The Science of Pen Chromatography

Chromatography is a powerful technique used by scientists to separate chemical mixtures. When we use it on ink or food colouring, we can see the results with our own eyes as the hidden “ingredients” of a colour begin to reveal themselves. While we can see dyes easily, this same method is used in labs to separate invisible compounds like sugars or amino acids, which are then “tagged” with stains so they can be identified.

The process relies on a mobile phase (the solvent, like water) moving through a stationary phase (the paper). As the water travels, it carries the ink mixture with it. Different components of the ink move at different speeds based on their solubility and their “affinity” for the paper versus the solvent. Essentially, it’s a race: molecules that are more hydrophilic (water-loving) stay in the water longer and travel further, while those that are more attracted to the paper fibres lag behind.

The water climbs the paper against gravity thanks to capillary action. This is powered by two forces: adhesion (water molecules sticking to the paper fibres) and cohesion (water molecules sticking to each other). Together, they pull the liquid upwards through the tiny gaps in the paper.

Scientific Myth-Busting: A common misconception is that ink separates based on the size of the molecules (with smaller ones travelling further). In paper chromatography, it is actually about chemical polarity and solubility. However, size is a major factor in gel electrophoresis, a different technique used to separate DNA and proteins!

Variables to test

Find out more on variables here.

• Substrate Porosity
  Try coffee filter paper versus paper towelling or even newspaper. Does the thickness or “tightness” of the paper fibres change how clearly the colours separate?
• Solvent Properties
  Permanent markers won’t budge in water because their ink is non-polar. Try using methylated spirits, isopropyl alcohol, or turpentine. Which solvents “unlock” which inks?
• Ink Composition
  Do biro pens or highlighters work? Compare water-based “washable” markers to “professional” art pens to see which brands use the most complex blends of dyes.
• Environmental Factors
  Does the temperature of the solvent affect the speed of the capillary action? Does hot water make the “race” happen faster?

Extension: Forensic Investigation

Put your skills to the test in a mock forensic lab! You can use chromatography to “fingerprint” a pen used in a crime. By comparing the chromatography pattern (the chromatogram) of a ransom note to pens found on different suspects, you can identify the exact match. No two brands of black pens are quite the same!