Solving a "Crime" with Ink Chromatography

A note has been left on the teacher’s desk: “The exam papers are hidden. Find them if you can.” It’s written in black felt-tip pen. Four suspects each had a black pen in their bag that morning. Your job is to find out which pen wrote the note, using the same principle forensic scientists use to compare inks: chromatography.

This practical works well as a lesson, a science club activity or a careful home experiment. It teaches technique, observation, Rf calculations and, most importantly, how to reason from evidence.

The science in brief

Most black inks aren’t one substance; they’re mixtures of several coloured dyes that together look black. In paper chromatography, a solvent moves up a strip of paper and carries the dyes with it. Each dye is attracted to the paper and dissolves in the solvent to a different extent, so each one travels a different distance. The mixture separates into a pattern of coloured spots, a chromatogram, that acts like a fingerprint for that ink.

If the note’s ink produces the same pattern as one suspect’s pen, that pen is the likely source. See paper chromatography and chromatography explained.

Equipment

• chromatography paper (or filter paper / coffee filters cut into strips about 2 cm × 15 cm)
• the “ransom note” (a piece of paper with writing in one of the pens)
• four black water-soluble felt-tip pens, labelled A, B, C, D (use different brands)
• scissors
• a pencil and ruler
• a tall beaker or glass jar for each strip (or one wide tank for a whole sheet)
• a lid, or cling film, for each container
• water (or a 50:50 mixture of water and ethanol, if the inks don’t move well in water)
• paper clips, and a pencil or wooden splint to hang each strip
• a hairdryer (optional)

Safety

• Wear eye protection if using ethanol; it’s highly flammable, so keep it away from flames. See lab safety rules.
• Water is safe, but clear up spills to avoid slips.
• Don’t put pens or paper in your mouth.

Preparing the samples

The note sample needs special treatment: you’re analysing ink that’s already on paper.

1. Cut a small piece of the note containing a letter or thick line of ink. Cut it so the ink is close to one end.
2. Alternatively, cut out a tiny square with ink on it and stick it to the baseline of your chromatography strip with a small drop of water.

For the suspect pens, you draw directly on the chromatography strip.

Method

1. Draw a pencil baseline about 2 cm from the bottom of each strip (or across a single wide sheet). Pencil doesn’t dissolve, so it won’t interfere.
2. Label each strip at the top in pencil: Note, A, B, C, D.
3. Apply the inks: on each suspect strip, make a small, dark dot of the pen’s ink on the baseline. Let it dry and go over it again two or three times to concentrate it, keeping the dot small (2–3 mm).
4. Prepare the note sample as described above, with its ink on the baseline.
5. Add solvent to each container to a depth of about 1 cm, below the baseline.
6. Hang the strips so the bottom edge dips into the solvent and the strips don’t touch the sides of the containers.
7. Cover each container to keep the air saturated with solvent vapour.
8. Wait until the solvent is about 1–2 cm from the top (typically 10–30 minutes).
9. Remove the strips and immediately mark the solvent front in pencil.
10. Dry the strips (hang them up or use a hairdryer on low).

Recording results

Draw or photograph each chromatogram. Record the colour of each spot and measure:

• the distance from the baseline to the centre of each spot
• the distance from the baseline to the solvent front

Then calculate:

Rf = distance moved by spot ÷ distance moved by solvent front

See Rf value calculations.

Sample results

Solvent front moved 10.0 cm on every strip.

Sample	Spot colours (bottom to top)	Distances (cm)	Rf values
Note	purple, blue, yellow	2.5, 5.4, 8.1	0.25, 0.54, 0.81
Pen A	blue, pink	5.3, 7.0	0.53, 0.70
Pen B	purple, blue, yellow	2.4, 5.5, 8.0	0.24, 0.55, 0.80
Pen C	purple, blue	2.5, 5.4	0.25, 0.54
Pen D	black spot stays on baseline	0	0.00

Analysis

• Pen B matches the note: same three colours, same order, and Rf values within ±0.01 of each other.
• Pen C has two of the same dyes but no yellow, so it can’t have written the note.
• Pen A has a pink dye absent from the note.
• Pen D’s ink didn’t move at all. It’s probably a permanent (water-insoluble) ink. It didn’t write the note, because the note’s ink did move in water.

Conclusion: the evidence is consistent with Pen B writing the note.

Evaluating the evidence

A good scientist, like a good detective, asks what the evidence doesn’t prove.

• Does a match prove Pen B wrote the note? No. It shows the ink is the same type. Every pen of that brand and model would probably give the same chromatogram. The evidence supports the conclusion but can’t prove it alone. See TV forensics vs real forensic chemistry.
• Could two different inks give the same pattern in water? Possibly. Repeating the test in a second solvent (such as a water–ethanol mix) would make a match much stronger.
• Were the conditions the same for all samples? Running all samples on one wide sheet at the same time is better than separate strips, because the solvent and time are identical.

Sources of error and improvements

Problem	Effect	Improvement
Spots too large	Spots overlap and smear; Rf hard to measure	Apply small dots, repeatedly, drying between
Solvent above baseline	Ink dissolves into the solvent reservoir	Check depth before hanging strips
Strip touches container side	Uneven solvent front	Hang centrally
Solvent front not marked	Rf can’t be calculated	Mark immediately on removal
Measuring to spot edge	Inconsistent Rf values	Measure to the centre of each spot
Note ink is less concentrated	Faint spots	Use a thicker section of writing

Questions to answer

1. Why is the baseline drawn in pencil, not ink?
2. Why must the solvent level be below the baseline?
3. Why does black ink separate into several colours?
4. Pen D’s ink didn’t move. Suggest why, and how you could test it.
5. Explain why a matching chromatogram doesn’t prove a particular person wrote the note.
6. Suggest one way to make the comparison more reliable.

Answers in brief: (1) pencil doesn’t dissolve in the solvent; (2) otherwise the ink dissolves into the solvent in the container; (3) it’s a mixture of dyes that move different distances; (4) it’s insoluble in water, so try ethanol or propanone as the solvent (with suitable safety precautions); (5) many pens share the same ink; (6) repeat in a second solvent, run all samples on one sheet, or compare with more pens of the same brand.

Extension ideas

• Two solvents: compare Rf values in water and in a water–ethanol mix.
• Coloured pens: do green or brown pens also contain several dyes?
• Food dyes: analyse sweets’ coatings against known food colourings.
• TLC version: for older students, compare ink separation on TLC plates. See TLC vs paper chromatography.
• Real forensics: find out how document examiners also use spectroscopy and microscopy on inks. See forensic chemistry.

Key takeaways

• Black inks are usually mixtures of dyes that separate by paper chromatography.
• Matching colours, order and Rf values link an unknown ink to a pen type.
• Careful technique (pencil baseline, small spots, solvent below baseline, lid, marked front) is essential.
• A match supports a conclusion but doesn’t prove who wrote the note; testing in a second solvent strengthens it.
• Evaluating what evidence can and can’t show is as important as the result itself.