Summary — What This Chapter Covers
- TLC: stationary phase, mobile phase, baseline, solvent front and Rf values.
- GLC: retention times and percentage composition from peak areas.
- Carbon-13 NMR: one peak per carbon environment; chemical shift shows the type of carbon.
- Proton NMR: chemical shift, peak area ratio and n + 1 splitting give the structure.
- TMS is the reference standard, CDCl3 is the solvent, and a D2O shake identifies O–H and N–H protons.
1. Thin-Layer Chromatography
37.1
Terms
- Stationary phase: a thin layer of solid such as aluminium oxide (or silica) on a solid support (a glass, plastic or aluminium plate).
- Mobile phase: the solvent, which can be polar or non-polar, moving up the plate.
- Baseline: a pencil line near the bottom where the samples are spotted. It must be above the solvent level in the tank.
- Solvent front: the furthest point the solvent reaches; mark it as soon as the plate is removed.
- Rf value: distance moved by the spot ÷ distance moved by the solvent front, both measured from the baseline. Always between 0 and 1.
Retardation factor
Rf = distance travelled by spot / distance travelled by solvent front
Interpreting Rf values
Under the same conditions (same stationary phase, solvent and temperature) a compound always has the same Rf. Compare unknown spots with known samples run on the same plate. More spots than expected means the sample is impure. Colourless spots can be seen under UV light or with a locating agent.
Why Rf values differ
Each component is distributed between the two phases. A component that interacts strongly with the stationary phase (e.g. a polar compound forming hydrogen bonds or dipole attractions with polar aluminium oxide) is held back and has a low Rf. A component that is more soluble in the mobile phase travels further and has a high Rf.
Worked Example
On a TLC plate the solvent front is 8.0 cm from the baseline. Spot A moves 3.6 cm and spot B moves 6.4 cm. Calculate their Rf values. The stationary phase is polar; which compound is more polar?
2. Gas–Liquid Chromatography
37.2
Terms
- Stationary phase: a high boiling point, non-polar liquid coated on a solid support, packed into or lining a long column inside an oven.
- Mobile phase: an unreactive carrier gas such as nitrogen or helium.
- Retention time: the time between injecting the sample and that component reaching the detector (the peak maximum).
Retention times
interacts more with (is more soluble in) the stationary liquid
longer retention time
Percentage composition
The area under each peak is proportional to the amount of that component. Peak area can be estimated as ½ × base × height for a triangular peak.
Composition from a chromatogram
3. Carbon-13 NMR Spectroscopy
37.3