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TLC Ink Separation Interactive Diagram

Run the TLC Ink Separation Diagram Fullscreen

About This MicroSim

A pen line that looks like a single color is usually a mixture of several dyes. Thin-layer chromatography (TLC) pulls that mixture apart: a tiny spot of ink is placed near the bottom of a coated plate, the plate stands in solvent, and as the solvent front climbs, it carries each dye a different distance. The result is a ladder of separated spots that acts like a fingerprint for that ink.

This MicroSim lets you run the separation, read the Rf value of each dye, flip to UV light to uncover invisible components, and compare a questioned ink to three reference lanes to decide which formulation it matches — exactly what a questioned-document examiner does to detect a forgery or an altered date.

How to Use It

  1. The plate on the left has four lanes: Q (questioned) and R1, R2, R3 (references). The dashed line at the bottom is the origin; the blue line is the solvent front.
  2. Press Run Chromatography to watch the solvent front rise and the dye spots separate and climb to their resting positions.
  3. Press Calculate Rf to label each spot with its Rf value — the spot's distance divided by the solvent-front distance. The right panel shows the numbers.
  4. Press Toggle UV to switch to ultraviolet light. The plate darkens and any fluorescent components (like optical brighteners) light up — some are invisible in white light.
  5. Use the Questioned ink dropdown to change the unknown pen, and the Reference set dropdown to swap the reference lanes.
  6. Read the Match Determination box: it reports which reference the questioned ink matches and how many components agree.

What You Can Learn

  • Explain why a single ink produces several spots on a TLC plate.
  • Calculate an Rf value and use it to compare dyes objectively.
  • See how UV light reveals components that white light misses.
  • Decide whether two inks match by comparing their components one by one — not just their overall color.

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/tlc-ink-separation/main.html"
        width="100%" height="602" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 8–12 minutes Bloom level: Understand (L2) — explain.

Worked example. With the default ballpoint ink and Reference set 1, press Run Chromatography, then Calculate Rf. Notice that lane R2 has spots at the same heights (same Rf values) as the questioned lane — the Match Determination reads 3/3. Now press Toggle UV: a green fluorescent spot at Rf ≈ 0.62 appears in both Q and R2, a hidden component that confirms the match.

Guided questions:

  • A dye traveled 26 mm while the solvent front traveled 50 mm. What is its Rf?
  • Why is Rf a better way to compare two inks than just looking at their color?
  • Switch to Reference set 2. Why does the Match Determination now say "no full match"? Which lane is closest, and why isn't it good enough?
  • What can UV light show that white light cannot, and why does that matter when comparing inks on a questioned document?

Extension. Change the Questioned ink to the gel pen, then to the fountain pen. Describe how the number and spacing of spots differs between a pigment-based gel ink and a dye-based ballpoint, and what that tells you about the ink's composition.

References

Specification

This MicroSim was generated from a specification in Chapter 14: Document Examination and Forgery Detection.

Design note: the inks, dye colors, and Rf values are illustrative teaching data chosen to make the comparison clear, not measurements of specific commercial pens. Real ink analysis uses calibrated plates, standards, and often instrumental methods alongside TLC, and is reported by a qualified examiner.