AQA A-Level Chemistry: Analytical Techniques — Complete Revision Guide (7405)
AQA A-Level Chemistry: Analytical Techniques — Complete Revision Guide (7405)
Analytical chemistry is the "how do we know?" half of A-Level Chemistry. Every structural formula you draw in an organic mechanism, every product you isolate in a synthesis, every concentration you quote in a kinetics question — somebody, somewhere, had to prove that the substance was what they claimed and present at the level they claimed. Mass spectrometry, infrared spectroscopy, proton NMR, carbon-13 NMR, chromatography and the wet chemical tests are the toolkit. Together they identify essentially any unknown organic compound and quantify essentially any analyte. On AQA 7405, this material is examined heavily across all three papers — and Paper 3 always carries a multi-mark deduction question where you are handed three or four spectra and asked to work out a structure.
This guide walks through analytical techniques topic by topic. For each, you will find the underlying principle, the kind of information it gives, the common pitfalls, a worked example and a link into the LearningBro Analytical Techniques course.
Where This Fits in the Wider Course
Analytical chemistry is the most synoptic topic on the spec. Mass spectrometry builds directly on the mass-spectrometer model from atomic structure — same instrument, but now the sample is a molecule rather than an element. Infrared spectroscopy depends on bond polarity and bond strength, both from bonding; polar bonds absorb, non-polar bonds do not. Every spectrum you interpret depends on knowing functional groups from organic foundations and organic advanced; without that recall, the absorptions and peaks tell you nothing. The chemical tests draw on transition-metal redox chemistry and qualitative ion analysis from inorganic chemistry. Time spent on this section consolidates everything else.
Guide Overview
The ten sub-topics below cover the full AQA 7405 analytical content: mass spectrometry of organic compounds, infrared spectroscopy, proton NMR, carbon-13 NMR, chromatography (TLC, GC and HPLC), the standard chemical tests, two sections on combining techniques to deduce structures, quantitative analysis, and Required Practical 12 (the analysis practical). The order matches the natural learning sequence — single techniques first, then combinations, then quantification, then the practical anchor.
What the AQA 7405 Specification Covers
AQA A-Level Chemistry (7405) is examined through Paper 1 (Inorganic and Physical, 2h, 105 marks), Paper 2 (Organic and Physical, 2h, 105 marks) and Paper 3 (synoptic + practical, 2h, 90 marks). Analytical techniques sit in specification sections §3.3.15 (Nuclear Magnetic Resonance Spectroscopy) and §3.3.16 (Chromatography), with mass spectrometry of organic compounds in §3.3.5 and infrared in §3.3.6. The chemical tests are distributed across the organic sections but are pulled together in the analysis practical.
| Sub-topic | Spec area | Typical paper weight |
|---|---|---|
| Mass spectrometry of organic compounds | §3.3.5 | 3-6 marks |
| Infrared spectroscopy | §3.3.6 | 3-5 marks |
| Proton NMR | §3.3.15 | 5-9 marks |
| Carbon-13 NMR | §3.3.15 | 3-5 marks |
| Chromatography (TLC, GC, HPLC) | §3.3.16 | 4-7 marks |
| Chemical tests for functional groups | §3.3.7-§3.3.14 | 3-6 marks |
| Combined-technique structural deduction | Synoptic | 8-12 marks |
| Quantitative analysis | §3.1.2 + §3.3.16 | 4-6 marks |
These weights are estimates modelled on the AQA 7405 paper format. What is reliable is that an extended structural deduction question — typically 8 to 12 marks combining MS, IR and NMR — appears on essentially every Paper 3.
Mass Spectrometry of Organic Compounds
A mass spectrum of an organic compound shows the masses of the molecular ion M+ and the fragment ions produced when the molecular ion breaks apart. The molecular ion peak (highest m/z under normal conditions) gives the relative molecular mass. Fragment ions reveal which bonds were weakest and which groups were attached.
The molecular ion forms when an electron is knocked off the parent molecule: M + e- → M+ + 2e-. M+ is a radical cation; it is inherently unstable and fragments. A bond breaks, giving a smaller cation (which appears in the spectrum) and a neutral radical (which does not). Only positive ions are detected.
Worked example. A spectrum shows M+ at m/z 60, with fragments at m/z 45 and 15. Mass 60 suggests C3H8O (60 g/mol). The fragment at m/z 15 is CH3+; loss of 15 from 60 gives 45, which is C2H5O+. The compound is plausibly propan-1-ol or propan-2-ol; the m/z 45 fragment (CH3CHOH+) supports propan-2-ol.
Common diagnostic losses: loss of 15 (CH3), 17 (OH), 18 (H2O), 28 (CO or C2H4), 29 (CHO or C2H5), 45 (COOH). The M+1 peak arises from the 1.1 percent natural abundance of carbon-13; its height relative to M tells you the number of carbons (about 1.1n percent for n carbons).
A common pitfall is to confuse the molecular ion with the base peak (most intense peak). They are usually different. Another is to forget that fragment ions are cations — the neutral radical lost is invisible.
See the mass spectrometry lesson.
Infrared Spectroscopy
Infrared (IR) spectroscopy measures absorption of infrared radiation by molecular vibrations — stretches and bends of polar bonds. The energy of absorption depends on bond strength and the masses of the bonded atoms, so each bond type absorbs at a characteristic wavenumber (cm-1). The spectrum is a plot of transmittance against wavenumber, conventionally with wavenumber decreasing left to right.
Only polar bonds absorb. A homonuclear bond (H-H, O=O) is IR-inactive because the dipole moment does not change during vibration. The more polar the bond, the more intense the absorption.
| Bond | Wavenumber range (cm-1) | Notes |
|---|---|---|
| O-H (alcohol) | 3230-3550 | Broad if H-bonded |
| O-H (carboxylic acid) | 2500-3300 | Very broad |
| N-H (amine) | 3300-3500 | Two peaks for -NH2 |
| C-H | 2850-3100 | Always present |
| C=O | 1680-1750 | Sharp, strong |
| C=C | 1620-1680 | Weak |
| C-O | 1000-1300 | Multiple absorptions |
The fingerprint region (400-1500 cm-1) is unique to each compound. You do not interpret individual peaks here; you match the pattern against a database (the AQA data booklet shows extracts).
Worked example. A spectrum shows a broad absorption at 2500-3300, a sharp peak at 1715 and absorptions in the fingerprint region. The broad band plus C=O strongly suggest a carboxylic acid (the O-H of COOH is far broader and lower than an alcohol O-H).
A common pitfall is to call any peak near 3000 an O-H — the C-H stretch is there too. Another is to forget that ester, ketone, aldehyde, amide and acid all have C=O in slightly different ranges; the surrounding features distinguish them.
See the infrared spectroscopy lesson.
Proton NMR (¹H NMR)
Proton NMR identifies hydrogen environments in a molecule. Hydrogens in different chemical environments resonate at different frequencies (different chemical shifts, δ, measured in parts per million from tetramethylsilane TMS at δ = 0). Each peak gives four pieces of information: chemical shift (what environment), integration (how many H), splitting (how many neighbours), and (sometimes) coupling constant.
Chemical shift correlates with electron density at the proton. Electron-poor environments (next to electronegative O or in aromatic rings) deshield the proton and shift it downfield (higher δ). The AQA data booklet provides a shift table.
| Environment | δ (ppm) |
|---|---|
| R-CH3 alkane | 0.7-1.6 |
| R-CH2-R' | 1.2-1.4 |
| R-CO-CH3 | 2.0-2.9 |
| R-O-CH3 | 3.3-3.9 |
| Ar-H | 6.0-9.0 |
| R-CHO | 9.0-10.0 |
| R-COOH | 10.0-12.0 |
Integration (the area under each peak, given as a step height on the integration trace) is proportional to the number of hydrogens. Ratios — not absolute heights — are what matter.
Splitting (multiplicity) follows the n + 1 rule: a proton with n equivalent neighbours appears as an (n+1)-peak multiplet. So -CH3 next to -CH2- splits into a triplet (2 + 1); the -CH2- splits into a quartet (3 + 1). Equivalent protons do not split each other.
Worked example. Ethanol CH3CH2OH gives three peaks: -OH at δ 2-5 (singlet, 1H, exchangeable with D2O), -CH2- at δ 3.7 (quartet, 2H, three CH3 neighbours), -CH3 at δ 1.2 (triplet, 3H, two CH2 neighbours).
A common pitfall is to count equivalent protons separately — the three H of CH3 are one environment, not three. Another is to forget that OH (and NH) protons exchange and often appear as a broad singlet; running the sample in deuterated solvent removes them. See the proton NMR lesson.
Carbon-13 NMR (¹³C NMR)
Carbon-13 NMR identifies carbon environments. Because only carbon-13 (1.1 percent natural abundance) is NMR-active, peaks are usually shown without splitting (broadband decoupling removes complications). The spectrum is therefore much simpler: one peak per distinct carbon environment.
Chemical shifts span a much wider range (0-220 ppm) than proton NMR, which makes assignments easier. Typical regions: alkane C 0-50, C-O (ether/alcohol) 50-90, C=C 110-150, aromatic C 110-160, C=O (ester/acid) 160-185, C=O (ketone/aldehyde) 190-220.
Worked example. Propan-2-ol (CH3)2CHOH has two carbon environments — the two methyl carbons are equivalent. Spectrum: two peaks, one near δ 25 (the methyls), one near δ 65 (the C-O carbon).
Carbon-13 NMR is most useful for counting distinct carbon environments quickly, especially in aromatic compounds where symmetry can collapse what looks like a complex structure to a handful of peaks. A para-disubstituted benzene with two identical substituents shows only two aromatic carbons; a mono-substituted benzene shows four (ipso, ortho, meta, para).
A common pitfall is to expect splitting in C-13 spectra — under standard AQA conditions, peaks are singlets. Another is to forget that equivalent carbons (by symmetry) give a single peak, not multiple.
See the carbon-13 NMR lesson.
Chromatography (TLC, GC and HPLC)
Chromatography separates a mixture by partitioning components between a stationary phase and a mobile phase. Components with stronger affinity for the stationary phase travel more slowly. Three forms appear on AQA 7405.
Thin-layer chromatography (TLC): stationary phase is a thin layer of silica gel on a plate; mobile phase is an organic solvent. Sample spots travel up the plate as the solvent rises by capillary action. Each component has a characteristic retention factor Rf = distance moved by component / distance moved by solvent front. Rf is reproducible under identical conditions, so unknowns can be identified by comparing Rf to known standards run on the same plate.
Gas chromatography (GC): mobile phase is an inert carrier gas (helium or nitrogen), stationary phase is a high-boiling-point liquid coated on the inside of a long capillary column. The sample is vaporised, carried through the column and detected at the end. Each component has a retention time characteristic of its volatility and affinity for the stationary phase. GC is excellent for volatile mixtures (alcohols, esters, hydrocarbons) and is the basis of breath alcohol testing and forensic analysis.
High-performance liquid chromatography (HPLC): mobile phase is a high-pressure liquid, stationary phase is a fine solid (often modified silica) packed into a metal column. Components elute at characteristic retention times. HPLC handles thermally labile compounds (drugs, proteins, biomolecules) that GC cannot vaporise.
In all three, the chromatogram shows peaks at retention times; peak area is proportional to amount. Coupling GC or HPLC to a mass spectrometer (GC-MS, LC-MS) gives a mass spectrum of each component as it elutes — a hugely powerful analytical method used routinely in drug-testing and environmental labs.
A common pitfall is to assume Rf values are absolute identifiers — they depend on plate, solvent and temperature. Always run standards alongside unknowns. See the chromatography lesson.
Chemical Tests for Functional Group Identification
Before NMR became routine, organic chemists identified functional groups by wet chemical tests. AQA still expects fluency with the standard set.
| Functional group | Reagent | Observation |
|---|---|---|
| Alkene (C=C) | Bromine water | Decolourised (orange to colourless) |
| Primary or secondary alcohol | Acidified potassium dichromate(VI) | Orange to green |
| Aldehyde | Fehling's solution | Blue to brick-red ppt |
| Aldehyde | Tollens' reagent | Silver mirror |
| Ketone | Brady's reagent (2,4-DNPH) | Orange-yellow ppt (also aldehydes) |
| Carboxylic acid | Sodium carbonate or hydrogencarbonate | Fizzing (CO2) |
| Halogenoalkane | Aqueous silver nitrate (in ethanol, warm) | Cream/white/yellow ppt depending on halide |
| Phenol | Iron(III) chloride | Purple colour |
The Fehling's, Tollens' and Brady's tests are named after the chemists who developed them. Tollens' reagent — ammoniacal silver nitrate — distinguishes aldehydes (which reduce silver(I) to silver metal, depositing a mirror) from ketones (which do not). Fehling's solution — copper(II) tartrate complex — works the same way: aldehydes reduce copper(II) to red copper(I) oxide, ketones do not. Brady's reagent (2,4-dinitrophenylhydrazine) gives a yellow-orange precipitate with any carbonyl (aldehyde or ketone), and the precipitate's melting point can identify the specific compound.
Worked example. An unknown gives an orange-yellow ppt with Brady's reagent, a silver mirror with Tollens', and decolourises bromine water. Conclusion: the compound contains both an aldehyde group and a C=C double bond (e.g. propenal CH2=CHCHO).
A common pitfall is to write Tollens' as just "silver nitrate" — it must be ammoniacal silver nitrate. Another is to claim Fehling's distinguishes aldehydes from carboxylic acids; it does not test acids directly. See the chemical tests lesson.
Combining Techniques 1: Identifying Simple Compounds
Real structural deduction questions hand you two or three pieces of evidence — a molecular formula, an IR spectrum, a proton NMR — and ask for the compound. The approach is the same every time. Step 1: get the molecular formula and the degrees of unsaturation. Step 2: read the IR for the major functional group. Step 3: read the NMR for the carbon skeleton. Step 4: assemble.
Worked example. A compound has molecular formula C3H6O, M+ at m/z 58, IR shows a strong peak at 1715 cm-1 and no broad O-H, and proton NMR shows a single peak at δ 2.2 (singlet, 6H). The C=O absorption identifies a carbonyl. No O-H rules out alcohol and carboxylic acid. The single NMR peak as a 6H singlet means all six hydrogens are equivalent and have no neighbours — two equivalent CH3 groups with no adjacent CH protons. The structure is propanone CH3COCH3.
Worked example. Molecular formula C2H4O2, IR shows a broad peak 2500-3300 plus a sharp peak at 1710 cm-1, and proton NMR shows a singlet at δ 2.1 (3H) and a singlet at δ 11.5 (1H). Broad O-H plus C=O identifies a carboxylic acid; the δ 11.5 singlet confirms COOH. Three hydrogens of a methyl with no neighbours fit CH3COOH (ethanoic acid).
A common pitfall is to skip the molecular formula step — without knowing how many of each atom there are, you have no constraint on the skeleton. Another is to expect every problem to give a unique answer; sometimes the data fit two isomers and you need an extra test (carbon-13 NMR is often decisive).
See the combined techniques 1 lesson.
Combining Techniques 2: Complex and Multi-functional Compounds
Larger molecules and molecules with two functional groups need more care. The strategy adds carbon-13 NMR and integration ratios as decisive evidence.
Worked example. Molecular formula C4H8O2, IR with sharp C=O at 1735 cm-1 and no broad O-H, proton NMR shows a quartet at δ 4.1 (2H), a singlet at δ 2.0 (3H) and a triplet at δ 1.2 (3H). C=O without O-H suggests an ester. The triplet plus quartet pattern is the classic -OCH2CH3 group (ethyl ester). The singlet at δ 2.0 is a -COCH3 next to no other protons. Total integration 2+3+3 = 8 matches molecular formula. Structure: ethyl ethanoate CH3COOCH2CH3.
Worked example. Molecular formula C8H10O, IR with sharp O-H near 3500, proton NMR shows aromatic peaks at δ 6.8-7.3 (4H) plus aliphatic peaks at δ 4.5 (2H, singlet) and δ 2.3 (3H, singlet). Four aromatic H plus two methyl/methylene signals fit a disubstituted benzene. The 2H singlet near δ 4.5 is -CH2-O-, the 3H singlet at δ 2.3 is Ar-CH3 on the ring. Structure: methylbenzyl alcohol (e.g. 4-methylbenzyl alcohol, para isomer if aromatic peaks are a simple AA'BB' pattern).
For aromatic systems, carbon-13 NMR clinches substitution pattern. Para-disubstituted benzene gives two aromatic carbon peaks; meta gives four; ortho gives four (but different shifts).
A common pitfall is to ignore total integration — it must match the molecular formula. Another is to forget that ester C=O is at higher wavenumber (~1735) than ketone or aldehyde C=O (~1715), a useful diagnostic.
See the combined techniques 2 lesson.
Quantitative Analysis
Quantitative analysis means measuring how much, not just what. Three methods feature on AQA 7405.
Titrimetry: a known volume of analyte is reacted with a standard solution of titrant. Volume at end point gives moles, hence concentration. Acid-base, redox (manganate(VII), thiosulfate) and complexometric (EDTA) titrations all appear.
Spectrophotometry (Beer-Lambert): the absorbance of a coloured solution at a chosen wavelength is proportional to concentration: A = εcl, where ε is the molar extinction coefficient, c is concentration in mol dm-3 and l is path length in cm. A calibration curve of A against c for known standards is used to read off unknown concentrations. This underlies UV-visible spectrophotometry of transition-metal complexes, dyes and some organic analytes. The relationship is associated with Beer and Lambert.
Chromatographic peak area: in GC or HPLC, peak area is proportional to amount of component. Quantification needs either an internal standard or a calibration curve of peak area against known concentration.
Worked example. A calibration curve for Fe(III)-thiocyanate complex at 480 nm gives a straight line of A = 245c (c in mol dm-3, l = 1 cm). An unknown gives A = 0.49. Concentration = 0.49 / 245 = 2.0 × 10-3 mol dm-3.
A common pitfall is to forget the path length — most cuvettes are 1 cm, but if a problem specifies 2 cm, the absorbance doubles for the same concentration. Another is to plot absorbance against concentration in different units in calibration vs unknown.
See the quantitative analysis lesson.
Required Practical 12: Analysis
Required Practical 12 is the AQA analysis practical — a hands-on synthesis or separation followed by analysis. Typical RP12 variants include: preparing aspirin by acetylation of salicylic acid, recrystallising the crude product, measuring melting point, running TLC against pure standards, and recording an IR spectrum of the product. Examination questions on RP12 ask about purification techniques (filtration, recrystallisation, melting point as a purity test), TLC procedure (spotting, developing chamber, visualisation under UV or with iodine), and interpretation of the IR spectrum.
Worked example question. A student prepares aspirin. Their crude product melts at 130-134 C; pure aspirin melts at 135 C. TLC shows two spots — one matching aspirin, one matching salicylic acid. What does this tell you about the sample? The melting point is depressed and broadened, indicating impurity. The second TLC spot confirms unreacted salicylic acid contamination. Recrystallisation from hot ethanol/water should remove it; rerun TLC to confirm a single spot before quoting yield.
A common pitfall is to claim a single TLC spot proves purity — it shows no detectable impurity by TLC, but small amounts may still be present. Always confirm purity with a second method (melting point, IR). See the Required Practicals: Analysis lesson.
Common Mark-Loss Patterns
- Calling any peak near 3000 cm-1 an O-H without checking width and context.
- Forgetting that equivalent protons in NMR are one environment, not several.
- Mis-applying the n+1 rule by counting non-equivalent neighbours.
- Treating Tollens' as "silver nitrate" without specifying ammoniacal.
- Reporting Rf without context (plate, solvent, standards run alongside).
- Skipping the molecular formula at the start of a deduction question.
- Confusing ester C=O (~1735) and ketone/aldehyde C=O (~1715).
- Forgetting the carbon-13 abundance argument for M+1 peaks.
- Quoting integration as absolute number rather than ratio.
- Forgetting the path length in Beer-Lambert.
How to Revise This Topic
- Build a spectroscopy data card with IR wavenumbers, proton NMR shifts and carbon-13 NMR shifts. Carry it. Refer to it for every practice question.
- Drill twenty deduction questions from past papers. The pattern repetition is what builds speed.
- Memorise the chemical tests table with reagent, observation and what each test rules in or out.
- Draw mass-spectrum fragmentations for ten representative compounds. Knowing what to expect from a ketone, an alcohol, an ester before you see the spectrum makes interpretation seconds rather than minutes.
- Practise quantitative calculations using Beer-Lambert and titration data; one of the more reliable mark sources on Paper 3.
- Use the LearningBro practice quizzes under timed conditions, then check using the AI tutor feedback to spot interpretation errors.
Linking to Other Topics
Analytical techniques connects everything else on 7405. The mass spectrometer model comes from atomic structure. Bond polarity underlying IR comes from bonding. Functional groups for every test and every spectrum come from organic foundations and organic advanced. The qualitative ion tests overlap with inorganic chemistry. The historical X-ray diffraction work of Bragg, the rotation experiments of Pasteur on optical isomers, and the Beer-Lambert relationship from physical chemistry all sit in the wider analytical tradition you are joining. Time spent here returns in every paper.
Final Word
Analytical techniques rewards consistent practice more than any other section. The data tables (IR wavenumbers, NMR shifts, mass-spec fragmentations, chemical tests) are short and memorisable; the interpretation skills come from rep work. Build the data card, drill twenty deduction questions, and an 8-12 mark Paper 3 question turns from intimidating to routine. The full LearningBro Analytical Techniques course walks through every sub-topic with worked spectra, AI tutor feedback and Required Practical drill. Get this section fluent and you have the language to talk about every other section of the course.