Atomic absorption and emission spectroscopy (SACE Stage 2 Chemistry)

Syllabus dot point

“Explain the principles of atomic absorption spectroscopy (AAS) and atomic emission spectroscopy, and their use in determining trace metal concentrations.”

9 min readUpdated 2 Jun 2026

Quick answer

Atomic absorption spectroscopy (AAS) measures trace metal concentrations by atomising a sample in a flame and shining light from a lamp made of the target element. Ground-state atoms absorb light at wavelengths matched to their own electron energy gaps; the amount absorbed is proportional to concentration. Atomic emission works in reverse: excited atoms emit characteristic wavelengths as electrons fall back. Both are element-specific because every element has a unique set of energy levels.

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What this dot point is asking
Lead worked calculation
How AAS works
How atomic emission works
The calibration-curve method
Strengths and limits
Why it matters for monitoring
Exam-style questions

What this dot point is asking

SACE expects you to explain the electron transitions behind both techniques, why each is element-specific, and how a calibration curve converts an absorbance reading into a concentration.

Lead worked calculation

Concentration from a calibration curve

Standard copper solutions give absorbances $A = 0.090$ at $1.0\ \text{ppm}$ , $0.180$ at $2.0\ \text{ppm}$ and $0.360$ at $4.0\ \text{ppm}$ . A diluted sample reads $A = 0.225$ . The sample was diluted by a factor of $10$ before measurement. Find the copper concentration in the original sample.

Establish the calibration line

The points lie on a straight line through the origin, so $A = kc$ . The gradient is $k = \dfrac{0.090}{1.0} = 0.090\ \text{absorbance per ppm}$ .

Read off the diluted sample

c_\text{diluted} = \frac{A}{k} = \frac{0.225}{0.090} = 2.5\ \text{ppm}

Correct for dilution

c_\text{original} = 2.5 \times 10 = 25\ \text{ppm}

The original sample contains $25\ \text{ppm}$ of copper. Always reverse any dilution at the end.

How AAS works

Atomic absorption

A sample solution is aspirated into a flame, which evaporates the solvent and breaks compounds into free, neutral, ground-state atoms. A hollow-cathode lamp made of the target metal emits light at exactly the wavelengths that element absorbs. As this light passes through the flame, ground-state atoms absorb photons and electrons jump to higher energy levels: $\text{atom} + \text{photon} \rightarrow \text{atom}^*$ . A detector measures how much light is absorbed; more atoms means more absorption.

The absorbed wavelengths are set by the energy gap $\Delta E$ between levels, related to wavelength by $E = \dfrac{hc}{\lambda}$ . Because each element has unique energy gaps, the absorbed wavelengths are a fingerprint, which is why a lamp made of the specific element is required and why other metals in the sample do not interfere at that wavelength.

How atomic emission works

In atomic emission, the flame or plasma supplies enough energy to excite electrons to higher levels. As they fall back, they emit photons of characteristic wavelengths: $\text{atom}^* \rightarrow \text{atom} + \text{photon}$ . The emitted colours identify the element (the basis of the flame test: sodium yellow, potassium lilac, copper green-blue), and the emission intensity is proportional to concentration. AAS measures light removed; emission measures light given out. AAS is generally better for trace levels because measuring a small decrease against a bright source is more sensitive than detecting faint emission.

The calibration-curve method

Beer-Lambert proportionality

A = \varepsilon\, l\, c \quad\Rightarrow\quad A \propto c

At fixed path length

l

and for a given element (fixed

\varepsilon

), absorbance is directly proportional to concentration. In practice you measure several standards of known concentration, plot

A

against

c

, and read the unknown's concentration from the line of best fit.

The standard procedure: prepare a series of standards spanning the expected range, measure each, plot the calibration curve, then read the unknown off the linear portion. Reasons this works: the line passes through the origin (zero concentration gives zero absorbance), and proportionality holds only within the linear range, so very concentrated samples must be diluted back into range.

Strengths and limits

AAS detects metals at parts-per-million and parts-per-billion levels, far below what titration can reach, making it ideal for trace contaminants such as lead, cadmium and mercury in water. Its main limitations are that it measures one element at a time (one lamp per element), needs matrix-matched standards, and loses proportionality above the linear range.

Forgetting the dilution factor and the linear range

A frequent SACE error is reading the concentration from the calibration line but forgetting that the sample was diluted before measurement; multiply back by the dilution factor. Another is using a sample whose absorbance lies above the linear range of the standards, where $A$ is no longer proportional to $c$ ; such samples must be diluted and remeasured. Also keep AAS (absorption, uses a lamp) distinct from emission (atoms emit their own light).

Why it matters for monitoring

AAS and atomic emission are the workhorses of trace-metal monitoring, quantifying toxic metals in drinking water, soil and biological samples at concentrations far too low for titration. Their element specificity lets analysts target a single contaminant in a complex environmental sample.

Exam-style questions

Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

SACE 2022-style4 marks

Standard lead solutions gave the following AAS absorbances:

2.0\ \text{ppm} \rightarrow 0.110

;

4.0\ \text{ppm} \rightarrow 0.220

;

6.0\ \text{ppm} \rightarrow 0.330

;

8.0\ \text{ppm} \rightarrow 0.440

. A water sample gave an absorbance of

0.275

. Using the calibration data, determine the concentration of lead in the sample and state one assumption you make.

Show worked answer →

Step 1: the standards show a straight line through the origin; the gradient is $\dfrac{0.110}{2.0} = 0.0550\ \text{absorbance per ppm}$ . (2 marks)

Step 2: rearranging $A = (\text{gradient}) \times c$ , $c = \dfrac{A}{\text{gradient}} = \dfrac{0.275}{0.0550} = 5.0\ \text{ppm}$ . (1 mark)

Assumption: the sample lies within the linear (calibration) range and the matrix of the sample matches the standards so that absorbance is directly proportional to concentration. (1 mark)

SACE 2020-style3 marks

Explain why atomic absorption spectroscopy is specific for a single element, and why a different lamp is required to measure a different metal. Refer to electron transitions in your answer.

Show worked answer →

Each element has a unique set of electron energy levels, so the energy gaps between levels, and therefore the wavelengths of light its atoms absorb, are characteristic of that element alone. (1 mark)

In AAS the source is a hollow-cathode lamp made of the element being measured; it emits light at exactly the wavelengths that element's atoms can absorb, when an electron is promoted from the ground state to a higher level: $\text{atom} + \text{photon} \rightarrow \text{atom}^*$ . (1 mark)

Because another metal absorbs at different wavelengths matched to its own energy gaps, its atoms would not absorb the first lamp's light, so a lamp made of the new element is needed. This element-specific source is what makes AAS selective. (1 mark)

Practise this

Sources & how we know this

Stage 2 Chemistry subject outline (2025 onwards) — SACE Board
Stage 2 Chemistry: external assessment — SACE Board