Summary
Highlights
The video introduces the experiment: analyzing two unknown analgesic powders using TLC to identify them. The first unknown, number nine, is used as an example. A small amount of the unknown powder, equivalent to half a tablet, is scooped into a test tube. A 50/50 mixture of ethanol and dichloromethane (5ml) is added to dissolve the active ingredients, while inactive binders like starch and cellulose remain undissolved. The solution is then capped and gently shaken for several minutes.
A TLC plate is prepared by holding it by the edges to avoid smudging. The dull, silica gel side is used for the experiment. Two light pencil lines are drawn across the bottom and top of the plate, about one centimeter from each edge, to avoid scraping off the silica gel. Five equally spaced hash marks are then made on the bottom line to indicate spotting positions. Each spot is labeled with a letter (A, B, C, D, E) to keep track of the samples.
Micro-pipettes are made from glass capillary tubing using a Bunsen burner. The tubing is heated in the hottest part of a clean, blue flame until it softens, then pulled apart into two thin, elongated sections once removed from the flame. It's crucial to pull the tubing apart *after* removing it from the flame to ensure the ends are open and functional. Incorrect technique (pulling in the flame) can result in unusable, closed-end pipettes.
The TLC plate is spotted with four analgesic standards and the prepared unknown solution. Acetaminophen, aspirin, and caffeine are spotted once in lanes A, B, and C, respectively, using separate micro-pipettes. For ibuprofen (lane D), multiple applications are made to ensure a sufficiently strong spot, as it typically yields a weaker result. The unknown solution is spotted once in lane E. Samples are drawn into the pipettes via capillary action and gently touched to the plate.
After spotting, the plate's spots are checked for concentration using UV light. The TLC plate contains a fluorescent indicator, causing spots to appear as dark circles under UV illumination. The room lights are dimmed to enhance visibility. All spots should be clearly visible as dark circles and be approximately 1-2 millimeters in diameter. If any spots are too weak, additional applications can be made, but over-spotting can lead to smearing and indistinct blobs.
A 250ml beaker is used as a developing chamber. Filter paper is folded and placed inside to act as a wick and saturate the atmosphere with solvent vapor, preventing evaporation from the plate. Ethyl acetate, the mobile phase, is added to saturate the filter paper and form a shallow pool at the bottom. The TLC plate is carefully placed in the chamber, ensuring the solvent level is below the spots to prevent them from dissolving. A watch glass covers the beaker to further prevent solvent evaporation. The solvent moves up the plate by capillary action, carrying the analgesic spots at different rates based on their interaction with the stationary phase and mobile phase. The process is sped up for the video, but usually takes longer. The plate is removed and dried once the solvent front reaches the top pencil line.