Optimizing Paper Chromatography for Forensic Ink Identification: A Comprehensive Laboratory Experiment

Categories: Science

In Figure 2, a chromatography trial setup is depicted, where each dot on the starting line represents a sample.

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The chromatography paper is immersed in a liquid mobile phase, causing the samples to migrate up the paper based on their polarity. While paper chromatography is utilized in forensic ink identification, spectroscopy is an alternative method for distinguishing inks. In this experiment, inks are examined using a spectrophotometer and infrared cameras for visual evidence.

Figure 3 illustrates a sample chromatogram with varying distances traveled, component separations, and a final ink showing two components. Achieving an optimal chromatogram involves conducting numerous trials to identify the best-fit conditions. This chromatogram is then used to identify unknown inks, ensuring that they match the preferred chromatogram. Adjustments, such as changing the polarity of the mobile phase, are made to enhance the chromatogram's quality by minimizing component spreading and maximizing migration differences.

The procedure involves an initial base trial with a 2:1 propanol/water solvent, and adjustments are made based on the findings. Chromatography paper is prepared with marked ink samples, stapled, and placed in a petri dish with the prepared mobile phase. After capillary action promotes upward movement, the paper is dried, and component distances are analyzed. RF values, crucial for assessing the separation, are calculated using a formula.

Recognizing the need for a different mobile phase after the incoherence of the base trial, additional trials are conducted with different solvents like methanol, 2:1 water/methanol, 1:1 water/methanol, and 2:1 water/propanol. Trial 4, using a mobile phase of 2:1 water/propanol, is identified as suitable for unknown ink identification. In the final step, this mobile phase is applied to test a set of unknowns (set B), and the resulting chromatogram is visually and numerically compared to the preferred chromatogram and RF values for conclusive identification.

The objective of this laboratory experiment is to identify unknown ink samples through chromatographic analysis. Chromatography is a technique that separates and identifies components in a mixture based on their movement through a stationary phase and a mobile phase. In this experiment, we aim to analyze the chromatograms obtained from Trial 4 and compare them to an Unknown Trial to identify the unknown ink samples.

Materials and Methods:

Chromatography Paper
Unknown Ink Samples (Set B)
Mobile Phase: 2:1 Water/Propanol (Polarity Index: 9, 4)
Ruler
Visual inspection of chromatograms

Chromatogram Analysis: Table 2 and Table 3 provide information on the distances traveled by different ink samples during Trial 4 and the Unknown Trial, respectively. The mobile phase was a constant 3.8 cm from the starting line up the paper. To calculate the Retention Factor (RF) values, we used the formula:

Distance traveled by componentDistance traveled by mobile phaseRF=Distance traveled by mobile phaseDistance traveled by component

RF values were calculated for each component of the unknown ink samples based on the distances provided in Table 3.

Results and Discussion: The chromatograms from Trial 4 and the Unknown Trial exhibited clear differences, primarily attributed to the variation in the mobile phase. Trial 4 employed a mobile phase of 2:1 water/propanol, which proved to be more effective in achieving separation without excessive spreading.

The relationship between the polarity of the mobile phase and the speed of its movement up the chromatography paper was established. A more polar solvent, as evidenced by the 2:1 water/propanol mixture, resulted in a slower movement, preventing smearing and ensuring clear visual differences between samples.

Identification of Unknown Samples: The identification process involved comparing visual evidence between Trial 4 and the Unknown Trial. RF values were then compared for similarities. Unknown samples were identified based on the visual characteristics and RF values. For instance:

Unknown 1 (Red Pilot V-Ball) showed similarities in color components and travel distance compared to known red inks.
Unknown 2 (Blue Pilot Easytouch) exhibited two components with spreading, matching Trial 4's spot 7.
Unknown 3 (Black Pilot V-Ball) displayed characteristics similar to Trial 4's spot 11, confirming its identification.
Unknown 4 (Blue Staples) resembled Trial 4's spot 8, leading to its accurate identification.

The experiment successfully demonstrated the importance of selecting an appropriate mobile phase for chromatographic analysis. The use of a 2:1 water/propanol mixture in Trial 4 provided clear and distinct chromatograms, enabling the identification of unknown ink samples through visual inspection and RF value comparisons.

Future experiments should explore the impact of different mobile phase compositions on chromatographic separation to optimize conditions for various ink samples. Additionally, refining the calculation methods for RF values could enhance the accuracy of sample identification.

This laboratory report summarizes the experimental procedure, results, and conclusions drawn from the chromatographic analysis of unknown ink samples. The insights gained contribute to the understanding of chromatography as a powerful analytical technique for forensic applications.