Spring SAVY 2017, Day 2- Chromatography 101
This week in Chromatography 101 we began by reviewing the three components of chromatography and discussing some of the limitations of paper chromatography (the technique we used last week to separate ink components and determine which pen had forged part of a check). The students pointed out that not only were there limitations to what conclusions could be drawn from ink analysis (e.g. someone could have stolen someone else’s pen to forge the check), but also that if the chemical components of a mixture were invisible, we would not be able to detect them using this method.
The latter situation proved the case this week when the students were called upon to use chromatography to determine if explosives, as opposed to some natural cause, were responsible for the burning of an abandoned barn. (Do not be alarmed—no actual explosive materials were used in the laboratory experiment.) When these “explosives” are dissolved in a solvent such as methanol, they give colorless solutions. In order to determine which explosives (if any) were present at the crime scene, we learned a new technique called thin layer chromatography (TLC). In TLC, the stationary phase is a silica gel-coated glass plate and the mobile phase composition is optimized experimentally to provide the best separation between mixture components. The components are invisible when spotted on the plates and invisible post-chromatography as well. However, many of these explosives absorb UV light, so the TLC plates are coated such that they appear green under UV light and dark spots (where the compounds have absorbed this light) indicate where the different components of the mixture have migrated. Ask your student about how he/she designed his/her experiment and about the results. We had a lively discussion about why this experiment was more difficult than the previous one and the students brainstormed some ways to make their results less ambiguous were they to perform the experiment again; I encourage you to ask them to share these ideas with you.
Finally, I posed the following question to the students: If you were to create a kit based on this technique (TLC) for use by U.S. soldiers on the ground in the Middle East, how might you modify it to suit their particular needs? In fact, the U.S. military does use TLC for detection of explosives and illicit drugs in the field, and research scientists from Lawrence Livermore National Laboratory won an award a few years ago for their development and optimization of a TLC technique for just such a purpose (https://str.llnl.gov/october-2014/reynolds). I encourage you to read and discuss this article with your child.
See you next week!