How do chromatography and NMR separate and identify the parts of a molecule?
The principles of chromatography including thin-layer and gas chromatography and the use of Rf values, and the principles of nuclear magnetic resonance spectroscopy including chemical shift, the number of environments, integration and spin-spin splitting.
A CCEA A-Level Chemistry answer on chromatography and NMR, covering the principles of thin-layer and gas chromatography with Rf values, and the principles of nuclear magnetic resonance including chemical shift, the number of environments, integration and spin-spin splitting.
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What this dot point is asking
CCEA wants you to explain the principles of chromatography (thin-layer and gas chromatography) using values, and the principles of nuclear magnetic resonance spectroscopy, including chemical shift, the number of environments, integration and spin-spin splitting.
Chromatography
All chromatography relies on a dynamic equilibrium in which each component partitions between the two phases. A component that spends more time bound to the stationary phase (because it adsorbs strongly or is more soluble in it) moves slowly; one that prefers the mobile phase moves quickly, so the components separate as they travel.
In thin-layer chromatography (TLC) the stationary phase is a thin layer of silica or alumina on a plate, and the mobile phase is a solvent that rises by capillary action. Spots are applied on a pencil baseline, the plate is developed in the solvent, and colourless spots are revealed under UV light or with a locating agent such as ninhydrin (for amino acids). The value, a number between and with no units, is characteristic of a substance under fixed conditions and is matched against reference compounds to identify each spot.
Gas chromatography (GC) vaporises the sample and carries it through a long heated column packed with (or coated with) a non-volatile stationary phase, using an inert carrier gas as the mobile phase. Each component has a characteristic retention time (the time to pass through the column), and the peak areas give the relative amounts of the components. GC separates volatile mixtures and is often coupled to a mass spectrometer (GC-MS) so each separated component is also identified.
Nuclear magnetic resonance
NMR works because nuclei such as and behave like tiny magnets and, in a strong magnetic field, absorb radio-frequency energy at a frequency that depends on their electronic surroundings. Hydrogens in different chemical environments (for example a next to a carbonyl versus a next to oxygen) are shielded to different extents and so resonate at different chemical shifts, measured in parts per million () relative to the reference tetramethylsilane (TMS) at . TMS is chosen because it gives a single sharp peak, is inert, volatile (easily removed) and has highly shielded hydrogens that fall outside the range of most signals. The integration trace measures the area under each peak, which is proportional to the number of hydrogens in that environment, giving their whole-number ratio.
Spin-spin splitting
The splitting arises because the small magnetic field of each neighbouring hydrogen adds to or subtracts from the applied field, producing several slightly different resonance frequencies. Reading the splitting patterns together with the chemical shifts and the integration lets a complete structure be pieced together, which is why NMR is the single most powerful tool for organic structure determination.
Examples in context
TLC is used in the lab to monitor a reaction: a spot of the reaction mixture is run alongside spots of the starting material and an authentic product, and the disappearance of the starting-material spot shows the reaction is complete. In hospital and forensic toxicology, GC-MS separates and identifies drugs and their metabolites in blood. NMR underpins the structural confirmation of every new pharmaceutical: a research chemist who has made a target molecule confirms its identity by checking that the number of peaks, the chemical shifts, the integration ratios and the splitting patterns all match the proposed structure, the same reasoning a CCEA student uses to assign a spectrum.
Try this
Q1. State how the value is calculated in thin-layer chromatography. [1 mark]
- Cue. Distance moved by the spot divided by the distance moved by the solvent.
Q2. A proton has two hydrogens on the adjacent carbon. State the splitting pattern of its peak. [1 mark]
- Cue. A triplet ().
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20184 marksA sample of an alcohol gives a proton NMR spectrum with two peaks in the integration ratio . The peak near is a doublet and there is a peak near . Deduce what the integration, chemical shifts and splitting tell you about the molecule.Show worked answer →
Two peaks means two hydrogen environments. The integration ratio gives the ratio of hydrogen atoms in each environment, so one environment has three times as many hydrogens as the other.
Chemical shift identifies the type of environment: a peak near is a in an alkyl group (such as ), while a peak near to region fits hydrogens near an electronegative atom such as or next to oxygen.
Splitting follows the rule: the doublet near has , so , meaning that group has one hydrogen on the adjacent carbon.
Markers reward (1) two environments from two peaks, (2) integration giving the hydrogen ratio, (3) chemical shift identifying environment type, (4) the doublet meaning one neighbouring hydrogen ( rule).
CCEA 20203 marksIn a thin-layer chromatography experiment a spot moves while the solvent front moves . Calculate the value and explain how it is used to identify the component.Show worked answer →
The retention factor is .
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An value is characteristic of a substance for a given stationary phase and solvent, so it is compared with the of known reference compounds run under the same conditions: a match identifies the component.
Markers reward (1) the correct formula, (2) the value with no units, (3) comparing with known reference values under identical conditions.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)