VKC Notes · Analytical chemistry · Geo-Scientist P-III
13.8 Spectroscopic Instruments: UV–Vis, IR, Beer–Lambert, AAS and Flame Emission
★★★ 2021–2025: 28 parts, 220 marks, all in Geo-Scientist Paper III. Every paper has 4–6 parts from this unit: a Beer’s-law numerical, an instrument sketch and an AAS interference question.
Syllabus line. Geo-Scientist P-III: “Lambert-Beer’s law and its limitations. UV-visible spectroscopy: basic principles of UV-Vis spectrophotometer, instrumentation consisting of source, monochromator, grating and detector, spectrophotometric determinations (estimation of metal ions from aqueous solutions, determination of composition of metal complexes using Job’s method of continuous variation and mole ratio method). Infra-red spectrometry: basic principles of instrumentation (choice of source, monochromator and detector) for single and double beam instruments, sampling techniques. Flame atomic absorption and emission spectrometry … background correction, sources of chemical interferences and methods of removal … Flame-AAS, VG-AAS and GF-AAS … Hollow Cathode Lamp (HCL) and Electrodeless Discharge Lamp (EDL)”. Structure from IR/NMR/MS: 13.5; UV of conjugated systems: 13.1.
Two-page short notes for revisionAll VKC notes
Download PDF PLAN
PDF edition, print-ready. It opens with the VKC plan, together with every locked answer, note and test on the site.
CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes
13.8.1 Beer–Lambert law ★★★ numerical every year
(13.8.1)
Draw it in 60 s: two small graphs: a curve falling from 1 (T); a straight line from the origin (A) with a dashed curve bending below it at high c.
- ε (L mol⁻¹ cm⁻¹) depends on the substance, wavelength and solvent only; b in cm, c in mol L⁻¹. Absorbance, not transmittance, is proportional to c; absorbances of different species add.SN
- 60 % T, 1.0 × 10⁻³ M, 1 cm: A = −log 0.60 = 0.2218; ε = 0.2218/(1 × 10⁻³) = 222 L mol⁻¹ cm⁻¹ Geo 2021 Q1(c) 5m.
- Aminobenzene, 4.00 × 10⁻⁴ M, A = 0.504 (1 cm): ε = 1260; at 1.50 × 10⁻³ M in 0.5 cm, A = 1260 × 0.5 × 1.50 × 10⁻³ = 0.945, T = 10−0.945 = 0.114 (11.4 %) Geo 2024 Q1(c) 5m.SN
- Mn in steel: 0.55 g dissolved, Mn → MnO₄⁻ in 100 mL, A₅₃₀ = 0.42, ε = 4.2 × 10³: c = 1.0 × 10⁻⁴ M, Mn = 1.0 × 10⁻⁵ mol × 55 = 0.55 mg, 0.10 % Mn Geo 2021 Q5(e) 5m.SN
13.8.2 Deviations from Beer's law ★★
| Kind | Cause | Cure |
|---|---|---|
| Real | c > 0.01 M: solute particles interact, refractive index changes | work in dilute solution |
| Chemical | analyte associates, dissociates or reacts (unbuffered indicator HIn ⇌ In⁻; Cr₂O₇²⁻ ⇌ CrO₄²⁻) | buffer; excess reagent |
| Instrumental | polychromatic light (unequal ε across the band pass), stray light, mismatched cells | measure at λmax, narrow slit, matched cells |
- Polychromatic light: with two wavelengths of ε′ ≠ ε″, the less-absorbed one makes up a growing share of the transmitted light as c rises, so the measured A falls below εbc: negative deviation, curving towards the c axis (Fig. 13.8.1). Keep the band pass about one-tenth of the absorption band width, at λmax where the spectrum is flat Geo 2023 Q5(a) 5m.SN
- Unbuffered indicator (Ka = 2.5 × 10⁻⁵, c = 3.0 × 10⁻⁴ M): x²/(c − x) = Ka gives [In⁻] = 7.5 × 10⁻⁵, [HIn] = 2.25 × 10⁻⁴ M; A₄₃₀ = (4.0 × 10³)(2.25 × 10⁻⁴) + (3.0 × 10²)(7.5 × 10⁻⁵) = 0.900 + 0.0225 = 0.923. Without ionisation it would be 1.200: a chemical deviation Geo 2022 Q4(a) 15m.SN
- Precision is best for A ≈ 0.2–0.8 (minimum relative error near A = 0.434 for a constant error in T).
13.8.3 The UV–visible spectrophotometer ★★★
| Component | Function | Examples |
|---|---|---|
| Source | steady continuous radiation | D₂ lamp 160–380 nm; tungsten–halogen 320–2500 nm; xenon arc |
| Wavelength selector | isolates a narrow band | filter; monochromator: entrance slit → collimating mirror → grating (or prism) → focusing mirror → exit slit |
| Sample cell | holds solution, fixed path b | quartz or fused silica (UV); glass or plastic (visible only) |
| Detector | light → electrical signal | phototube, photomultiplier, silicon photodiode, diode array |
| Readout | amplifies, shows A or %T | meter, computer |
Draw it in 60 s: five boxes: source → monochromator → chopper, which splits to sample and reference cells → detector; write A = log(P0/P) beside it.
- Single beam: blank and sample read one after the other; lamp or detector drift causes error. Double beam (in time, by a rotating chopper; or in space, by a beam splitter and two detectors): the ratio is taken continuously, so drift cancels Geo 2021 Q4(b) 10m.SN
- Slit width to resolve two lines: effective band width = w × reciprocal linear dispersion D⁻¹; to resolve Δλ it must be ½Δλ. Na lines 588.9950 and 589.5924 nm: Δλ = 0.5974 nm; w = ½ × 0.5974/0.67 = 0.446 mm Geo 2022 Q2(c) 5m.SN
- Na resonance line from 2.10 eV: λ = hc/E = 12 330/2.10 = 5871 Å = 587.1 nm (the yellow D line) Geo 2024 Q5(b)(i) 5m.
13.8.4 Determinations, Job's and mole-ratio methods ★★★
- Non-absorbing analyte: convert it into a coloured species with excess chromogenic reagent at fixed pH and time, measure at λmax against a reagent blank, read c from a calibration line. Fe²⁺ + 1,10-phenanthroline (510 nm); PO₄³⁻ → molybdenum blue; NH₃ → indophenol blue; Ti(IV) + H₂O₂ (410 nm). Or measure the fall in colour of a reagent (F⁻ bleaching the Zr–SPADNS lake) Geo 2025 Q1(c) 5m.SN
- Cr and Mn in steel: dissolve in H₂SO₄–H₃PO₄ (H₃PO₄ decolourises Fe³⁺), then HNO₃; oxidise with persulfate + Ag⁺ (or periodate): Cr → Cr₂O₇²⁻, Mn → MnO₄⁻; dilute to volume; read A at 440 and 545 nm and solve two simultaneous equations Aλ = εCr,λbcCr + εMn,λbcMn, with ε values from pure standards Geo 2022 Q2(a) 15m.SN
- Standard addition (matrix effects): urine 2.00 mL → A = 0.428; same + 0.150 mg phosphate → 0.538. 0.110 per 0.150 mg, so x = 0.428 × 0.150/0.110 = 0.584 mg in 2.00 mL: 0.292 mg mL⁻¹ Geo 2024 Q5(b)(ii) 5m.SN
Draw it in 60 s: axes: mole fraction of metal 0 → 1 and absorbance; plot the points, draw a straight line up from the origin and one down to x = 1; drop a line from where they meet.
- Job's method of continuous variation: mix equimolar M and L solutions in different ratios, keeping the total moles constant; plot the corrected absorbance against mole fraction. The complex is largest where the mixture has its stoichiometric ratio: maximum at xL = n/(n + 1).SN
- 2023 data: VM 10 → 90 mL in 100 mL, highest A = 0.69 at VM = 20 mL. Extrapolated lines meet at xM = 0.20, xL = 0.80: n = 0.80/0.20 = 4, ML₄ Geo 2023 Q3(a) 15m.SN
- Mole-ratio method: fix [M], increase [L]; A rises then levels off; the break in the plot of A against [L]/[M] gives n. A rounded break means a weak complex; then extrapolate the straight parts.SN
13.8.5 IR spectrometers and sampling ★★★
| Part | Choice and reason |
|---|---|
| Region | mid-IR, 4000–667 cm⁻¹ (2.5–15 µm), where fundamental vibrations absorb |
| Sources | Nernst glower: ZrO₂ with Y₂O₃/Er₂O₃, 1500–2000 °C, non-conducting when cold so it must be preheated, fragile. Globar: sintered SiC rod, 1300–1500 °C, conducts when cold (self-starting), rugged, more output beyond about 15 µm (contacts water-cooled). Nichrome coil: cooler, long life. Tungsten lamp (near IR), Hg arc (far IR) |
| Monochromator | reflection grating with front-surface mirrors; glass and quartz absorb IR, so any prism or window is NaCl, KBr or CsI (hygroscopic: keep dry). Today the interferometer (FTIR) has replaced it |
| Detectors | thermal: thermocouple, bolometer, Golay (pneumatic) cell, pyroelectric DTGS; photon: HgCdTe (MCT, cooled), PbS, InGaAs (near IR) |
| Beam | double beam (sample and reference beams chopped; ratio recorded): cancels atmospheric CO₂ and H₂O bands and source drift |
- Monochromator, role and parts: entrance slit → collimating mirror → dispersing element (grating or prism) → focusing mirror → exit slit; it selects a narrow band and scans it across the detector. Types: prism (Littrow, Cornu) and grating (Czerny–Turner, Littrow) Geo 2024 Q2(b) 10m Geo 2021 Q2(c) 15m.SN
- Globar vs Nernst glower: SiC rod heated by current to about 1300 °C radiates as a near-black body; it needs no preheating and is sturdier, with better output at long wavelengths; the Nernst glower is brighter but brittle and must be preheated Geo 2025 Q3(c) 10m Geo 2023 Q1(c) 5m.SN
- Sampling: gas in a 10 cm cell with KBr windows; neat liquid as a 0.01–0.1 mm film between NaCl plates; solution in CCl₄ or CS₂ (transparent windows); solid as a KBr pellet or Nujol mull; attenuated total reflectance (ATR) for films and pastes.
- Matching sources and detectors: IR → Nernst glower, DTGS, PbS, InGaAsP; XRF → Mo-target tube, photomultiplier (scintillation counter); AAS → electrodeless discharge lamp, photomultiplier; ICP-MS → quadrupole analyser, Faraday cup Geo 2022 Q1(c) 5m.
13.8.6 Flame atomic absorption and flame emission ★★★
(13.8.2)
- Almost all atoms in a flame are in the ground state: AAS measures them by absorption, so it is less sensitive to flame temperature than emission, which counts the tiny excited fraction (flame photometry suits easily excited Na, K, Li, Ca).SN
- Hollow-cathode lamp (HCL): sealed tube with Ne or Ar at 1–5 torr, cathode made of the analyte, W or Zr anode. The discharge ionises the gas; ions sputter metal atoms off the cathode; collisions excite them; they emit the element's own lines. Low pressure and temperature give lines narrower than the absorption line (about 0.002 nm), which a monochromator alone cannot achieve Geo 2021 Q1(g) 5m.SN
- Electrodeless discharge lamp (EDL): a little metal or salt in a sealed quartz bulb excited by radio-frequency or microwave power; 10–100 times brighter; used for volatile As, Se, Sb, Hg.
- Double-beam AAS: a mirrored chopper sends the lamp beam alternately through the flame (P) and round it (Pr); one detector reads both; A = log(Pr/P) cancels lamp and detector drift, but not flame background Geo 2022 Q5(c) 5m.SN
Draw it in 60 s: row of boxes HCL → chopper → flame → monochromator; down to PMT → amplifier; a dashed path from the chopper round the flame; a D2 lamp joining the beam.
| Fuel–oxidant | Temperature / °C | Burning velocity / cm s⁻¹ | Use |
|---|---|---|---|
| Natural gas–air | 1700–1900 | 39–43 | alkali metals (emission) |
| Acetylene–air | 2100–2400 | 158–266 | most metals |
| Acetylene–N₂O | 2600–2800 | 285 | refractory oxides: Al, Ti, V, Sr, Ca with phosphate |
| Acetylene–oxygen | 3050–3150 | 1100–2480 | too fast for a premix burner |
So, 1800 °C with about 40 cm s⁻¹: natural gas–air; about 3000 °C with about 2000 cm s⁻¹: acetylene–oxygen Geo 2022 Q3(c) 5m.
Higher in the flame atoms have had longer to form (rise), but in the outer, oxidant-rich zone easily oxidised metals form oxides (fall). Mg: maximum near the middle; Ag: rises to the top; Cr: falls from the base. Optimise the burner height for each element and keep it the same for standards and samples Geo 2025 Q1(d) 5m.SN
Draw it in 60 s: three curves from the origin: one rising steadily (Ag), one rising then falling (Mg), one falling from the start (Cr).
13.8.7 Atomisation: flame, furnace, vapour ★★
| Mode | How | Strength and limit |
|---|---|---|
| Flame AAS | pneumatic nebuliser → spray chamber → premix slot burner; only ≈ 5–10 % of the sample reaches the flame | fast, precise (≈ 1 %); mg L⁻¹ levels |
| Graphite furnace (GF-AAS) | 10–50 µL in an electrically heated graphite tube under Ar: dry (≈ 110 °C), ash (400–1000 °C), atomise (2000–3000 °C) | nearly all of the sample atomised: detection limits 100–1000 times lower (picograms), solids directly; poorer precision (5–10 %), slow, strong background (needs D₂ or Zeeman correction) |
| Cold vapour (Hg) | digest (HNO₃–H₂SO₄–KMnO₄); Hg²⁺ + Sn²⁺ → Hg⁰ + Sn⁴⁺; Ar sweeps Hg vapour into a quartz cell; read 253.7 nm at room temperature | ≈ 1 ng; Hg is the only metal with a useful vapour pressure at room temperature |
| Hydride generation (VG-AAS) | NaBH₄ in acid → AsH₃, SeH₂, SbH₃, BiH₃; decomposed in a heated quartz tube | analyte leaves the matrix: low interference, µg L⁻¹ |
GF-AAS vs flame Geo 2023 Q1(d) 5m · cold-vapour Hg Geo 2022 Q1(d) 5m.
13.8.8 Interferences and background correction ★★★
| Interference | Example | Remedy |
|---|---|---|
| Chemical (anion) | PO₄³⁻, SO₄²⁻, Al, Si hold Ca, Mg, Sr as involatile compounds | releasing agent La³⁺ or Sr²⁺ (takes the phosphate); protective agent EDTA, 8-hydroxyquinoline, APDC; hotter N₂O–acetylene flame |
| Ionisation | M ⇌ M⁺ + e⁻ in hot flames (alkali, alkaline-earth) | ionisation suppressor: large excess of K or Cs (≈ 1000 mg L⁻¹) |
| Spectral | overlapping lines; molecular bands (CaOH near Na 589 nm); scattering by particles | other line, narrower band pass; background correction |
| Physical (matrix) | viscosity, surface tension, solvent change uptake | matrix-matched standards; standard addition; internal standard (Li) |
| Self-absorption (emission) | cooler atoms absorb emitted line at high c | dilute to the linear range |
- Ca in phosphate: thermally stable calcium phosphate (pyrophosphate) atomises poorly, so A is low. Add La³⁺ (forms LaPO₄, freeing Ca), or EDTA, which binds Ca as a complex that breaks up easily in the flame (a protective agent), or use the N₂O–acetylene flame Geo 2025 Q3(a) 15m.SN
- K for Sr in N₂O–acetylene: at ≈ 2800 °C part of Sr (IE 5.69 eV) ionises and Sr⁺ does not absorb at 460.7 nm. K (IE 4.34 eV) floods the flame with electrons; Sr ⇌ Sr⁺ + e⁻ shifts back; A rises and the calibration becomes linear Geo 2023 Q4(c) 5m.SN
- Flame photometry interferences and their removal Geo 2024 Q4(b) 10m: spectral (filter or monochromator), ionisation (CsCl buffer), anion (LaCl₃ or EDTA), self-absorption (dilute), physical (matched standards, Li internal standard).
- Why background correction: molecular absorption (oxides, hydroxides, salt particles) and light scattering in the flame or furnace absorb broadly and add to the analyte's A; in emission the flame itself emits a continuum and bands under the line Geo 2021 Q3(a) 15m.SN
- Two-line method: measure A at the analyte line (analyte + background) and at a nearby non-absorbed line from the same lamp (fill-gas or another cathode line; background only); the difference is the analyte absorbance. Works only if the background is the same at both lines Geo 2025 Q3(b) 5m.SN
- Continuum-source (D₂ lamp): the broad D₂ beam is absorbed only by the background (the narrow atomic line removes a negligible part of it); the instrument subtracts it alternately from the HCL reading.SN
- Zeeman: a magnetic field splits the analyte line (π and σ components); a polariser lets the instrument read analyte + background, then background alone; best for furnaces. Smith–Hieftje: pulsing the HCL at high current broadens and self-reverses its line, so that reading measures background.
- In emission: scan the monochromator just off the line and subtract the emission there.
13.8.9 ★ Add-on beyond the syllabus
Why FTIR replaced the dispersive IR. An FTIR spectrometer uses a Michelson interferometer: all wavelengths reach the detector together (Fellgett or multiplex advantage, a full spectrum in about a second), there are no slits to throttle the light (Jacquinot or throughput advantage), and a He–Ne laser fixes the wavenumber scale (Connes advantage). Fourier transformation of the interferogram gives the spectrum. One line on this lifts any “IR instrument” answer.
13.8.10 How UPSC asks this
| Question type | Asked | What earns the marks | Time |
|---|---|---|---|
| Beer's law numericals, deviations | Geo 2021 Q1(c) Geo 2024 Q1(c) Geo 2021 Q5(e) Geo 2022 Q4(a) Geo 2023 Q5(a) Geo 2024 Q5(b)(ii) | Law stated with units; each step; why A (not T) is linear; sign of deviation | 4–12 min |
| UV–vis instrument, determinations, Job | Geo 2021 Q4(b) Geo 2022 Q2(c) Geo 2024 Q5(b)(i) Geo 2025 Q1(c) Geo 2022 Q2(a) Geo 2023 Q3(a) | Block diagram; component table; reactions and wavelengths; Job plot drawn with extrapolated lines | 4–12 min |
| IR instrumentation | Geo 2021 Q2(c) Geo 2024 Q2(b) Geo 2023 Q1(c) Geo 2025 Q3(c) Geo 2022 Q1(c) | Region, sources (with temperatures), grating and why not glass, detectors in two classes | 4–12 min |
| AAS and flame emission | Geo 2021 Q1(g) Geo 2022 Q5(c) Geo 2022 Q3(c) Geo 2025 Q1(d) Geo 2023 Q1(d) Geo 2022 Q1(d) | HCL working; double-beam sketch; flame table; GF vs flame | 4–8 min |
| Interferences, background correction | Geo 2025 Q3(a) Geo 2025 Q3(b) Geo 2024 Q4(b) Geo 2023 Q4(c) Geo 2021 Q3(a) | Kind → example → remedy; equilibrium argument for ionisation; each correction method's principle | 4–12 min |
Time at 0.8 min per mark for Geo-Scientist.
13.8.11 Traps
✗ Plotting %T against c as a straight line. Only A is linear in c; T falls exponentially.
✗ Using glass cells below 350 nm or glass optics in IR. Use quartz in the UV and NaCl/KBr (with mirrors) in the IR.
✗ Calling La³⁺ a protective agent and EDTA a releasing agent. La³⁺ releases (takes the anion); EDTA protects (binds the analyte).
13.8.12 Where to read
Any one book is enough.
- Book 1: G.D. Christian, P.K. Dasgupta and K.A. Schug, Analytical Chemistry, 7th ed.: Ch. 16 “Spectrochemical methods” (p. 477): interaction of radiation with matter (16.1, p. 478), IR absorption (16.3, p. 489), quantitative calculations (16.7, p. 494), instrumentation (16.8, p. 504), types of instruments (16.9, p. 519), FTIR (16.11, p. 523), deviations from Beer's law (16.14, p. 527); Ch. 17 “Atomic spectrometric methods” (p. 548): flame emission (17.2, p. 553), atomic absorption (17.3, p. 556), internal standard and standard addition (17.5, p. 567).
- Book 2: P.W. Atkins and J. de Paula, Physical Chemistry, 9th ed.: experimental techniques (12.1, p. 446), Further information 12.1 “Spectrometers”: sources, dispersing elements, detectors (p. 479), Beer–Lambert law (13.1, p. 490). For the theory behind the instruments.
Solved PYQs, short notes and books: vkcscience.comTelegram: t.me/upsc_chemistry_vkc