Separating Mixtures by Differential Migration
Chromatography is a family of separation techniques that exploit differences in how strongly various components of a mixture interact with two phases: a stationary phase (a solid or liquid fixed in place) and a mobile phase (a liquid or gas that flows past the stationary phase). Components that interact strongly with the stationary phase move slowly; those that interact weakly move quickly. This differential migration separates the mixture into its individual components.
Invented by the Russian botanist Mikhail Tsvet in 1900 to separate plant pigments (hence "chromatography," from the Greek for "color writing"), the technique has evolved into one of the most powerful and versatile tools in chemistry. Modern chromatographic methods can separate and identify substances present at concentrations below one part per billion.
Thin-Layer Chromatography (TLC)
TLC is the simplest and fastest chromatographic technique, making it the standard method for monitoring reactions, checking purity, and selecting conditions for column chromatography.
The setup: A thin layer (0.2-0.25 mm) of adsorbent — usually silica gel (SiO2) or alumina (Al2O3) — is coated onto a glass, plastic, or aluminum plate. A small spot of the sample solution is applied near the bottom of the plate using a capillary tube. The plate is placed upright in a shallow pool of solvent (the mobile phase) inside a closed developing chamber.
The process: Solvent rises through the adsorbent by capillary action, carrying the sample components upward at different rates depending on their relative affinity for the stationary phase versus the mobile phase. When the solvent front reaches approximately 75% of the plate height, the plate is removed and the solvent front is immediately marked with a pencil.
Visualization: Many compounds are not visible to the naked eye on the plate. Common visualization methods include:
- UV light (254 nm) — Many TLC plates contain a fluorescent indicator (F254). Compounds that absorb UV appear as dark spots against a green fluorescent background.
- Iodine chamber — Iodine vapor reversibly stains organic compounds brown.
- Chemical stains — KMnO4 (general, oxidizes most organics), anisaldehyde (good for alcohols, aldehydes, terpenes), ninhydrin (specific for amino acids, appears purple).
Rf Values
The retention factor (Rf) is the ratio of the distance traveled by a compound to the distance traveled by the solvent front:
Rf = distance traveled by compound / distance traveled by solvent front
Rf values range from 0 (compound did not move) to 1 (compound moved with the solvent front). They are characteristic of a specific compound under specific conditions (same stationary phase, mobile phase, temperature, and chamber saturation). An Rf of 0.3-0.5 is generally ideal for TLC analysis — compounds are well-separated from both the origin and the solvent front.
Important: Rf values are reproducible only under identical conditions. Different plate brands, solvent batches, temperatures, or chamber saturation levels can shift Rf by 0.05-0.10. Always run a known reference standard alongside your sample for reliable identification.
Solvent Selection for TLC
The polarity of the mobile phase determines how far compounds travel. On silica gel (a polar stationary phase):
- Non-polar solvents (hexane, petroleum ether) — Compounds move slowly. Polar compounds barely leave the origin.
- Polar solvents (ethyl acetate, methanol, water) — Compounds move quickly. Non-polar compounds travel near the solvent front.
In practice, binary solvent mixtures are used to fine-tune the separation. A common starting system is ethyl acetate : hexane in ratios from 1:9 (mostly non-polar) to 9:1 (mostly polar). Adjust the ratio until the compounds of interest have Rf values between 0.2 and 0.5 with visible separation between spots.
Eluotropic series (increasing elution strength on silica gel): Pentane < hexane < toluene < dichloromethane < diethyl ether < ethyl acetate < acetone < ethanol < methanol < water
Column Chromatography
Column chromatography is the preparative extension of TLC — it separates gram-scale quantities of material for further use. The stationary phase (typically silica gel, 40-63 micrometer particle size) is packed into a vertical glass column, and the sample is applied to the top.
Packing the column: A slurry of silica in the starting solvent is poured into the column and allowed to settle under gravity. The silica bed should be uniform with no cracks or air channels. A layer of sand on top of the silica prevents disruption when solvent is added.
Loading the sample: Dissolve the mixture in the minimum amount of a low-polarity solvent or, preferably, pre-adsorb it onto silica (dry loading) and add it as a powder on top of the column. Concentrated loading gives sharper bands.
Elution: Solvent is added to the top of the column and allowed to flow through under gravity or gentle air pressure. Fractions are collected in test tubes or vials. The column is typically eluted with solvents of gradually increasing polarity (gradient elution) to bring all components off the column.
Monitoring: TLC is used to analyze individual fractions. Fractions containing the same compound (same Rf on TLC) are combined and the solvent is removed by rotary evaporation.
Rule of thumb: Use approximately 30-50 grams of silica per gram of crude mixture for straightforward separations, and up to 100 grams per gram for difficult separations.
Paper Chromatography
Paper chromatography uses cellulose filter paper as the stationary phase. Water adsorbed onto the cellulose fibers acts as the actual stationary phase, making this a partition chromatography technique (components distribute between two liquid phases — adsorbed water and the mobile phase).
Paper chromatography is inexpensive and requires minimal equipment, making it popular for educational demonstrations and qualitative analysis of polar compounds like amino acids, sugars, and water-soluble dyes. It lacks the resolving power and speed of TLC for most applications, but its simplicity makes it valuable for fieldwork and resource-limited settings.
Practical Tips for Better Chromatography
- Spot size matters in TLC. Apply the smallest spot possible. Large, diffuse spots give poor resolution regardless of the solvent system.
- Chamber saturation — Line the developing chamber with filter paper wetted with the solvent. This saturates the atmosphere, preventing uneven evaporation from the plate edges that causes curved solvent fronts.
- Column flow rate — Faster is not better. Gravity flow gives the best separation. Excessive air pressure compacts the silica and creates channels.
- Never let a column run dry — Air entering the silica bed creates irreparable channels. Always maintain a layer of solvent above the silica.