Paper Chromatography
Separating the invisible
Two black pens look identical. A detective needs to know if the same pen wrote two different notes. How could you possibly tell them apart — just from the ink — without a microscope?
Think about what "black" ink actually is.
Take a black marker and draw a spot on filter paper. Dip the edge of the paper in water. Watch what happens: the "black" ink separates into a rainbow — blue, red, yellow, violet — as the different dye components travel up the paper at different speeds. Black pen ink is actually a mixture of several coloured dyes. Chromatography reveals what's hidden in plain sight.
How Paper Chromatography Works
Paper chromatography separates a mixture of substances based on how strongly each component is attracted to the paper (stationary phase) vs. how soluble it is in the solvent (mobile phase).
The two phases:
- Stationary phase: The chromatography paper (which holds water in its fibres)
- Mobile phase: The solvent (water, ethanol, or a mixture) that moves up the paper by capillary action
What happens:
As the solvent travels up the paper, it carries the components of the mixture with it. Components that are more attracted to the paper move slowly. Components that are more soluble in the solvent move quickly. The result: the mixture separates into distinct spots at different heights.
AI Generation Prompt
A rectangular white chromatography paper strip on a dark background showing a completed paper chromatography experiment. At the bottom: a faint pencil line (origin) with a single black dot (ink spot applied). Above it, distinct coloured bands separated at different heights: a yellow-orange band low (β-carotene), a yellow-green band mid (xanthophyll), a bright green band higher (chlorophyll a). At the very top: a dashed line labeled "Solvent front". Two red brackets with Rf value labels: one pointing to carotene band showing "Rf = 0.98", another to chlorophyll showing "Rf = 0.65". Clean, precise, educational chemistry style on dark background. Orange labels.
The Rf Value — Retention Factor
Loading simulator…
Applications:
- Identifying dyes in food colourings (food safety testing)
- Testing purity of drugs (FSSAI, pharmaceutical labs)
- Forensic analysis — identifying inks on disputed documents
- Testing blood plasma and urine samples
- Identifying plant pigments (chlorophyll a, b, carotenoids) in botany
A student runs paper chromatography on a black water-colour marker and a black permanent marker using water as the solvent. The water-colour ink separates into blue, red, and yellow spots. The permanent marker shows only one black spot that barely moves from the baseline. Why does the permanent marker behave differently?
Q1.In paper chromatography, the Rf value of a substance is calculated as:
Where You See This Every Day
Chromatography is so versatile that it appears in settings as different as food labs and crime scenes:
- Food Safety in India — FSSAI (Food Safety and Standards Authority of India) uses chromatography to detect illegal artificial food colours in sweets, snacks, and beverages. Cadmium yellow, Sudan Red, and Rhodamine B are banned but found in street food — they are identified by their unique Rf values in chromatography.
- Anti-Doping in Sports — NADA (National Anti-Doping Agency) tests athletes' urine using High-Performance Liquid Chromatography (HPLC, an advanced version of paper chromatography). Prohibited substances leave a unique chromatographic fingerprint that cannot be masked.
- Forensic Science — The CBI and state forensic labs use chromatography to analyse ink in disputed documents (comparing Rf values of questioned ink with known samples), detect poisons in blood, and identify accelerants in arson cases.
- Blood Plasma Proteins — Hospitals separate blood plasma proteins by chromatography to diagnose conditions like multiple myeloma (abnormal protein bands on the chromatogram are diagnostic).
- Plant Pigments — Every time a leaf changes colour in autumn, it is revealing the chromatographic truth: the green chlorophyll was masking the yellow and orange carotenoids all along.
🔬 Real-World Impact
The ink on your Class 10 Board exam answer sheet can be forensically verified by chromatography. The Supreme Court has upheld cases where disputed signatures were authenticated by ink chromatography analysis. A technique invented in 1903 by a Russian botanist is now admissible evidence in the highest court of the land.
Image needed — generation prompt:
A close-up square photograph of a scientist's gloved hand holding a paper chromatography strip showing vivid separated colour bands — pink, yellow, blue — against a soft-focus lab bench background. Clean laboratory lighting. Square (1:1) format. No text overlay.
Two black pens look identical. A detective needs to know if the same pen wrote two different notes. How could you possibly tell them apart — just from the ink — without a microscope?
Think about what "black" ink actually is.
Take a black marker and draw a spot on filter paper. Dip the edge of the paper in water. Watch what happens: the "black" ink separates into a rainbow — blue, red, yellow, violet — as the different dye components travel up the paper at different speeds. Black pen ink is actually a mixture of several coloured dyes. Chromatography reveals what's hidden in plain sight.
How Paper Chromatography Works
Paper chromatography separates a mixture of substances based on how strongly each component is attracted to the paper (stationary phase) vs. how soluble it is in the solvent (mobile phase).
The two phases:
- Stationary phase: The chromatography paper (which holds water in its fibres)
- Mobile phase: The solvent (water, ethanol, or a mixture) that moves up the paper by capillary action
What happens:
As the solvent travels up the paper, it carries the components of the mixture with it. Components that are more attracted to the paper move slowly. Components that are more soluble in the solvent move quickly. The result: the mixture separates into distinct spots at different heights.
AI Generation Prompt
A rectangular white chromatography paper strip on a dark background showing a completed paper chromatography experiment. At the bottom: a faint pencil line (origin) with a single black dot (ink spot applied). Above it, distinct coloured bands separated at different heights: a yellow-orange band low (β-carotene), a yellow-green band mid (xanthophyll), a bright green band higher (chlorophyll a). At the very top: a dashed line labeled "Solvent front". Two red brackets with Rf value labels: one pointing to carotene band showing "Rf = 0.98", another to chlorophyll showing "Rf = 0.65". Clean, precise, educational chemistry style on dark background. Orange labels.
The Rf Value — Retention Factor
Loading simulator…
Applications:
- Identifying dyes in food colourings (food safety testing)
- Testing purity of drugs (FSSAI, pharmaceutical labs)
- Forensic analysis — identifying inks on disputed documents
- Testing blood plasma and urine samples
- Identifying plant pigments (chlorophyll a, b, carotenoids) in botany
A student runs paper chromatography on a black water-colour marker and a black permanent marker using water as the solvent. The water-colour ink separates into blue, red, and yellow spots. The permanent marker shows only one black spot that barely moves from the baseline. Why does the permanent marker behave differently?
Q1.In paper chromatography, the Rf value of a substance is calculated as:
Where You See This Every Day
Chromatography is so versatile that it appears in settings as different as food labs and crime scenes:
- Food Safety in India — FSSAI (Food Safety and Standards Authority of India) uses chromatography to detect illegal artificial food colours in sweets, snacks, and beverages. Cadmium yellow, Sudan Red, and Rhodamine B are banned but found in street food — they are identified by their unique Rf values in chromatography.
- Anti-Doping in Sports — NADA (National Anti-Doping Agency) tests athletes' urine using High-Performance Liquid Chromatography (HPLC, an advanced version of paper chromatography). Prohibited substances leave a unique chromatographic fingerprint that cannot be masked.
- Forensic Science — The CBI and state forensic labs use chromatography to analyse ink in disputed documents (comparing Rf values of questioned ink with known samples), detect poisons in blood, and identify accelerants in arson cases.
- Blood Plasma Proteins — Hospitals separate blood plasma proteins by chromatography to diagnose conditions like multiple myeloma (abnormal protein bands on the chromatogram are diagnostic).
- Plant Pigments — Every time a leaf changes colour in autumn, it is revealing the chromatographic truth: the green chlorophyll was masking the yellow and orange carotenoids all along.