โš— CAPE Chemistry โ€” Unit 2, Module 2

Chromatographic Methods
of Separation

An interactive learning suite for university and advanced-level students. Explore, simulate, and master chromatography.

4Simulator Types
40+Real Substances
25Quiz Questions
8.1โ€“8.6CAPE Objectives
Notes

Comprehensive notes aligned to CAPE objectives 8.1โ€“8.6 and 9.1. Covers principles, types, applications, and terminology.

Simulator

Run interactive simulations of TLC, Paper, Column, and Gas-Liquid Chromatography with real-world substance data.

Quiz

Test your knowledge with 25 exam-style questions covering all CAPE chromatography objectives.

CAPE Objectives Covered
8.1 โ€” Theoretical principles of chromatographic methods (adsorption & partition)
8.2 โ€” Terms: retention factor (Rf), retention time (tR), visualising agent, solvent front
8.3 โ€” Basic steps in separating and quantifying mixture components
8.4 โ€” Examples of stationary phases (cellulose, silica gel, alumina)
8.5 โ€” Separating amino acids, plant pigments, and food colourings
8.6 โ€” Wide applications: pesticide analysis, forensic testing, natural product purification
What is Chromatography?

"Chromatography is the non-destructive separation of chemical substances by selective sorption โ€” the differential distribution of components between a stationary phase and a mobile phase."

๐Ÿ”ฌ Core Principle
Components move at different rates because they have different affinities for the stationary and mobile phases. The greater the affinity for the stationary phase, the slower the movement.
๐Ÿ’ก Did You Know?
The word chromatography literally means "colour writing" โ€” derived from two Greek words. In its earliest days, the technique was only used to separate coloured plant pigments. Today it is one of the most powerful analytical tools in science, used from pharmaceutical testing to space exploration.

The Four Types We Explore

๐Ÿงช TLC โ€” Thin Layer

Silica or alumina on a glass/plastic plate. Very fast, inexpensive. Used for amino acids, dyes, drugs. Components identified by Rf value after visualisation.

๐Ÿ“„ Paper Chromatography

Cellulose paper as stationary phase. Classic technique for separating plant pigments, amino acids, and food dyes.

๐Ÿ› Column Chromatography

Silica or alumina packed into a vertical column. Preparative scale. Components elute in fractions for collection.

๐Ÿ“ˆ Gas-Liquid (GLC)

Carrier gas through a stationary liquid phase. Identifies volatile compounds by retention time. Coupled to MS for confirmation.

Comprehensive Notes

Expand each section to read detailed notes aligned with CAPE Unit 2, Module 2 objectives.

8.1Theoretical Principles of Chromatography
โ–ผ

CAPE Objective Explain the theoretical principles upon which chromatographic methods are based, in terms of adsorption and partition between the mobile and stationary phases.

๐Ÿ”ฌ Core Principle
Chromatography separates substances using two competing forces: the tendency of a substance to remain on the stationary phase versus its tendency to dissolve in and move with the mobile phase. Components that interact more strongly with the stationary phase move slowly; those with greater affinity for the mobile phase travel faster.

The Two Phases

Mobile Phase: The carrier โ€” a liquid or gas that moves through or over the stationary phase, carrying the analyte with it. Its polarity is critical in determining separation quality.

Stationary Phase: The fixed material โ€” a solid or a liquid supported on a solid. The analyte components interact with it to varying extents, causing differential migration.

Two Types of Separation Mechanism

โš— Adsorption Chromatography
The stationary phase is a solid (e.g., silica gel, alumina). Components form bonds of varying strength with the solid surface. Molecules more strongly adsorbed move slowly; those weakly adsorbed move faster with the solvent. The equilibrium is:

Substance(adsorbed) โ‡Œ Substance(dissolved in mobile phase)

Used in TLC and column chromatography with solid stationary phases.
๐Ÿ’ง Partition Chromatography
The stationary phase is a liquid trapped on a solid support (e.g., water adsorbed on cellulose fibres). Components distribute between the two liquid phases according to their partition coefficient โ€” their relative solubility in each phase. Components more soluble in the stationary liquid are retarded; those more soluble in the mobile phase travel faster. Used in paper chromatography and GLC.

Types of Chromatography by Phase

The CAPE syllabus specifically requires reference to: paper, column, thin layer, and gas-liquid chromatography.

๐Ÿ’ก Historical Note
Chromatography was invented in 1900 by the Russian botanist Mikhail Tsvet, who used calcium carbonate columns to separate leaf pigments. He named it "chromatography" after the coloured bands he observed.
8.2Key Terms: Rf, tR, Visualising Agent, Solvent Front
โ–ผ

CAPE Objective Explain the terms: retention factor (Rf) and retention time (tR); visualising agent; solvent front.

Retention Factor (Rf)

๐Ÿ“ Formula
Rf = Distance moved by substance from baselineDistance moved by solvent front from baseline

Rf values range from 0 to 1. They are dimensionless and are characteristic for a given substance under the same conditions (solvent, stationary phase, temperature).

Interpreting Rf:

  • High Rf (closer to 1.0): substance is less polar, travels far โ€” greater affinity for the mobile phase
  • Low Rf (closer to 0): substance is more polar, stays near origin โ€” stronger interaction with the stationary phase
๐Ÿ“Œ Important Note on Rf Values
Rf values are only reproducible when temperature, solvent composition, stationary phase, and technique are kept constant. The same substance will have different Rf values with different solvents! Always report conditions alongside Rf values.

Retention Time (tR) โ€” Used in GLC

In gas-liquid chromatography, the retention time is the time elapsed from injection of the sample to the peak maximum of a component in the detector output. It is used instead of Rf to identify components. Like Rf, it is characteristic for a substance under fixed conditions (carrier gas, flow rate, column, temperature).

Solvent Front

The solvent front is the farthest point reached by the mobile phase (solvent) on the chromatogram. It defines the maximum possible distance of travel. In TLC and paper chromatography, the solvent front must be marked immediately when the run ends, as it evaporates quickly.

Visualising Agent (Locating Agent)

Many compounds are colourless and cannot be seen on a chromatogram. A visualising agent is used to reveal their positions. Common methods:

  • Ninhydrin spray: Reacts with amino acids to give purple/brown spots
  • Iodine chamber: Iodine vapour stains many organic compounds brown/yellow (CAPE practical: iodine chamber specifically mentioned)
  • UV lamp: Fluorescent TLC plates glow under UV; compounds absorbing UV appear as dark spots
  • Potassium permanganate: For reducing substances
  • Fluorescamine: For amines
๐Ÿงช CAPE Practical Suggestion
Use TLC to investigate mixtures and pure compounds (plant extracts, dyes, inks). Visualisation may be done using an iodine chamber. Calculate Rf values. Suitable mixtures: amino acids, plant pigments, food colouring.
8.3Separating and Quantifying Mixture Components
โ–ผ

CAPE Objective Describe the basic steps involved in separating and quantifying the components of a mixture. Use of Rf values and retention times in the quantitation of substances is required.

TLC / Paper Chromatography Procedure

  1. Draw a pencil baseline 1โ€“2 cm from the bottom of the plate/paper (never ink โ€” ink contains dyes that will be separated themselves!)
  2. Apply small spots of the sample and known reference compounds on the baseline using a capillary tube. Keep spots small (โ‰ˆ1 mm diameter).
  3. Place the plate/paper in a covered chamber with the appropriate solvent at a depth below the baseline.
  4. Allow the solvent to rise by capillary action. Cover the chamber to prevent solvent evaporation (this also ensures vapour saturation).
  5. When the solvent front nears the top, remove the plate and immediately mark the solvent front in pencil.
  6. Allow to dry. Visualise spots if colourless (UV, iodine, ninhydrin).
  7. Measure distances from baseline to centre of each spot and to the solvent front. Calculate Rf values.
  8. Identify components by comparing Rf values with known standards run simultaneously.
โš– Quantification via TLC
For precise quantification, spots can be cut from the TLC plate, the compound dissolved out with a solvent, and the resulting solution analysed by UV-Vis spectrophotometry or mass spectrometry. This combines the separation power of TLC with the quantitative precision of spectroscopy.

Column Chromatography Procedure

  1. Pack the column with the stationary phase (slurry method ensures no air gaps)
  2. Add the mixture to the top of the column as a concentrated solution
  3. Continuously add fresh solvent (eluent) to the top
  4. Collect fractions (1โ€“100 cmยณ) at the bottom in test tubes
  5. Each component elutes as a separate band at a different time
  6. Analyse fractions by UV-Vis, measure absorbance to determine amount in each fraction

GLC Quantification

In GLC, each component produces a peak. The area under each peak is proportional to the amount of that component.

๐Ÿ“ GLC Composition Formula
% of component A = (Peak area of A / Sum of all peak areas) ร— 100%

Since peaks are approximately triangular: Peak area โ‰ˆ ยฝ ร— base ร— height

8.4Stationary Phases: Cellulose, Silica Gel, Alumina
โ–ผ

CAPE Objective Name examples of commonly used stationary phases, including cellulose, silica gel, and alumina.

๐ŸŒฟ Cellulose (Paper)

The cellulose fibres in chromatography paper trap water which acts as the stationary phase. Very polar. Used in paper chromatography. Best for hydrophilic compounds: amino acids, sugars, plant pigments.

๐Ÿชจ Silica Gel (SiOโ‚‚)

Finely powdered silicon dioxide. Polar, slightly acidic. Most common stationary phase for TLC and column chromatography. Excellent for organic compounds. Large surface area (200โ€“500 mยฒ/g).

๐Ÿงฑ Alumina (Alโ‚‚Oโ‚ƒ)

Aluminium oxide. Polar, slightly basic. Good for acid-sensitive compounds. Also used as a thin film or narrow column. More reactive than silica โ€” requires care with unstable compounds.

๐Ÿ“Š Comparison Summary
Phase Type Polarity Best For
Cellulose Partition High (polar) Amino acids, sugars, pigments
Silica Gel Adsorption Medium-High Most organic compounds
Alumina Adsorption Medium-High Basic/neutral compounds
Hydrocarbon oil Partition (GLC) Low Volatile organic compounds
๐Ÿ’ก Why Fine Powder?
Adsorbents are used as fine powder to present a very large surface area to the component molecules. This maximises adsorption and allows the equilibrium to be established rapidly as the mobile phase flows through. Silica gel has a surface area of up to 500 mยฒ per gram!
8.5Separating Amino Acids, Plant Pigments & Food Colouring
โ–ผ

CAPE Objective Separate the components of mixtures. Suitable mixtures include amino acids, plant pigments, food colouring.

Practical Perform simple experiments using paper and column chromatographic techniques.

Amino Acids

Amino acids are polar molecules. On silica TLC with n-butanol:acetic acid:water (3:1:1) solvent system, their Rf values depend on their polarity and side chain. Visualised with ninhydrin spray (gives purple to brown colours). Key values:

๐Ÿ“Š Amino Acid Rf Values (silica, n-BuOH:AcOH:Hโ‚‚O 3:1:1)
Amino AcidRfPolarity
Lysine0.14Very polar (charged)
Glycine0.20Polar
Aspartic acid0.25Very polar
Alanine0.40Moderately polar
Valine0.53Slightly nonpolar
Phenylalanine0.68Nonpolar (aromatic)
Leucine0.73Nonpolar (aliphatic)

Plant Pigments

Spinach leaves contain multiple pigments separable by paper or TLC using petroleum ether:acetone as solvent. Rf values reflect polarity (carotene least polar, xanthophylls most polar):

๐Ÿƒ Plant Pigment Rf Values (petroleum ether:acetone 92:8)
PigmentColourRf
ฮฒ-Caroteneโ–  Orange0.98
Pheophytinโ–  Olive/Grey0.81
Chlorophyll aโ–  Blue-green0.59
Chlorophyll bโ–  Yellow-green0.42
Xanthophyll 1โ–  Yellow0.28
Xanthophyll 2โ–  Yellow0.15
๐Ÿงช Experiment โ€” Plant Pigments by Paper Chromatography
  1. Grind spinach leaves in acetone to extract pigments
  2. Apply concentrated spot to chromatography paper
  3. Use petroleum ether:acetone (92:8) as mobile phase
  4. Run in covered chamber for 20โ€“30 minutes
  5. Mark solvent front, measure bands and calculate Rf
  6. Identify bands by colour and Rf value comparison

Food Colouring (Dyes)

Food dyes in sweets and drinks can be separated on cellulose or silica TLC. The dyes are naturally coloured, so no visualising agent is needed. This makes food dye TLC an ideal beginner experiment.

๐Ÿ’ก Two-Dimensional Chromatography
When two compounds have similar Rf values and cannot be separated in one run, 2D chromatography is used. After the first run, the paper is rotated 90ยฐ and a second run is performed using a different solvent. This greatly increases resolution.
8.6Applications of Chromatographic Separation
โ–ผ

CAPE Objective Cite the wide applications of chromatographic methods of separation. Refer to pesticide analysis, forensic testing, purification of natural products.

๐ŸŒพ Pesticide Analysis

GLC and HPLC are used to detect and quantify pesticide residues in food, water, and soil at parts-per-billion concentrations. Retention times are matched against known standards. GLC-MS provides definitive identification.

๐Ÿ” Forensic Testing

Drug identification in blood/urine samples; ink analysis (forgery detection via TLC of pen inks); explosives residue analysis; arson investigation (GLC of accelerants). GLC retention times matched against drug libraries.

๐ŸŒฟ Purification of Natural Products

Column chromatography at large scale purifies plant-based drugs (e.g., limonene), essential oils, and pharmaceutical compounds. Each fraction collected is analysed separately and pure fractions combined.

๐Ÿ“‹ Other Important Applications
  • Clinical chemistry: Amino acid profiling in urine for metabolic disease diagnosis
  • Food science: Separating and identifying flavour compounds, antioxidants, vitamins
  • Pharmaceutical: Quality control of drug purity; identifying degradation products
  • Environmental monitoring: Detecting pollutants in air and water
  • Sports doping: Testing blood/urine of athletes for performance-enhancing drugs
  • Petroleum industry: Analysing fuel compositions by GLC
GLCGas-Liquid Chromatography โ€” Detailed
โ–ผ

How GLC Works

Gas-Liquid Chromatography (GLC) is a form of partition chromatography. The stationary phase is a high-boiling point liquid (e.g., long-chain hydrocarbon oil) supported on a porous inert solid such as silica or alumina, packed into a long spiral tube. The mobile phase is an unreactive carrier gas (nitrogen, helium, or argon).

๐Ÿ”ง The GLC Apparatus
  • Sample injection port: Sample vaporised and injected into carrier gas stream
  • Spiral column: Long tube (1โ€“50 m) containing the stationary phase, inside a temperature-controlled oven
  • Variable temperature oven: Temperature programming allows better separation of complex mixtures
  • Detector: Usually thermal conductivity detector (TCD) or flame ionisation detector (FID)
  • Recorder/Computer: Plots the chromatogram โ€” signal vs. time

Separation Principle

As the vaporised mixture is carried through the column, components that are more soluble in the stationary oil travel more slowly; those less soluble in the oil travel faster with the carrier gas. Components exit the column at different times (retention times) and are detected sequentially.

๐Ÿ“ GLC Requirements

The substance injected must be able to form a vapour easily โ€” i.e., it must be a gas, liquid, or volatile solid at the operating temperature. This limits GLC to relatively small, volatile organic molecules (molecular mass typically <800 g/mol).

Reading a GLC Chromatogram

A GLC trace shows signal intensity vs. retention time. Each peak represents one component. The peak appears at the component's characteristic retention time. Peak area is proportional to concentration.

๐Ÿ“Š GLC Retention Times โ€” Typical Organic Compounds
CompoundtR (min)Notes
Hex-1-ene3.5Low boiling alkene
Heptane8.5Alkane reference
Pentan-2-one14.0Ketone, polar
Ethanol4.2Short-chain alcohol
Propan-1-ol6.1Polar alcohol
Butan-1-ol9.3Polar alcohol
๐Ÿ’ก GLC + Mass Spectrometry
One important limitation of GLC is that similar compounds have similar retention times. This is overcome by coupling GLC with a mass spectrometer (GLC-MS). The MS provides a structural fingerprint that uniquely identifies each component โ€” used heavily in drug testing, environmental analysis, and food authenticity testing.
COLColumn Chromatography โ€” Detailed
โ–ผ

Principle

Column chromatography is a form of adsorption chromatography on a preparative scale. Silica, alumina, or a resin is packed into a vertical glass tube (the column). The mixture is added to the top and a solvent (eluent) is run through continuously. Components separate as distinct bands that travel down the column at different rates.

โš— The Equilibrium
Substance(adsorbed on silica) โ‡Œ Substance(dissolved in solvent)

Materials strongly adsorbed travel slowly (remain adsorbed most of the time). Weakly adsorbed materials spend more time dissolved in the solvent and travel faster.

Key Advantage Over TLC

Column chromatography allows separation of milligrams to grams of material, making it preparative (for collecting pure substances), not just analytical. Columns can be up to 3.2 metres wide and 15 metres high for industrial-scale purification.

๐Ÿงช Classic Experiment โ€” Spinach Pigments by Column
Using a silica column with petroleum ether:acetone gradient, spinach pigments elute in this order: ฮฒ-carotene (most nonpolar, fastest) โ†’ Pheophytin โ†’ Chlorophyll a โ†’ Chlorophyll b โ†’ Xanthophylls (most polar, slowest).

Quantification in Column Chromatography

Each fraction collected can be analysed separately: proteins by ninhydrin + visible spectroscopy at 570 nm; amino acids by ninhydrin colorimetry; chlorophylls by absorbance at 664 nm (Chl a) or 647 nm (Chl b). This allows a concentration vs. volume profile to be constructed.

Interactive Chromatography Simulator

Simulate real chromatography experiments with authentic substance data.

TLC Controls
Ready
TLC Plate
Rf Data โ€” Bidirectional
#SubstanceColourRfdist (cm)
Substance:โ€”
Rf value:โ€”
Distance (spot):โ€”
Solvent front:10.0 cm
Polarity:โ€”
Calc: Rf =โ€”
Paper Chrom. Controls
Ready
Hovered:โ€”
Rf:โ€”
Band colour:โ€”
Paper Chromatography Strip
Rf Results
Pigment / CompoundColourRf
Column Controls
Ready
ComponentElution OrderVol (cmยณ)
Column
Elution Profile (UV absorbance)
GLC Controls
150ยฐC
Ready โ€” awaiting injection
Current t
0.0 min
Peaks
0
Signal
0.00
Last Peak
โ€”
GLC Chromatogram โ€” Signal vs. Retention Time
ComponenttR (min)Peak Area% Composition

Chromatography Calculations

Interactive calculators with step-by-step solutions.

Rf Value Calculator
Formula
Rf = dsubstance รท dsolvent front
Retention Time Analysis (GLC)
Formula
% A = (Peak areaA รท ฮฃ all peak areas) ร— 100%

Enter peak dimensions (triangular approximation: area = ยฝ ร— base ร— height)

Unknown Identification Tool

Enter an experimental Rf value to identify the most likely substance from our library.

Purity Assessment (TLC)

A pure compound gives exactly ONE spot on TLC. Use this tool to analyse your spots.

Number of spots observed:

Substance Library

Search and explore chromatographic data for common substances. Click any card to see full details.

Knowledge Assessment

25 exam-style questions covering all CAPE chromatography objectives.

References & Further Reading

Curated academic and educational resources for deeper study.