CSIR-NET Analytical Techniques — Chromatography and Thermal Analysis
"Analytical techniques" is a broad heading in CSIR-NET Part C, but spectroscopic methods (UV-Vis, IR, NMR, mass spectrometry) and electrochemical methods (potentiometry, voltammetry) are large enough topics that they earn their own dedicated treatment elsewhere. This article covers the other pillar that appears just as often: separation techniques (chromatography) and thermal analysis (TGA, DSC) — with the exact calculations examiners ask for, worked out step by step.
Chromatography — Rf and the retention factor k′
Two different numbers get confused constantly, and NET loves testing exactly that confusion: the Rf value from thin-layer or paper chromatography, and the retention factor k′ (sometimes called the capacity factor) from column-based techniques such as HPLC.
k′ = (tR − t₀) ÷ t₀ where tR = retention time of the analyte, t₀ = dead time (unretained solute)
Rf is dimensionless and always between 0 and 1 — it describes a spot's position on a TLC plate relative to the solvent front. k′ is dimensionless too, but it has no upper bound of 1 — it compares how much longer a retained compound takes than an unretained one, so k′ = 0 means "not retained at all" and larger k′ means stronger retention on the column.
Worked example 1 — Rf from a TLC plate
Q. On a TLC plate, a compound spot travels 4.2 cm while the solvent front travels 6.0 cm. Calculate Rf.
Rf = 4.2 ÷ 6.0 = 0.70
An Rf of 0.70 means the compound is fairly non-polar in this solvent system (it moved most of the way with the mobile phase). A polar compound in the same system would show a much smaller Rf, since it interacts more strongly with a polar stationary phase such as silica.
Worked example 2 — retention factor in HPLC
Q. An HPLC run shows a dead time (unretained peak) t₀ = 1.2 min and the analyte elutes at tR = 6.0 min. Calculate k′.
k′ = (6.0 − 1.2) ÷ 1.2 = 4.8 ÷ 1.2 = 4.0
A k′ of 4.0 sits comfortably in the practically useful range (roughly 1–10); a k′ far below 1 means the analyte is barely retained and risks co-eluting with the solvent front, while a very large k′ means an unnecessarily long run time.
Thermal analysis — TGA measures mass, DSC measures heat
This is the single most-tested distinction in the thermal-methods section: TGA (thermogravimetric analysis) records mass change against temperature or time — it tells you when a sample loses water of crystallisation, decomposes, or oxidises. DSC (differential scanning calorimetry) records heat flow — it tells you about melting points, glass transitions, crystallisation and reaction enthalpies, none of which necessarily involve any change in mass at all.
Worked example 3 — confirming a hydrate formula from a TGA curve
Q. A 1.000 g sample of hydrated copper(II) sulfate, CuSO₄·xH₂O, is heated in a TGA experiment. The mass plateaus at 0.639 g once dehydration is complete. Determine x.
Step 1. Mass lost as water = 1.000 − 0.639 = 0.361 g, i.e. 36.1% of the original mass.
Step 2. Molar mass of anhydrous CuSO₄ = 63.55 + 32.06 + 4(16.00) = 159.61 g/mol. If the residue (0.639 g) corresponds to this anhydrous formula, the original sample's molar mass was 1.000 g ÷ (0.639/159.61 mol) = 249.8 g/mol.
Step 3. Mass of water per mole = 249.8 − 159.61 = 90.2 g, and 90.2 ÷ 18.02 (molar mass of H₂O) = x ≈ 5.
This confirms the familiar pentahydrate, CuSO₄·5H₂O — and the 36.1% water-loss figure is exactly what a genuine TGA trace of this compound shows, which is why this specific salt is such a common thermal-analysis teaching example.
Common mistakes that cost marks
- Confusing Rf with k′. They answer different questions and are not interchangeable — Rf ranges strictly 0–1; k′ does not.
- Forgetting the dead time (t₀) in an HPLC calculation. Using the raw retention time tR alone, instead of (tR − t₀), overstates how strongly a compound is actually retained.
- Treating TGA and DSC as interchangeable. A melting transition shows up clearly in DSC (a sharp endotherm) with essentially no TGA mass change — a student who expects a mass drop at every DSC event will misread the data.
- Ignoring accuracy vs precision. A method can be highly precise (reproducible) while being inaccurate (systematically off from the true value) — these are independent properties of a measurement, not synonyms.
Where this appears in CSIR-NET
| Technique | What it measures | Typical NET use |
|---|---|---|
| TLC / paper chromatography | Position relative to solvent front (Rf) | Qualitative identification, purity check |
| HPLC / GC | Retention time, retention factor k′ | Quantitative separation, resolution problems |
| TGA | Mass vs. temperature | Hydrate/formula determination, decomposition studies |
| DSC | Heat flow vs. temperature | Melting point, phase transitions, reaction enthalpy |
Check the arithmetic behind a hydrate-formula or percent-composition question instantly. The Molar Mass & Composition calculator handles hydrated formulas such as CuSO₄·5H₂O directly, so you can confirm a TGA-style calculation before writing it into an exam answer.
Open the ABC Chemistry Calculator Suite →Preparing for CSIR-NET, IIT-JAM, GATE or CUET-PG? ABC Chemistry runs dedicated competitive-exam batches at the coaching centre and online across India — details at abcchemistry.in.