The Shape of Analysis Part I

We did the “extraction of caffeine from tea” and “extraction of caffeine from beverages” this spring in the introduction to chemistry course. The caffeine extraction lab is one of my standard experiments and seems to be adaptable to different levels of students. For the analysis of the students’ extracted caffeine we did TLC which is a good basic technique to practice if not terribly exciting. The students were fascinated by the sublimation of caffeine this year. I don’t remember that so much from previous years. On the first “tea” report, I had the students consider a series of UV traces taken between 210 and 300 nm. I presented 1) tea, 2) aqueous tea after extraction, 3) the dicholoromethane extract of tea, and 4) a caffeine standard. I asked them “is there any evidence that caffeine is present in tea but not in tea after extraction with dichloromethane?” That answer to this was rather ambiguous because the UV trace was not done quantitatively. The “hump” between 250 and 290 nm was a little flatter in the “after extraction” spectrum but not very much. The key was for the students to realize that they were supposed to compare spectrum 1) and spectrum 2) to answer the question. The other question I asked was “is there any evidence that caffeine is extracted from tea with dichloromethane?” For this one, the students were supposed to compare spectrum 3) and spectrum 4). This was noticeably difficult for students. First to figure out which spectra they were supposed to consider and second to make sense of those UV sloping curves. We had a UV-vis lecture in class that week so they could have drawn on that information as well. The ability to grasp the significance and limitations of UV spectra is challenging at the introductory level, but I felt that it was necessary for them to look at the shapes rather than the numbers (lambda max and/or Absorbance) that can be treated mathematically by Beer’s Law.

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