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CHEM3120 · Environmental and Analytical Chemistry

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Chapter 2 of 10 · CHEM3120

Analytical Techniques: GC, AAS & Flame Photometry

Lectures 3, 6 and 7 cover the workhorse instruments of CHEM3120: gas chromatography (separation on a stationary phase, with TCD, FID and electron-capture detectors), atomic absorption spectroscopy (hollow-cathode lamp, flame atomisation, Beer-Lambert quantitation) and flame photometry (emission from thermally excited atoms). This is the heaviest source of Part B multiple-choice questions, and the quantitative core — calibration curves, Beer-Lambert A = εcl and standard addition — appears as Part A short-answer.

In this chapter

What this chapter covers

01Gas chromatography: stationary phase (non-volatile coating) vs mobile phase (inert carrier gas); selective retardation and separation
02GC detectors: TCD (thermal-conductivity, filament resistance), FID (CH· + O → CHO+ + e-), electron-capture; GC-MS and single-ion monitoring
03FID limitation: responds to C-H organics only, not heteroatoms (O, S, N, P, Cl) and not CO
04Atomic absorption spectroscopy: hollow-cathode lamp, flame atomisation, absorption by ground-state atoms, chopper background correction
05Flame photometry (emission): thermal excitation then emission of a characteristic line; filters not a monochromator; Na, K, Li, Ca
06Beer-Lambert law A = -log(I/I0) = εcl for quantitation
07Calibration: external standard curve vs standard addition (for matrix effects)
08Detection limit and sensitivity of a technique

Worked example · free

Potassium in water by flame photometry — the method of standard additions

Q [4 marks]. Two identical 0.500 mL aliquots of a water sample are each made up to a fixed volume with a flame-photometry buffer. The second aliquot is spiked with 10.0 µL of 0.0500 mol/L KCl. The net emission signals are 40.0 (unspiked) and 60.0 (spiked). Assuming signal is proportional to the moles of K+ present, find the concentration of K+ in the original sample. (4 marks)

+1Moles of K+ added by the spike: n(spike) = 10.0 µL × 0.0500 mol/L = 10.0×10^-6 L × 0.0500 mol/L = 5.0×10^-7 mol.
+1Let x = moles of K+ from the sample in an aliquot. Because signal ∝ moles at fixed final volume: (unspiked signal)/(spiked signal) = x/(x + n(spike)), i.e. 40.0/60.0 = x/(x + 5.0×10^-7).
+1Solve: 0.6667(x + 5.0×10^-7) = x → 0.6667x + 3.333×10^-7 = x → 0.3333x = 3.333×10^-7 → x = 1.0×10^-6 mol of K+ in the 0.500 mL aliquot.
+1Convert to a concentration in the original sample: [K+] = 1.0×10^-6 mol ÷ 0.500×10^-3 L = 2.0×10^-3 mol/L.

[K+] = 2.0×10^-3 mol/L. Standard addition works here because the sample matrix (which can suppress or enhance emission) is present in both readings, so it cancels when you take the signal ratio.

Sia tip — Use standard addition when the matrix might change the signal — spike the real sample rather than a clean standard. Keep the final volume the same for both readings so signal stays proportional to moles; then it is one ratio equation. Ask Sia to re-derive the ratio step if it slips.

Glossary

Key terms

Stationary phase (GC): The non-volatile liquid coating on the column's solid support; analytes that partition into it more strongly are retarded more and elute later.
Mobile phase (GC): The inert carrier gas that sweeps volatilised analyte through the column; it does not interact chemically, it just transports.
Flame ionization detector (FID): Burns the eluent in H2/air so C-H fragments form CH· that react as CH· + O → CHO+ + e-; the ions carry a current proportional to organic carbon. Blind to heteroatoms and CO.
Hollow-cathode lamp: The light source in AAS: inert-gas ions sputter analyte atoms from a cathode made of that element, which then emit the element's own sharp line — exactly the wavelength its ground-state atoms in the flame will absorb.
Flame photometry: Atomic EMISSION: the sample is atomised in a flame, a Boltzmann fraction of atoms is thermally excited, and the characteristic light they emit on relaxing is measured. No external lamp; wavelength selected by filters.
Standard addition: A calibration method for matrix-affected samples: known amounts of analyte are spiked into the sample itself, and the signal increase is used to back-calculate the original amount.

FAQ

Analytical Techniques: GC, AAS & Flame Photometry FAQ

What is the difference between AAS and flame photometry?

Both atomise the sample in a flame, but AAS measures ABSORPTION by ground-state atoms of light from a hollow-cathode lamp (it needs an external light source), while flame photometry measures the EMISSION from the small fraction of atoms that the flame thermally excites (no lamp). Emission suits easily excited elements like Na, K and Li; AAS is more general and uses Beer-Lambert, A = εcl, for quantitation.

Why does an FID not respond to every compound?

The FID counts ions formed when C-H fragments burn (CH· + O → CHO+ + e-), so it only responds to organics containing carbon-hydrogen bonds. It is essentially blind to permanent gases, water, and heteroatom species with no C-H, and it will not detect carbon monoxide. That is why a detector like TCD (which responds to any thermal-conductivity difference) or electron capture (sensitive to halogens) is chosen when the FID would miss the analyte.

When should I use standard addition instead of a normal calibration curve?

Use standard addition when the sample matrix itself changes the signal — for example flame emission that is enhanced or suppressed by other ions in the water. Because you spike known amounts of analyte into the real sample, the matrix is present in every reading and cancels out when you take the ratio. A plain external-standard curve assumes the standards and sample respond identically, which fails under strong matrix effects.

Can AI help me revise the analytical techniques in CHEM3120?

Yes. Sia can walk you through a Beer-Lambert or standard-addition calculation line by line, contrast GC detectors or AAS versus flame photometry for a given analyte, and quiz you on the MCQ-style distinctions this block is famous for. It explains the method and checks your reasoning; it does not do your graded quiz, assignment or exam, and University of Sydney academic-integrity rules apply.

Study strategy

Exam move

This block dominates Part B multiple-choice, so drill the qualitative distinctions until they are reflexes: stationary vs mobile phase, TCD vs FID vs electron capture, AAS (absorption, needs a lamp) vs flame photometry (emission, no lamp), and why the chopper corrects flame background. Then secure the two quantitative routines — Beer-Lambert A = εcl and the method of standard additions — by working each as a single clean chain of substitutions with units carried through. Because the weekly quizzes sample this material early, keep it warm rather than leaving it to STUVAC. In the closed-book final a formula sheet is provided, so practise applying the relationships fast rather than memorising them. Confirm the exam date, room and permitted materials on Canvas.

Working through Analytical Techniques: GC, AAS & Flame Photometry in CHEM3120? Sia is AskSia’s AI Chemistry tutor — ask any CHEM3120 Analytical Techniques: GC, AAS & Flame Photometry question and get a clear, step-by-step explanation grounded in how CHEM3120 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.

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