Quick questions on Chromatography techniques: TLC, GC and HPLC (QCE Chemistry Unit 4)

Every chromatography technique has the same three elements:

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.

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.

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.

A typical IA3 design or EA short response asks why a particular technique was chosen for a particular analyte. The decision tree:

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.

A calibration curve plots peak area (or detector response) against known concentration of standards. The relationship is typically linear at low concentration:

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.

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.

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.

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.

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.

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).

Qualitative only (rough Rf comparisons; not suitable for concentration measurement). Limited resolution. Cannot handle volatile samples.

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).