Experiment 3 - TLC - Discussion

Discussion: According to the experiment, two solvent systems are employed in order to dissolve and assist the mixture compounds to move along the silica plate by capillary action. There are system A (70:30 mixture of hexane: acetone) and system B (50:50 mixture of hexane: acetone). In this study, we intended to determine the unknown chemical compound from the known compounds which are acetaminophen, aspirin and ibuprofen by using the thin layer chromatography (TLC). In this case, there are four chemical compounds have to be separated, thereby, choosing a suitable solvent as a mobile phase is crucial before running the TLC. In solvent system A, there are mixture of 70% of hexane and 30% of acetone. Therefore, this solvent system showed less polarity compared with solvent system B due to the presence of 70 part of non-polar hexane in 30 part of polar acetone which slightly decreases the polarity of the overall solvent. In other words, solvent system A provided a low eluting power in comparison with solvent system B (Weaver, 2013). From the resulting spots appeared on the TLC plate associated with solvent system A, we can deduce that 70:30 mixture of hexane: acetone solvent has a low eluting power compared with solvent system B. This is because the chemical compounds in solvent system A ascended much slower than solvent system B in a given time. As we known, the TLC plate is made up of silica gel, a very polar adsorbent which is likely to absorb the polar compound strongly. In this condition, when a less polar solvent is used, the movement rate of solvent along the TLC plate is reduced because only 30 part of acetone in the solvent is able to absorb properly with polar stationary phase and therefore, the chemical compound which moved along with the solvent will also slow down. In other words, solvent system A is unable to elute the chemical compounds which absorbed with the adsorbent effective in the TLC plate due to its low polarity. As a result, all the resulting spots on the silica plate in system A are located near the baseline. If we insisted to apply this system for separation of the chemical compounds, perhaps we required an additional time to running the TLC (Clark, 2007). When parallel TLC are performed in this experiment, we are able to observe that the spots showed on the silica plate with solvent system B are well-separated compared

with solvent system A. This is because solvent system B is considered more polar and demonstrating a high eluting power which is able to eluted the chemical compound out of the plate faster. In the case of solvent system A, it might be useful for separating the less polar chemical compounds and less polar TLC plate. As a result, less polar solvent is able to move effectively along the TLC plate and giving a more desire result (Weaver, 2013).

Reference: Weaver, S, L., 2013. Thin Layer and Column Chromatography. Available at: http://www.xula.edu/chemistry/crsorgleclab/Organic_Lab_1_2230L_Web_Files/10_TLC _&_%20Column_notes.pdf. [Accessed 22 June 2013]. Clark, J., 2007. Thin Layer Chromatography. Available at http://www.chemguide.co.uk/analysis/chromatography/thinlayer.html. [Accessed 22 June 2013].