HSC Exam Practice

Sample response. The stationary phase is the fixed medium through which the mobile phase passes. It causes separation because different components of the mixture have different affinity for it: a component that interacts more strongly with the stationary phase moves more slowly through the system, while a component with lower affinity moves more quickly with the mobile phase, creating spatial separation over time.

Marking notes. 1 mark for defining the stationary phase as the fixed/immobile medium. 1 mark for explaining that separation occurs because components have different affinity for it (not equal attraction), so they move at different rates.

Sample response. Rf = distance compound travels ÷ distance solvent front travels = 4.2 ÷ 7.0 = 0.60. Since Rf = 0.60, the compound has greater affinity for the mobile phase: a value above 0.5 means the compound has moved more than halfway to the solvent front, indicating it spends more time in the mobile phase than on the stationary phase.

Marking notes. 1 mark for the correct formula or working (numerator = compound distance). 1 mark for the correct numerical answer of 0.60 (or 3/5). 1 mark for correctly stating that the compound has greater affinity for the mobile phase and justifying this from the Rf value (Rf > 0.5).

Sample response. The conditions that must be kept identical are the stationary phase (same plate material and thickness) and the mobile phase (same solvent composition). This is essential because Rf depends on the relative affinity of the compound for the two phases; any change to either phase changes the affinity ratios, and the same compound will produce a different Rf, making comparisons meaningless.

Marking notes. 1 mark for identifying that same plate (stationary phase) AND same solvent (mobile phase) must be used. Accept either alone for the mark if accompanied by reasoning. 1 mark for explaining why: Rf is a ratio that depends on both phases, so a change to either changes the Rf of every compound.

Sample response. TLC is an analytical technique primarily used to check purity or identify components by comparing Rf values with standards; the separated spots remain on the plate and cannot be collected. Column chromatography is a preparative technique: the mobile phase carries components through a packed column and they exit the column at different times, allowing the separated fractions to be collected as individual solutions for further use or analysis.

Marking notes. 1 mark for TLC — analytical use (purity/identity check); separated spots cannot be collected. 1 mark for column chromatography — preparative use; separated fractions can be collected from the column exit. 1 mark for explicitly comparing the fate of the separated components (plate vs collected liquid fractions).

Sample response. HPLC is quantitative because the area under each peak in the chromatogram is proportional to the concentration of that component; by comparing peak areas to a calibration curve, the exact amount of active ingredient and impurity can be calculated. In pharmaceutical testing this means specifying, for example, that a tablet contains 500 mg of paracetamol ± 2%. HPLC is highly sensitive because the combination of high-pressure fine-particle columns and instrument-based detectors (UV, fluorescence, or mass spectrometry) can detect compounds at very low concentrations — sometimes parts per billion — allowing trace impurities that pose a health risk to be detected before a product is approved for sale.

Marking notes. 1 mark — defines quantitative: peak area proportional to concentration. 1 mark — applies quantitative to pharmaceutical context (determines exact amount of active ingredient or impurity). 1 mark — defines sensitive: can detect very small amounts / low concentrations. 1 mark — applies sensitivity to pharmaceutical context (detects trace impurities at safety-relevant concentrations).

Sample response. Rf is the ratio of how far the compound travels to how far the solvent front travels. A compound with a low Rf value travels a short distance relative to the solvent front. This happens because it is strongly attracted to (adsorbed onto) the stationary phase, spending more time bound to the plate and less time moving with the mobile phase. A compound with high Rf travels further because it has less affinity for the stationary phase and more for the mobile phase, so it is carried further by the solvent.

Marking notes. 1 mark — low Rf means the compound travels less distance relative to the solvent; links this to stronger affinity for the stationary phase. 1 mark — explains the mechanism: strongly attracted to the stationary phase means it spends more time bound there, slowing its movement through the system.

Sample response. The main peak in the tablet extract chromatogram appears at a retention time of 3.2 min, which is the same as the paracetamol standard peak at 3.2 min, under identical conditions. Matching retention times support the identification of this peak as paracetamol. The peak is also the dominant (largest area) component in the extract, consistent with paracetamol being the major active ingredient.

Marking notes. 1 mark — states that the main peak in the extract has the same retention time as the paracetamol standard (3.2 min). 1 mark — explains that matching retention time under identical conditions supports identification as paracetamol.

Sample response. The peak at 5.1 min indicates the presence of at least one additional component in the tablet extract that is separate from paracetamol. Because 5.1 min is a different retention time from the paracetamol standard (3.2 min), this component cannot be paracetamol; if it were paracetamol, it would elute at the same time as the standard under identical conditions. The 5.1 min peak could represent a manufacturing impurity, an excipient or a degradation product, but the standard run does not include a reference at that retention time, so it cannot be specifically identified from this data alone.

Marking notes. 1 mark — states the peak at 5.1 min indicates an additional component (impurity, excipient or degradation product). 1 mark — explains it cannot be paracetamol because its retention time (5.1 min) differs from the paracetamol standard (3.2 min). 1 mark — states or implies that specific identification requires a matching reference standard at that retention time; this component is therefore unidentified from this data alone.

Sample response. In HPLC, peak area is proportional to the amount (and hence concentration) of the compound reaching the detector during a run: a larger peak area means more of that compound is present. To estimate concentration from peak area, a calibration curve must be constructed using known concentrations of the same compound (the impurity) under identical instrument conditions, so that peak area can be converted to concentration. One condition that must be met is that the calibration standards must be the same compound as the impurity (not paracetamol) and measured under exactly the same instrument conditions (column, mobile phase, flow rate, detector settings).

Marking notes. 1 mark — states that peak area is proportional to concentration / amount of the compound. 1 mark — explains that a calibration curve or reference standard of the same compound at known concentrations is needed to convert peak area to concentration. 1 mark — identifies one specific required condition: same compound as calibrant; OR same instrument conditions (column, mobile phase, flow rate, detector); OR that the detector response factor must be the same.

Sample response. Tablet X has one spot with Rf = 0.60, which matches the paracetamol standard (Rf = 0.60). This supports the conclusion that Tablet X contains paracetamol only as its active ingredient. Tablet Y has two spots with Rf values of 0.60 and 0.82, which match the paracetamol standard (0.60) and the ibuprofen standard (0.82) respectively. This supports the conclusion that Tablet Y contains both paracetamol and ibuprofen — consistent with a combination analgesic tablet.

Marking notes. 1 mark — Tablet X contains paracetamol (Rf = 0.60 matches standard) with justification. 1 mark — Tablet Y contains both paracetamol and ibuprofen (both Rf values match the respective standards). 1 mark — explicit justification using Rf matching for at least one identification.

Sample response. Tablet Z shows two spots, one at Rf = 0.60 (matching paracetamol) and one at Rf = 0.74 (not matching either standard). The presence of two spots indicates that the extract is a mixture of at least two components. The first spot is supported as paracetamol by its matching Rf. The second spot at Rf = 0.74 is a component different from both paracetamol and ibuprofen under these conditions; it may be a second active ingredient, an excipient or an impurity, but it cannot be identified without running an appropriate reference standard at Rf = 0.74 under the same conditions.

Marking notes. 1 mark — two spots indicate a mixture with at least two components; one matches paracetamol. 1 mark — the second spot (Rf = 0.74) does not match either standard and therefore cannot be identified without a matching reference, but is a distinct compound different from paracetamol and ibuprofen under these conditions.

Sample response. The claim that HPLC has made TLC and column chromatography obsolete overstates the case. While HPLC is unquestionably the most powerful of the three techniques for many applications, each retains contexts in which it is the most appropriate choice. HPLC is faster than column chromatography, more sensitive than TLC, and quantitative where the others are not. These properties make it the preferred method for pharmaceutical purity testing (e.g. TGA release testing of paracetamol tablets) and environmental trace-contaminant monitoring (e.g. PFAS quantification in groundwater near RAAF Base Williamtown, NSW). However, HPLC requires expensive instrumentation, trained operators and a stable laboratory environment. TLC remains the most appropriate technique for rapid, inexpensive reaction monitoring in synthetic chemistry labs — a chemist running a multi-step synthesis can spot-check each stage on a TLC plate in minutes without committing to a full HPLC run. It is also practical for simultaneous screening of many samples on one plate, and is deployable in field or resource-limited settings. Column chromatography remains the only practical technique when the goal is preparative separation — physically isolating and collecting grams of purified product in a synthesis. Neither TLC nor HPLC allows this on a preparative scale. On sensitivity, HPLC detects far lower concentrations than TLC. On quantitative capability, HPLC provides precise peak area data while TLC is semi-quantitative at best. On cost and accessibility, TLC is far cheaper and more portable. On preparative scale, only column chromatography isolates quantities large enough for further use. The claim is therefore rejected: the three techniques are complementary rather than competitive. HPLC has not rendered the others obsolete because it cannot replicate the speed and accessibility of TLC screening, or the preparative function of column chromatography. Each technique solves a different practical problem in chemistry.

Marking criteria:

1 mark — States an explicit overall evaluative judgement rejecting the “obsolete” claim (e.g. “complementary rather than competitive”).

1 mark each (max 3) — Compares the three techniques on three valid criteria: sensitivity (HPLC > TLC ≥ column); quantitative capability (HPLC yes, TLC semi at best, column no); speed (TLC fast, HPLC fast, column slow); cost/accessibility (TLC cheapest, column moderate, HPLC expensive); preparative capability (only column).

1 mark — Identifies at least one specific analytical context where TLC is most appropriate (e.g. reaction progress monitoring in synthetic chemistry; field screening; rapid multi-sample check).

1 mark — Identifies at least one specific analytical context where column chromatography is most appropriate (e.g. preparative isolation of a product in a multi-step synthesis).

1 mark — Identifies at least one specific analytical context where HPLC is most appropriate, using an Australian example (TGA pharmaceutical release testing; AWRI wine analysis; NSW EPA PFAS monitoring).