Technique for Checking Ink Used in Documents: Thin Layer Chromatography

Introduction
In a court case, the judge suspected that some changes had been made to the documents presented as evidence. To verify this suspicion, the judge requested the forensic department to examine the ink used at two different places on the documents. Various techniques can be used to analyze the ink composition, but the most suitable technique in this case would be thin layer chromatography (TLC).

Explanation
1. Solvent Extraction
Solvent extraction involves dissolving the ink in a suitable solvent to separate its components. However, this technique may not be effective in this case because it does not provide a clear separation of the ink components. It would not be able to distinguish between different inks used at two different places on the document.

2. Distillation
Distillation is a technique used to separate liquids based on their boiling points. However, it is not suitable for analyzing ink composition as it does not provide the required resolution to identify and compare the components of the ink.

3. Thin Layer Chromatography (TLC)
TLC is a widely used technique for analyzing complex mixtures such as ink. It involves spotting a small amount of the ink sample onto a thin layer of adsorbent material (typically silica gel or alumina) on a glass or plastic plate. The plate is then placed in a solvent chamber, and the solvent rises up the plate by capillary action, carrying the ink components with it.

Key points about TLC:
- TLC provides excellent separation of components in a mixture, making it ideal for analyzing inks.
- The relative movement of the ink components on the TLC plate allows for comparison and identification of different inks used at two different places on the document.
- The separation is based on the different affinities of the ink components towards the adsorbent material and the solvent used.
- The separated components can be visualized by exposing the TLC plate to various detection methods such as UV light, iodine vapor, or chemical reagents specific to ink components.
- The Rf (retention factor) value, which represents the distance traveled by a component divided by the distance traveled by the solvent, can be used to compare and identify different ink components.

Conclusion
In conclusion, among the given options, thin layer chromatography (TLC) is the most suitable technique for analyzing the ink used at two different places on the documents presented in court. TLC provides excellent separation and identification of ink components, allowing for a clear comparison between the inks used.