← Unit 4: Structure, synthesis and design
Topic 2: Chemical synthesis and design
Describe the principles and apply chromatographic techniques (thin-layer chromatography (TLC), gas chromatography (GC) and high-performance liquid chromatography (HPLC)) to separate, identify and quantify the components of a mixture
A focused answer to the QCE Chemistry Unit 4 dot point on chromatography. Explains the stationary/mobile phase principle, Rf values in TLC, retention times in GC and HPLC, and the use of calibration curves for quantification. Includes the canonical food and pharmaceutical IA3 / EA contexts.
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What this dot point is asking
QCAA wants you to describe how chromatography separates a mixture into its components by differential partitioning between a stationary phase and a mobile phase, identify the three techniques in the syllabus (TLC, GC, HPLC), interpret the diagnostic measurements (Rf for TLC; retention time and peak area for GC and HPLC), and apply chromatography to identify and quantify compounds in a mixture. The dot point is the experimental-skill bridge between Unit 4 spectroscopy and IA3 / IA2 design work; pharmaceutical and food chemistry IA3 contexts make heavy use of these techniques.
The answer
Chromatography is a family of separation techniques that distinguish compounds in a mixture by how strongly each interacts with a stationary phase and a mobile phase. Compounds that interact more strongly with the stationary phase move slowly; those that interact more strongly with the mobile phase move quickly. The separation depends on the relative polarities and chemical compatibility of the compounds with each phase.
The general principle
Every chromatography technique has the same three elements:
- Stationary phase. A solid or liquid-coated solid that does not move (silica, alumina, octadecyl-bonded silica, liquid film on a column wall).
- Mobile phase (eluent). A liquid or gas that flows over or through the stationary phase, carrying the sample with it.
- Sample. A mixture introduced onto the stationary phase and carried through by the mobile phase.
Components in the sample partition between the two phases. A component that prefers the stationary phase (e.g. polar molecule on polar silica) spends more time stuck and moves slowly. A component that prefers the mobile phase (e.g. non-polar molecule with non-polar solvent) moves quickly with the eluent. After a fixed time or distance, components separate into bands.
Thin-layer chromatography (TLC)
Apparatus. A thin layer of silica (or alumina) on a glass or aluminium plate, dipped vertically into a shallow pool of mobile phase. Sample is spotted near the bottom (above the solvent line); the solvent rises by capillary action.
Phases.
- Stationary: polar silica (Si-O-H surface).
- Mobile: typically a less polar organic solvent or solvent mixture (hexane, ethyl acetate, methanol).
Measurement: Rf value. When the solvent front has risen close to the top of the plate, the plate is removed and the position of each spot is measured.
Rf is dimensionless, between 0 and 1, and reproducible under identical solvent and temperature conditions.
Visualisation. Coloured compounds are seen directly. Colourless compounds are visualised under UV light (silica plates often contain a UV indicator that fluoresces, leaving dark spots) or by staining with iodine, ninhydrin, or other reagents.
Identification. Compare Rf with known standards run on the same plate. Co-spotting (mixing the sample with a standard at the spotting line) confirms identity if the combined spot appears as a single spot rather than two.
Applications. Quick monitoring of reactions (is starting material consumed?); identification of food colourings, pigments, amino acids in protein hydrolysates; purity check (a single spot vs multiple spots).
Limitations. Qualitative only (rough Rf comparisons; not suitable for concentration measurement). Limited resolution. Cannot handle volatile samples.
Gas chromatography (GC)
Apparatus. A long narrow column (typically 10 to 30 m, coiled inside an oven), with a liquid film stationary phase coated on the inside wall. The column is heated to vaporise the sample; an inert carrier gas (He, N2, H2) sweeps the sample through. A detector at the column exit records signal vs time.
Phases.
- Stationary: liquid film (varies; common are silicone-based polymers of differing polarity).
- Mobile: inert gas.
Measurement: retention time. The time taken for a compound to travel from the injector to the detector. Each compound has a characteristic retention time under fixed conditions (column, oven program, flow rate).
Detector. Flame ionisation detector (FID) is most common; gives peak area proportional to the amount of carbon. Mass spectrometer (GC-MS) is the gold standard combination, identifying each peak by its MS fragmentation.
Applications. Volatile organic compounds: hydrocarbons, esters (food flavours, fragrances), alcohols, low-Mr drugs. Blood alcohol testing. Petroleum analysis. Air quality monitoring.
Limitations. Sample must vaporise without decomposing. Useless for thermally labile, ionic, or high-Mr compounds (polymers, proteins, sugars). Upper Mr limit around 500 in routine work.
High-performance liquid chromatography (HPLC)
Apparatus. A narrow column (typically 5 to 25 cm) packed with very small particles (3 to 10 micrometres) of stationary phase. A high-pressure pump forces the liquid mobile phase through. A detector (UV-visible absorbance, fluorescence, or MS) records signal vs time at the column exit.
Phases.
- Stationary: most commonly C18-bonded silica (non-polar; "reverse-phase HPLC"). Less commonly, bare silica (polar; "normal-phase HPLC").
- Mobile: aqueous-organic mixtures (water + methanol or acetonitrile), with optional buffer salts to control pH.
Measurement: retention time and peak area. Retention time identifies the compound (by comparison with a standard). Peak area is proportional to amount.
Quantification. Run standards of known concentration; plot peak area vs concentration; the slope is the calibration curve. Read the sample concentration off the curve.
Applications. Pharmaceutical analysis (purity, concentration of active ingredients); food analysis (caffeine, vitamins, additives); environmental analysis (pesticides in water); biomolecules (proteins, sugars).
Strengths over GC. Handles non-volatile and thermally sensitive samples. Wide range of polarities. Mr range up to several thousand.
Limitations. Slower than GC. Solvent consumption and waste (less green). Sample must be soluble in the mobile phase.
Choosing the right technique
A typical IA3 design or EA short response asks why a particular technique was chosen for a particular analyte. The decision tree:
- Is the analyte coloured and qualitative identification sufficient? TLC.
- Is the analyte volatile and thermally stable? GC (especially GC-MS for unknown identification).
- Is the analyte non-volatile, thermally sensitive, or in aqueous solution, and is quantitative concentration needed? HPLC.
Caffeine, paracetamol, aspirin, vitamins, pesticides, sugars, proteins -> HPLC. Volatile esters (food flavours), short-chain alcohols, petroleum -> GC. Plant pigments, dyes, amino-acid screening -> TLC.
Combined techniques
GC-MS combines GC separation with MS identification. The sample is separated into individual compounds in the GC column, then each compound passes into the mass spectrometer where its molecular ion and fragmentation pattern identify it. GC-MS is the standard forensic technique for drugs, accelerants and environmental contaminants.
HPLC-MS combines HPLC separation with MS detection. Standard in pharmaceutical, biological, and environmental analysis where compounds are non-volatile or polar.
QCAA does not require detailed coverage of combined techniques but expects you to know they exist and to identify why combination is useful (one technique separates, the other identifies).
Quantitative analysis: calibration curves
A calibration curve plots peak area (or detector response) against known concentration of standards. The relationship is typically linear at low concentration:
where m is the slope (sensitivity) and b is the intercept (typically near zero). For an unknown:
QCAA EA Paper 2 may give you a calibration table and ask for a sample concentration. The expected approach: plot or compute the slope, read off the unknown, report with appropriate significant figures.
Common traps
Confusing Rf with retention time. Rf is for TLC (dimensionless, distance ratio). Retention time is for GC and HPLC (in minutes, time from injection to detection peak). They measure analogous behaviours but on different scales.
Treating chromatography as a single technique. TLC, GC and HPLC differ sharply in apparatus, phases, sample type and use case. QCAA marks for picking the right technique.
Forgetting calibration. A peak area without a calibration curve is meaningless for quantification. Concentration calculations require standards run alongside the sample.
Using HPLC for volatile mixtures. Volatile compounds tend to evaporate from HPLC sample vials and give poor peak shape. Use GC for volatiles.
Citing "polar vs non-polar" without specifying phases. In normal-phase TLC and HPLC, the stationary phase is polar (silica); polar compounds bind more strongly. In reverse-phase HPLC, the stationary phase is non-polar (C18); non-polar compounds bind more strongly and elute later. The same compound has opposite behaviour on the two systems; QCAA EA expects this distinction.
In one sentence
Chromatography separates mixtures by partitioning components between a stationary phase and a mobile phase: thin-layer chromatography (silica plate, organic solvent) gives an Rf value for fast qualitative identification of coloured or UV-active compounds; gas chromatography (heated column, inert gas mobile phase) gives retention time and peak area for volatile compounds; high-performance liquid chromatography (packed column, aqueous-organic mobile phase, pump) gives retention time and peak area for non-volatile or thermally sensitive compounds and is the standard quantitative technique for pharmaceuticals and food.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2023 QCAA-style4 marksA food-chemistry IA3 investigates artificial food colourings in a sports drink. (a) Explain why TLC is a suitable initial technique for identifying which colourings are present. (b) Calculate the Rf value of a spot that travelled 4.2 cm when the solvent front travelled 6.0 cm. (c) Explain how the identity of the colourings is confirmed using TLC.Show worked answer →
A 4-mark answer needs the rationale, the Rf calculation, and the identity-confirmation step.
(a) Why TLC. Food colourings are coloured organic compounds with distinct polarities. TLC separates them by differential partitioning between a polar silica stationary phase and a less polar mobile phase (typically a water / ethanol / ammonia mix). Spots are visible without staining (the colourings are coloured), and the technique is fast (10 to 15 minutes), cheap and uses little sample. These features make TLC ideal as a screening or initial identification technique before HPLC quantification.
(b) Rf calculation.
(c) Identity confirmation. Run reference standards of known colourings (E102 tartrazine, E110 sunset yellow, E133 brilliant blue, etc.) on the same TLC plate, in parallel with the sample. A spot in the sample is identified as colouring X if it has the same Rf and the same colour as the X standard, under identical solvent and temperature conditions. Co-spotting (running the sample with a standard added) confirms identity by giving a single combined spot rather than two adjacent spots.
Markers reward the polarity-based separation mechanism, correct Rf arithmetic (with no units; Rf is dimensionless and always between 0 and 1), and the standards / co-spotting confirmation step.
2022 QCAA-style3 marksA pharmaceutical IA3 investigates caffeine content in three brands of energy drink. The chemist sets up an HPLC instrument with caffeine standards at 50, 100, 200 and 400 mg/L. The standards give peak areas of 1.2, 2.4, 4.8 and 9.6 (arbitrary units). The sample peak area is 5.5. (a) Sketch the calibration curve. (b) Calculate the caffeine concentration in the sample. (c) Justify the use of HPLC for this analysis (rather than TLC or GC).Show worked answer →
A 3-mark answer needs the linear calibration, the concentration, and the HPLC justification.
(a) Calibration curve. Peak area vs concentration is linear: passes through (50, 1.2), (100, 2.4), (200, 4.8), (400, 9.6). Slope = 0.024 (area / mg/L); intercept approximately zero. Equation: area = 0.024 x concentration.
(b) Sample concentration. Rearranging: concentration = area / 0.024 = 5.5 / 0.024 = 229 mg/L (3 significant figures).
(c) HPLC vs alternatives. Caffeine is non-volatile at room temperature (it does sublime at high temperature, so GC is possible but requires heating). Caffeine is also moderately polar, so it dissolves in aqueous-organic solvents typical of HPLC. HPLC gives quantitative peak areas with a calibration curve, suitable for the milligram-per-litre concentrations of interest. TLC is qualitative and would give an approximate Rf only; GC is possible but requires sample volatilisation, less efficient than HPLC for thermally sensitive or non-volatile compounds. HPLC is the routine pharmaceutical-industry method for quantifying caffeine, paracetamol, and similar analytes.
Markers reward the linear calibration and arithmetic (with sig figs), the volatility-based rationale ruling out GC, and the quantification-vs-qualitative distinction from TLC.
Related dot points
- Describe the principles and apply mass spectrometry and infrared (IR) spectroscopy to determine the molecular mass, molecular formula, structural features and functional groups of organic compounds
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- Describe the principles and apply proton (1H) nuclear magnetic resonance (NMR) spectroscopy to identify the number and types of hydrogen environments, peak ratios (integration) and splitting patterns to determine the structure of organic compounds
A focused answer to the QCE Chemistry Unit 4 dot point on proton NMR spectroscopy. Explains chemical environments, chemical shifts (with the QCAA reference table), the n+1 splitting rule, and integration. Walks through the 1H NMR of ethanol and ethyl ethanoate, the canonical IA3 / EA spectra.
- Describe the principles of green chemistry and apply them to evaluate the sustainability of industrial chemical processes, including atom economy, percentage yield, energy use, choice of solvents and catalysts, and waste management
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