Why shouldn't the start line for TLC be drawn in ink?

In thin layer chromatography (TLC), using ink to draw the start line is problematic because the ink may dissolve in the solvent used during the experiment. If the ink dissolves, it can lead to the creation of additional spots on the chromatogram. These unwanted spots can complicate the analysis, as they may interfere with the separation of the compounds being studied. Ideally, the start line should use a material that is inert to the solvent to ensure clarity in the results and avoid confounding factors that could affect the interpretation of the migration patterns of the analytes.

Using pencil, for example, is a common practice because the graphite does not dissolve in most solvents, ensuring that the start line remains stable throughout the TLC process. This helps maintain the integrity of the chromatogram, making it easier to analyze the separation of the compounds accurately.