After concerns about ageing pipes and trace metals in urban water supplies, chemists need methods that can do more than say “something is present”. They need methods that can identify a substance by its light interactions and quantify it at very low concentration, quickly enough to protect public health.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
A student says: “AAS works by detecting metal ions dissolved in water, and UV-Vis works only by looking at colour intensity with no real chemistry behind it.”
📚 Core Content
UV-Vis spectroscopy is built on a simple idea with powerful consequences: if a substance absorbs light at a particular wavelength, the amount of light absorbed can be linked to how much of that substance is present.
In UV-Vis spectroscopy, light is passed through a solution and the instrument measures how much of that light is absorbed. Coloured species absorb visible wavelengths selectively, while some substances absorb in the ultraviolet range.
The more absorbing particles present in solution, the more light is absorbed, as long as the system stays in a range where the relationship remains linear. That is the basis of quantitative UV-Vis analysis.
Wrong: The mole is a measure of mass.
Right: The mole is a measure of amount of substance; one mole contains Avogadro's number of particles.
l from Beer-Lambert law. The full relationship is A = εcl, not just A = εc.The instrument isolates a chosen wavelength, sends it through the sample, and measures the transmitted intensity. The absorbance value then links to concentration through Beer-Lambert law.
A UV-Vis result becomes defensible when the unknown is compared against standards of known concentration, not when a single absorbance number is viewed in isolation.
A calibration curve is made by preparing a set of standard solutions with known concentrations, measuring their absorbances at the same wavelength, and plotting absorbance against concentration. If Beer-Lambert behaviour is followed, the graph should be approximately linear.
AAS is powerful because each element absorbs light at its own characteristic wavelengths. That makes it highly specific for elemental analysis.
In AAS, the sample is introduced into a flame or graphite furnace where it is atomised. This step converts species in the sample into free ground-state atoms. Light of a characteristic wavelength for the element of interest is then passed through the atomised sample. If those atoms are present, they absorb that light.
The reduction in transmitted light is used to determine concentration, usually by comparison against calibration standards.
AAS uses a lamp matched to the element of interest. The sample is first atomised, then those free ground-state atoms absorb some of the characteristic light, allowing concentration to be determined from the reduced signal.
When public-health questions depend on very low concentrations, “can we see a colour change?” is no longer enough. Sensitivity and specificity become decisive.
AAS is widely used in environmental monitoring because it is:
This makes AAS suitable for analysing lead, copper, cadmium and other heavy metals in water, soil and food samples.
No analytical technique is universally best. A good chemist knows not just what a method can do, but where its limitations start to matter.
| Limitation | Why it matters |
|---|---|
| One element at a time | AAS usually measures one target element per run, so multi-element analysis can be slower |
| Matrix effects | Other substances in the sample can affect atomisation or absorption and alter the result |
| Instrument cost and setup | More specialised and expensive than simple wet-chemistry tests |
📊 Data Interpretation
A chemist prepares lead standards for AAS analysis and records the following absorbance data at the characteristic wavelength for Pb:
| Pb concentration / mg L-1 | Absorbance |
|---|---|
| 0.00 | 0.000 |
| 0.50 | 0.082 |
| 1.00 | 0.161 |
| 1.50 | 0.242 |
| 2.00 | 0.323 |
An unknown tap-water sample gives an absorbance of 0.201. This lies between the 1.00 mg L-1 and 1.50 mg L-1 standards, so the lead concentration is approximately 1.25 mg L-1 if the calibration remains linear.
✏️ Worked Example
Given: A coloured solution has absorbance 0.480 at a chosen wavelength. The cuvette path length is 1.00 cm and the molar absorptivity is 120 L mol-1 cm-1.
Find: Concentration of the coloured species.
Method: Rearrange Beer-Lambert law.
A = εcl c = A / (εl) c = 0.480 / (120 × 1.00) c = 0.00400 mol L-1Answer: The concentration is 4.00 × 10-3 mol L-1.
A = εcl🧠 Activities
1 Measuring the concentration of a blue copper(II) solution in a school laboratory.
2 Detecting trace lead in drinking water at very low concentration.
3 Explaining why AAS is more element-specific than UV-Vis for metal monitoring.
1 Explain why the calibration data supports a linear relationship between absorbance and concentration.
2 Estimate the concentration of the unknown sample with absorbance 0.201 and explain how you obtained it.
3 Why would an unknown absorbance far above the highest standard be less reliable to interpret directly from this calibration set?
1. What does Beer-Lambert law state?
What is NOT does Beer-Lambert law state?
2. Which statement about AAS is correct?
3. Which technique is generally more suitable for measuring trace lead in drinking water?
4. What is the main purpose of a calibration curve?
What is NOT the main purpose of a calibration curve?
5. Which is a real limitation of AAS?
1. Explain how a chemist would use UV-Vis spectroscopy and a calibration curve to determine the concentration of a coloured species in solution. 4 marks
2. Explain why the statement “AAS detects ions in solution” is incorrect. In your answer, describe what actually happens during the analytical process. 4 marks
3. Evaluate the suitability of AAS compared with UV-Vis spectroscopy for monitoring lead contamination in Sydney drinking water. In your answer, refer to sensitivity, specificity, and at least one limitation of AAS. 5 marks
Return to the misconception challenge from the start and rewrite it as a correct analytical explanation.
Use c = A / (εl).
c = 0.360 / (90.0 × 1.00) = 0.00400 mol L-1.
1. UV-Vis is suitable for a blue copper(II) solution because the species is coloured and absorbance can be related to concentration.
2. AAS is more suitable for trace lead in drinking water because it is highly sensitive and element-specific for metals at low concentrations.
3. AAS is more element-specific because each element absorbs characteristic wavelengths after atomisation, allowing targeted detection.
1. The data support linearity because equal increases in concentration produce approximately equal increases in absorbance.
2. The unknown concentration is about 1.25 mg L-1 because 0.201 lies about halfway between 0.161 and 0.242.
3. A value above the highest standard would require extrapolation beyond the calibrated range, which is less reliable because the relationship may no longer remain linear.
1. D — Beer-Lambert law is A = εcl.
2. B — AAS measures absorption by ground-state atoms after atomisation.
3. A — AAS is more suitable for trace lead in drinking water.
4. C — calibration curves relate known concentrations to instrument response so unknowns can be determined.
5. D — AAS usually measures one element at a time and can be affected by matrix effects.
Q1 (4 marks): A chemist first prepares standard solutions of known concentration and measures their absorbance at a selected wavelength using a UV-Vis spectrophotometer. These values are plotted on a calibration curve of absorbance against concentration. The absorbance of the unknown solution is then measured under the same conditions. The concentration of the unknown is found by reading across from its absorbance to the calibration line and then down to the concentration axis, or by using the line equation if given.
Q2 (4 marks): The statement is incorrect because AAS does not directly measure ions as they exist in solution. During AAS, the sample is atomised in a flame or furnace, producing free ground-state atoms. Light of a characteristic wavelength for the target element passes through this atomised sample. If those atoms are present, they absorb that light, and the instrument uses the decrease in transmitted light to determine concentration.
Q3 (5 marks): AAS is more suitable than UV-Vis for monitoring lead contamination in drinking water because it is both highly sensitive and highly specific for the element being measured. Trace lead may be present at concentrations too low for simple UV-Vis measurement, especially if the sample is not strongly coloured. AAS overcomes this by using characteristic absorption wavelengths for lead after atomisation. However, AAS still has limitations, including matrix effects and the fact that it generally measures one element at a time. Overall, AAS is the stronger choice for lead monitoring because the public-health task depends on detecting low concentrations of a specific metal reliably.
Tick when you've finished the activities and checked your answers.