How can the concentration of a metal ion be measured from the light it absorbs or emits?
Explain the principles of atomic absorption spectroscopy (AAS) and atomic emission spectroscopy, and their use in determining trace metal concentrations.
How AAS and atomic emission spectroscopy use the absorption and emission of specific wavelengths by gaseous atoms to identify and quantify trace metals using a calibration curve.
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What this dot point is asking
You must explain why each element interacts with specific wavelengths, describe how AAS works, and use a calibration curve to find an unknown concentration of a trace metal.
Why atoms absorb and emit specific wavelengths
Electrons in an atom occupy discrete energy levels. An electron can only jump between levels by absorbing or emitting a photon whose energy exactly matches the gap:
Because every element has a unique set of energy levels, it absorbs and emits a unique set of wavelengths - an atomic "fingerprint". This is the basis of all atomic spectroscopy.
How AAS works
- The sample solution is aspirated into a flame (or furnace), which evaporates the solvent and atomises the metal into free gaseous atoms.
- A hollow-cathode lamp made of the element being measured emits light at exactly that element's wavelengths.
- The light passes through the atomised sample. Ground-state atoms absorb some of it.
- A detector measures how much light reaches it. The more atoms present, the more light absorbed.
The calibration curve
AAS is a comparative technique. You prepare a series of standard solutions of known concentration, measure the absorbance of each, and plot absorbance against concentration. The graph is a straight line through the origin over the working range. You then measure the unknown's absorbance and read its concentration off the line.
Atomic emission spectroscopy
In flame emission (and flame tests), the flame excites atoms, and the wavelengths they emit on relaxing identify the element - sodium gives yellow, potassium lilac, copper blue-green. Measuring the intensity of an emission line allows quantification, again using a calibration curve.
Strengths and limits
- Specific: the hollow-cathode lamp emits only the target element's wavelengths, so other metals do not interfere.
- Sensitive: detects trace amounts.
- Comparative: needs standards run under identical conditions; results outside the linear range are unreliable.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2023 SACE Stage 23 marksAtomic absorption spectroscopy (AAS) can be used to measure the concentration of Ni2+ ions in wastewater. Explain why some wavelengths of radiation emitted and absorbed by Ni2+ are unique to nickel.Show worked answer →
For 3 marks, link the wavelengths to electron energy levels.
Electrons in an atom occupy discrete (quantised) energy levels.
An electron absorbs a photon and is promoted to a higher level only if the photon energy exactly matches the gap between two levels (E = hf), so only specific wavelengths are absorbed; the same wavelengths are emitted when the electron falls back down.
Each element has a unique set of energy levels (a unique electron arrangement), so the spacing between levels, and therefore the absorbed and emitted wavelengths, are unique to that element. This makes the pattern a fingerprint for nickel.
2023 SACE Stage 22 marksA simplified diagram of the components of an AAS apparatus is shown. Explain why the light that has passed through the flame must pass through a monochromator.Show worked answer →
The monochromator selects a single, specific wavelength of light (the characteristic wavelength for the element being measured, here nickel) and excludes all other wavelengths before the light reaches the detector.
This ensures the detector measures only the absorbance at the wavelength absorbed by the analyte atoms.
It removes interference from other wavelengths emitted by the flame or other species, so the measured absorbance accurately reflects the concentration of the target element. One mark each.
2024 SACE Stage 22 marksA lamp emitting light with a wavelength of 417 nm was used in this measurement. Explain why specific wavelengths of light are absorbed by elements atomised by a flame.Show worked answer →
Gaseous atoms have electrons in quantised energy levels. An electron can absorb a photon and jump to a higher energy level only when the photon energy exactly equals the difference between two energy levels.
Because the energy levels are fixed for a given element, only photons of specific energies, and therefore specific wavelengths, can be absorbed.
Light of other wavelengths does not match any energy gap and passes through unabsorbed. One mark for quantised levels, one for the photon-energy match.