The Shape of Analysis Part II

This is a second post referring to the “extraction of caffeine from tea” and “extraction of caffeine from beverages” that we did this spring in the introduction to organic chemistry course. On the first “tea” report, I had the students consider a series of HPLC chromatograms that paralleled the previously described UV traces. I presented 1) tea, 2) aqueous tea after extraction, 3) the dicholoromethane extract of tea, and 4) a caffeine standard. We have done TLC several times already, so I hoped that the HLPC would “make sense” as related chromatography method. I asked them “is there any evidence that caffeine is present in tea but not in tea after extraction with dichloromethane?” This was very much in evidence because the caffeine peak was much less intense in the second chromatogram. The key was for the students to realize that they were supposed to compare chromatogram 1) and chromatogram 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 chromatogram 3) and chromatogram 4). The chromatograms for “caffeine extracted from tea” and “caffeine standard” are both very clean and almost identical. The interesting thing is what students who are not familiar with column chromatography “see” in these chromatograms. Some considered the first set of three peaks (closest to the left of the chromatogram) to be the most significant.

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