17.6 Chromatography (HPLC, GLC, TLC, Ion Exchange) and Solvent Extraction
★★★ 2021–2025: 15 parts, 125 marks, all in Geo-Scientist Paper III, three every year: one numerical on plates, resolution, retention or extraction, and one or two on a technique (van Deemter, HPLC vs GLC, ion exchange, chelate extraction).
Syllabus line. Geo-Scientist P-III, Separation methods: “Chromatographic analysis: basic principles of chromatography (partition, adsorption and ion exchange), column chromatography, plate concept, plate height (HETP), normal phase and reversed phase concept, thin layer chromatography, frontal analysis, principles of High Performance Liquid Chromatography (HPLC) and Gas Liquid Chromatography (GLC), and ion-exchange chromatography. Solvent extraction: classification, principle and efficiency of the technique, mechanism of extraction, extraction by solvation and chelation, qualitative and quantitative aspects of solvent extraction, extraction of metal ions from aqueous solutions.”
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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes
17.6.1 Principles and kinds of chromatography ★★
- A mobile phase (gas or liquid) carries the mixture over a stationary phase (solid or liquid film); each solute distributes between the two, K = cS/cM. Solutes with larger K spend longer in the stationary phase and move slower: differential migration.SN
| Mechanism | Stationary phase | Examples |
|---|---|---|
| Adsorption | solid surface (silica, alumina) | column, TLC, GSC |
| Partition | liquid film on a support or bonded to it | GLC, HPLC (C18), paper |
| Ion exchange | resin with fixed ionic groups | ion chromatography, water softening |
| Size exclusion | porous gel | gel filtration and permeation (polymers, proteins) |
17.6.2 Retention: retention factor, relative retention, Rf value ★★★
- Partition ratio k′ (capacity factor): moles in the stationary phase ÷ moles in the mobile phase; how many times longer the solute stays put than it moves. Best 1–5; below 1 poor separation, above 10 long runs and broad peaks. Relative retention α (selectivity): later peak on top, so α ≥ 1; α = 1 means no separation however many plates Geo 2024 Q1(b) 5m.SN
- Rf in TLC: a ratio of two lengths, so no unit, 0–1. It depends on the solute, the adsorbent and its activity (moisture), the solvent, temperature and chamber saturation. Use: identification beside an authentic sample, purity, following a reaction, choosing the solvent for a column Geo 2021 Q2(a) 10m.SN
17.6.3 Plate concept, plate height, resolution ★★★ numerical every year
- Plate: one equilibration of the solute between the phases; a column of N plates. Plate height (HETP) H = L/N = σ²/L: smaller H, sharper peaks, better column. Packed GC ≈ 10³ plates m⁻¹; capillary GC ≈ 10⁵ in total; HPLC 3 µm ≈ 10⁵ m⁻¹.SN
- tR = 52.3 s, W½ = 5.3 (same units): N = 5.545 × (52.3/5.3)² = 540 plates: low, a broad peak for its retention, an inefficient column Geo 2024 Q4(a) 10m.SN
- W = 25 s at tR = 35 min = 2100 s: N = 16 × (2100/25)² = 16 × 84² = 1.13 × 10⁵; 3 m column: H = 3000 mm/112 896 = 0.0266 mm (26.6 µm) Geo 2023 Q2(b) 10m.SN
- Resolution, 30 cm column, tM 1.5 min; X 14.5 min (W 1.2), Y 15.2 min (W 1.3) Geo 2022 Q2(b) 10m:SN
- Rs = 2 × 0.7/2.5 = 0.56 (fused peaks; 1.0 → 2 % overlap; 1.5 → baseline).
- NX = 16(14.5/1.2)² = 2336; NY = 16(15.2/1.3)² = 2187; mean 2262; H = 30/2262 = 0.0133 cm.
- Rs ∝ √N ∝ √L: L₂ = 30 × (1.0/0.56)² = 95.7 cm; tR ∝ L: Y now at 15.2 × 3.19 = 48.5 min.
17.6.4 Rate theory: the van Deemter equation ★★
- A, eddy diffusion: molecules take paths of different length round the particles; independent of u; small for small, uniform, well-packed particles; zero in open capillaries.SN
- B/u, longitudinal diffusion: the band spreads along the column while it waits; ∝ DM; important at low u and in GC (gas diffusion fast), small in LC.SN
- Cu, mass-transfer resistance: equilibration between the phases lags behind the moving front; thin films, small particles and low viscosity reduce it; dominant at high u.SN
- Significance: it gives the optimum flow rate and tells how to build a better column (small uniform particles, thin film, capillary columns); sub-2 µm particles are the basis of UHPLC Geo 2025 Q1(b) 5m.
17.6.5 Column chromatography, TLC, frontal analysis ★
- Column (elution): adsorbent packed in a glass tube; sample on top; eluents of rising polarity (hexane → ethyl acetate → methanol) wash bands down; fractions collected. Least polar solute leaves first from silica or alumina.SN
- TLC: 0.25 mm silica gel G on glass or aluminium; spot 1 cm from the base; develop in a saturated chamber; locate by UV (F254 indicator), iodine or spray reagent; calculate Rf.
- Frontal analysis: the sample is fed continuously; only the least retained component emerges pure at first, then steps (“breakthrough fronts”) appear as each next component saturates the bed. Used to measure adsorption isotherms and for purification of the weakest-held solute, not for full separation. Displacement: a strongly held displacer pushes the components off in touching bands.
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