirrer at the non-marked end. Place each strip into the water so that the edge is submerged, but do not submerge to the ink line. Students using cups can place the coffee stirrers across the rim of the cup to hold the strips vertical. When using a tray, pile textbooks on either side to hold the stirrers. Have the students record what happens in their journals When the inks have finished separating have them match the sample inks (pens) to the sample from the unknown writing. Which sample 1-5 matched our written work? Compare dates of each sample to determine when the work was written. Introduce the Scientific Principle/Concept When scientists want to find out what a substance is made of, one of the techniques they can use is chromatography. Chromatography separates substances into their component parts. For example, it would separate the chlorophyll (the green pigment in plants) of a leaf into chlorophyll a, chlorophyll b, and other pigments. Chromatography works because the atoms and molecules of a given compound are attracted with different strengths to other atoms and molecules. Chromatography pushes the substance tested across another stationary substance for which it has an attraction. Molecules with a stronger attraction to the stationary substance are slowed down more than those with a weaker attraction. A measuring device is used to determine what substances are present based on how far they have traveled in a given time, or how fast they arrive at the detector after being placed in the chromatograph. Typically, the substance to be tested is pushed past the stationary substance in a liquid or a gas, which acts as a "carrier." In our experiment, we used liquid chromatography, where our line of ink was separated by a liquid and the component parts carried up a piece of paper. How far up the paper each dye (or component) in the ink traveled depended on how attracted each dye was to the wet paper—those with stronger attractions did not travel as far. This left bands of color, one band for each dye. Each pen, though producing black ink, is really made up of a number of dyes in combination. Not every ink uses the exact same combination of inks. That's why when they were allowed to separate only one matches our sample—the ink that was used to write it. Chromatography can be used in other scientific fields to identify chemicals. Gas chromatography was used to detect DDT amounts in bird eggs. Relate Activity and Concept Have the students try chromatography on different color ink pens. Were the colors used to make each pen what they expected? Which color had ink components that traveled he farthest? The shortest? Did the same pigments (component colors) always travel the same relative distance? Back to Classroom Ideas |
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