Chemistry

FORENSIC CHEMISTRY THIN LAYER CHROMATOGRAPHY (TLC)

In this experiment, two different drugs were used to demonstrate the separation mechanism of TLC. Cocaine and benzoylecgonine are drug substances with almost the same chemical structure. Their only difference is the ester functional group for cocaine, while carboxylic functional group for benzoylecgonine (Figure 1). The migration of the drugs were Figure 1. Chemical structure of Cocaine and Benzoylecgonine (Borges et al 2003) compared using a methanol-ammonium hydroxide developing solvent. Potassium iodoplatinate and UV 254 were used to visualise the spots. The Rf value provides information for the spots seen on the TLC plate. The Rf value is calculated as the ratio of the distance travelled by the spot with the distance travelled by the solvent. Rf values are expressed as decimals, and values should not exceed 1.0 (Thin-Layer Chromatography, n.d.). Rf values are expressed in two significant digits due to experimental uncertainties in their measurement (Reich and Schibli 2007). Method A silica gel TLC plate was used for this experiment. A line was drawn at the top and bottom part of the plate. The plate was divided to three, using a pencil to mark the position where the solutions were spotted. The solutions used were methanol (MeOH), cocaine (1 mg/mL in MeOH) and benzoylecgonine solution (1 mg/mL in MeOH). …
The plate was allowed to dry at room temperature then viewed under white light and UV 254nm. The spots for found under UV were marked using a pencil. The plate was then sprayed with acidified potassium iodoplatinate solution and the spots were marked accordingly. The distances travelled by the solvent and the spots were noted. Results and Discussion Cocaine and benzoylecgonine were the drugs used in the TLC experiment. Figure 2 presents the TLC plate after marking the spots. The spot labeled M stands for the methanol solution, spot C is the cocaine solution and B is the benzoylecgonine solution. The solvent front was marked to be able to calculate for the Rf value. It can be seen that the solvent front is slanted, contrary to the expected image that the solvent front will be a straight line. Figure 2. TLC plate after development and spraying of acidified potassium iodoplatinate The slanting may be because the plate was tilted when it was placed inside the developing tank, or the equilibrium inside the chamber was incomplete when the plate was immersed in the tank. It is important that the tank is completely vapor saturated before the plate is immersed (ed. Smith 2005). The Rf values of the drugs were calculated using the distances marked on the plates. Table 1 summarizes the data obtained from the experiment. Because of the slanted solvent front, two different distances of the solvents were used to calculate the Rf values of the drugs. Table 1. Calculation of Rf values for Cocaine and Benzoylecgonine. Solution Distance Travelled by the Spot (cm) Distance Travelled by the Solvent (cm) Calculated Rf Value Methanol No spot Not measured N/A Cocaine (1mg/mL) 4.5 6.5 0.69 Benzoylecgonine (1mg/mL) 1.9 6.2 0.31

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