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Notes/Chemistry/Paper 4/Analytical Techniques
CAIEA2 Level9701§37

Analytical Techniques

Read a TLC plate, a gas chromatogram and two kinds of NMR spectrum — the definitions, the calculations and the one-line explanations that score.

180 min read 9 sub-topics
155
question parts
2021–2025 · 36 papers
6 marks
per paper
≈ 6% of the paper
1.9/3
avg difficulty
moderate
#9
most examined
of 15 topics by marks

Analytical techniques is Paper 4's most reliable guest: across 2021–2025 it filled 155 examined parts worth 233 marks over 36 of the 37 sittings — typically one 6–10 mark analysis question per paper, built from a short menu of jobs. It is also one of the gentlest topics in the bank, mean difficulty 1.92 out of 3, because almost every mark is a definition said precisely, a ratio computed cleanly, or a one-sentence explanation with the right comparison in it. The catch is precision: "the distance moved by the solvent" scores nothing unless you say from the baseline to the solvent front; "the time taken" needs its injection-to-detection anchors; an Rf explanation must name which phase does what. This note drills that precision in four moves: §01 how thin-layer chromatography separates a mixture and the vocabulary of a plate, §02 Rf values — computing, comparing and explaining them, §03 gas/liquid chromatography and retention time, §04 turning chromatogram peak areas into percentage compositions, §05 counting carbon-13 environments through symmetry, §06 using those peak counts to pick between candidate structures, §07 proton chemical shifts and relative peak areas, §08 splitting patterns and the n + 1 rule, and §09 the closing trio of TMS, deuterated solvents and the D₂O shake.

Before you start you should be able to

  • AS energetics of intermolecular forces: hydrogen bonding, dipole–dipole and van der Waals forces — they decide how strongly a particle sticks to a surface or dissolves in a solvent

  • Functional-group recognition from the organic notes — alcohols, amines, carbonyls, arenes, esters — because every NMR assignment is 'which group is this H attached to?'

  • Drawing displayed formulae and naming organic molecules up to A2 level, including benzene derivatives

  • Ratios and percentages: every calculation in this topic is area ÷ total area × 100 or distance ÷ distance

By the end of this page you can

  • Describe TLC completely: the stationary phase (for example aluminium oxide on a solid support), the mobile phase (a polar or non-polar solvent), the baseline, the solvent front, and how an Rf value is calculated from distances measured between them

  • Interpret and explain Rf values: compare an unknown spot against reference values, and account for differences using attraction to the stationary phase and relative solubility in the mobile phase

  • Describe GLC completely: the high-boiling non-polar liquid stationary phase, the unreactive carrier gas, and retention time as the time from injection to detection

  • Interpret a gas chromatogram: use peak areas to calculate the percentage composition of a mixture, and explain retention-time order through interaction with the stationary phase

  • Predict and explain the number of peaks in a carbon-13 NMR spectrum by counting carbon environments, using molecular symmetry, and deduce possible structures from peak counts

  • Analyse proton NMR spectra three ways — chemical shift for the environment, relative peak areas for the numbers of protons, splitting pattern for the protons on the adjacent carbon — and predict all three for a given molecule

  • Describe TMS as the reference standard defining chemical shift zero, state why deuterated solvents such as CDCl₃ are used, and identify O–H and N–H protons by their disappearance on shaking with D₂O

01

Thin-layer chromatography: the separation

Syllabus requirement · §37.1

“

describe and understand the terms (a) stationary phase, for example aluminium oxide (on a solid support); (b) mobile phase; a polar or non-polar solvent; (c) Rf value; (d) solvent front and baseline.

”

Two phases, one race

Chromatography separates a mixture because its components spend different amounts of time in two places: stuck onto a stationary phase and dissolved in a mobile phase that moves. In thin-layer chromatography (TLC) the stationary phase is a thin layer of a solid — commonly aluminium oxide (Al2O3\text{Al}_2\text{O}_3) or silicon oxide (SiO2\text{SiO}_2, silica) — spread on a solid support such as a glass or plastic plate. The mobile phase is a liquid solvent, chosen to suit the mixture: it can be polar (water, ethanol) or non-polar (ethyl ethanoate). The sample is spotted near the bottom of the plate, the plate stands in a shallow pool of solvent, and capillary action carries the solvent up the plate — carrying each dissolved component with it, but only as fast as that component lets go of the stationary surface.

TLC platesolvent frontbaselinedrawn in pencil — the samples are spotted herePQsolvent risesup the platedistance movedby solute (Q)distance movedby solvent frontRf = distance moved by solute ÷ distance moved by solvent front

The vocabulary of a finished plate. Both distances are measured from the baseline: Q moved less far than P, so Q has the smaller Rf value.

term

what it is

how it appears in answers

stationary phase

the fixed solid the solvent climbs over

aluminium oxide or silicon oxide (silica/alumina) on a solid support

mobile phase

the moving solvent

a polar solvent (water, ethanol) or a non-polar solvent (ethyl ethanoate) — name one that suits the mixture

baseline

the pencil line where samples are spotted

drawn in pencil so it does not dissolve into the solvent or add its own spots

solvent front

the furthest point the solvent has reached when the run stops

mark its final position before the solvent reaches the top edge

Four terms, four definition marks. Learn the right-hand column phrasing — it is what mark schemes ask for.

Every component runs its own tug-of-war

At any moment during the run, particles of one component are either adsorbed on the stationary surface or dissolved in the moving solvent. A component that binds strongly to the stationary phase spends more time stuck, and crawls; one that dissolves readily in the mobile phase rides near the front. Separation happens because the components strike a different balance between these two fates — and everything you will be asked to explain about Rf values in §02 is just this balance restated.

Naming the phases for a given run (invented)

A few drops of a reaction mixture are spotted on the baseline of a silica plate. The plate is stood in a beaker containing ethyl ethanoate until the solvent has nearly reached the top edge.

Identify the mobile phase and the stationary phase in this experiment.

Show full working
  1. 1

    Ask what actually moves. The liquid climbing the plate is ethyl ethanoate, so the mobile phase is ethyl ethanoate — here acting as a non-polar-ish solvent.

    'Mobile' always names the moving medium; in TLC that is whatever liquid the plate stands in.

  2. 2

    Ask what stays put. The coating the solvent climbs over is the silica layer on the glass plate, so the stationary phase is silicon oxide, SiO2\text{SiO}_2 (or 'silica').

    Name the coating itself — 'the glass plate' is the support, not the stationary phase.

Answer

mobile phase: ethyl ethanoate · stationary phase: SiO2\text{SiO}_2 / silica (aluminium oxide / alumina would serve equally well on any plate).

Mark schemes accept SiO₂, Al₂O₃, silica or alumina — but not vague words like 'solid' or 'plate'. Give a substance.

Your turn

Definition marks are only free if the wording is exact. Write your answer first, then check it against the solution's phrasing.

  1. 19701/42 F/M 2022 Q6(e)(i)1 mark

    Lidocaine is analysed by thin-layer chromatography, with the plate standing in a shallow layer of ethyl ethanoate. Identify the substances used as the mobile and stationary phases in this experiment.

    Stuck? Show hint

    One answer is the solvent named in the question stem; the other is the standard plate coating.

    Show solution
    1. 1

      The mobile phase is the moving solvent — ethyl ethanoate, as named in the experiment.

      Whatever liquid the plate stands in IS the mobile phase; no further reasoning is needed for this mark.

    2. 2

      The stationary phase is the solid coating: SiO2\text{SiO}_2 / silica (or Al2O3\text{Al}_2\text{O}_3 / alumina).

      Both oxides are accepted because both are polar solids that adsorb solutes.

    Answer

    mobile: ethyl ethanoate · stationary: SiO₂/silica or Al₂O₃/alumina

  2. 29701/42 F/M 2023 Q6(a)(i)1 mark

    A student uses TLC to analyse a mixture of metal cations, with water as the solvent. Suggest a compound that could be used as the stationary phase in this experiment.

    Stuck? Show hint

    Think powder-coated-on-glass, not the beaker material.

    Show solution
    1. 1

      Any polar solid coating works; the expected answers are SiO2\text{SiO}_2 (silica) or Al2O3\text{Al}_2\text{O}_3 (alumina).

      Cations are held by ion–dipole attractions to a polar oxide surface, which is exactly what these coatings provide.

    Answer

    SiO₂ OR Al₂O₃ OR silica OR alumina (or equivalent wording)

Practise TLC phase questions straight from the bankReal past-paper questions · TLC: Rf values and phases

The rest of this note

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Can you do all of these?

  • Name stationary AND mobile phases for TLC (Al₂O₃ or SiO₂ on a solid support; polar or non-polar solvent) and for GLC (high-boiling non-polar liquid; unreactive carrier gas such as N₂)

  • Define Rf and retention time in full sentences with their anchors — baseline to solvent front; injection to detection

  • Calculate an Rf value from a plate drawing, and identify an unknown by matching Rf in more than one solvent

  • Explain an Rf difference through attraction to the stationary phase OR relative solubility in the mobile phase

  • Calculate a percentage composition from chromatogram peak areas (including simple triangle areas)

  • Count carbon environments using symmetry, then predict carbon-13 peak numbers for any given structure

  • Read a proton spectrum three ways: chemical shift → environment; relative peak area → ratio of proton numbers; splitting pattern → protons on the adjacent carbon

  • Predict shift ranges, relative areas and singlet/doublet/triplet/quartet/multiplet patterns for a drawn structure

  • State why TMS is used as the standard and why deuterated solvents (CDCl₃) are needed

  • Predict which peaks vanish when D₂O is added — and recognise when nothing changes because there is no O–H/N–H proton

Now do the questions
155 real Paper 4 parts from 2021–2025, sorted by difficulty, with mark schemes