Organic Chemistry – Some Basic Principles and Techniques · Lesson 11 of 15
Chromatography and Separation
“Read chromatograms by following how components divide between stationary and mobile phases.”
• Explain separation using stationary and mobile phases. • Distinguish adsorption and partition chromatography. • Interpret column, thin-layer, and paper chromatography. • Calculate retardation factors and explain how spots can be detected.
Two phases create different journeys
Chromatography separates components because they interact differently with a stationary phase and a moving phase. A component that is retained more strongly spends more time held back; one carried more readily by the mobile phase moves farther in the same time. The method can separate mixtures, isolate compounds, and provide evidence about identity or purity. It began with coloured plant mixtures, but it also works for colourless substances.
The stationary phase remains in place while the mobile phase moves through or over it. Components repeatedly interact with both phases and therefore migrate at different rates.
Adsorption is retention at a surface, such as silica gel or alumina. Different molecules adsorb to different extents. Partition is distribution between two phases, commonly two liquid environments in the examples here. The distinction concerns how retention occurs: adsorption at a surface is not the same as simply dissolving within a bulk liquid.
Column chromatography separates along a packed tube
A glass column is packed with an adsorbent stationary phase. The mixture is placed near the top and a liquid eluant flows through the column. Repeated adsorption and release separates the sample into bands. Less strongly retained components move down faster and emerge earlier; more strongly adsorbed components remain higher for longer. Separate fractions are collected as material leaves the column.
The column must contain an appropriate adsorbent and mobile phase for the compounds being separated. A solvent that moves every substance together gives little separation. The coloured bands in a diagram represent different sample components, not necessarily different colours that every real substance possesses. Colourless fractions require another detection method.
Thin-layer chromatography makes the journey visible
In TLC, a thin layer of silica gel or alumina is spread on a plate. A small sample spot is placed near the bottom, above the solvent level. The solvent rises through the layer by capillary action. Components move different distances because their adsorption differs. Marking the solvent front before it evaporates supplies the reference distance needed to compare migration.
The retardation factor, Rf, divides the distance travelled by a substance spot by the distance travelled by the solvent front, both measured from the same baseline. Use the centre of the spot for its distance. Because the distances have the same unit, the ratio has no unit. Under the usual developed-plate conditions a retained component has a value between zero and one.
Problem
A TLC solvent front moves 8.0 cm while the centre of one spot moves 5.0 cm. Find Rf.
- 1.Both distances are measured from the sample baseline, so they are directly comparable.
- 2.. The distance units cancel.
- 3.The spot has moved 62.5% as far as the front. This value is meaningful for the conditions used, rather than being a universal number independent of solvent and stationary phase.
Coloured spots may be seen directly. Some colourless substances fluoresce under ultraviolet illumination. Others can be revealed by iodine vapour, which gives visible brown regions for suitable compounds, or by a chemical spray reagent. Ninhydrin is used to reveal amino acids. A detection reagent may change the appearance chemically; it is not necessarily part of the original separation.
Problem
On the same TLC plate, spot A has Rf 0.25 and spot B has Rf 0.70. Which component is more strongly retained under those conditions?
- 1.A has moved a smaller fraction of the solvent-front distance.
- 2.It spends more of the separation held by the stationary phase relative to being transported by the solvent. A is more strongly retained in this comparison.
- 3.The inference applies to this plate and solvent system. A different eluant can change the migration behaviour.
The plate diagram illustrates migration distances rather than an isolated apparatus requirement. In an actual development chamber the solvent pool stays below the sample baseline, and the plate is removed before the solvent front reaches its top edge.
Paper chromatography uses partitioning
In paper chromatography, water held in the cellulose fibres acts as the stationary liquid phase in the chapter’s model. A suitable solvent moves up the paper as the mobile phase. Components repeatedly distribute between the stationary water and mobile solvent and consequently move at different rates. The developed strip or sheet is a chromatogram.
Place the sample above the initial solvent level so it is transported through the paper rather than simply dissolving into the solvent pool. The apparatus may hold a strip or a shaped sheet; the underlying principle remains partitioning. Coloured spots can be seen at different heights. Colourless substances can be located by ultraviolet light or suitable reagents, as with TLC.
Problem
A paper chromatogram gives three separated spots from one original sample dot. What does that suggest, and what does it not establish?
- 1.The sample contains at least three detectable components that separate under these conditions.
- 2.It does not establish that each spot is a completely identified pure substance. Different compounds may occasionally migrate together, and some components may remain undetected.
- 3.Comparison with standards under the same conditions can support identification, but a matching Rf alone is not an absolute proof of identity.
| Technique | Stationary phase in this chapter | Separation basis | Typical observation |
|---|---|---|---|
| Column | Packed silica or alumina | Differential adsorption | Bands and collected fractions |
| TLC | Thin adsorbent layer | Differential adsorption | Spots at different distances |
| Paper | Water retained in paper | Differential partitioning | Developed chromatogram |
Do not use the distance from the bottom edge when the baseline is higher up. Rf is not a speed and has no units. A single spot is useful purity evidence under one method, but it cannot rule out every unresolved or invisible impurity.
Quiz
Which component generally travels farther on the same adsorption TLC plate?
A spot moves 3 cm and the solvent front 6 cm from the baseline. What is Rf?
What is the stationary liquid phase in the paper-chromatography model used here?
Which reagent can reveal amino acids on a chromatogram?
Why is the sample baseline kept above the solvent pool?
Practice Problems
- Explain the difference between adsorption and partition chromatography in terms of retention.
- Sketch a column showing three separated components and predict their order of collection.
- Draw a developed TLC plate with a baseline, two spots, and a solvent front; show the measured distances.
- Calculate Rf values for spots at 2.4 cm and 5.6 cm when the solvent front is 8.0 cm from the baseline.
- Explain why Rf comparisons require matching chromatographic conditions.
- Describe paper chromatography and two methods for detecting colourless compounds.
Key Takeaways
• Different interactions with stationary and mobile phases cause separation. • Column and TLC examples use adsorption; paper chromatography uses partitioning. • Rf compares spot and solvent-front distances from the same baseline. • Detection and purity interpretation require attention to experimental conditions.