Chemistry • Year 12 • Module 8 • Lesson 4
Spectroscopic Analysis: UV-Vis & AAS
Lock in the core vocabulary, Beer-Lambert law, the structure of a calibration curve, and the key features that make AAS suited to trace metal analysis.
1. Term–definition match
The ten definitions below are shuffled. Write the matching term from this list in the right-hand column: UV-Vis spectroscopy, absorbance, Beer-Lambert law, calibration curve, molar absorptivity, path length, atomisation, hollow cathode lamp, AAS, matrix effects. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A technique that measures how much ultraviolet or visible light a solution absorbs to determine the concentration of a substance. | |
| 1.2 | The quantity A = log10(I0/I); a dimensionless measure of how much light a sample removes at a given wavelength. | |
| 1.3 | The relationship A = εcl, which states that absorbance is proportional to concentration when path length and molar absorptivity are constant. | |
| 1.4 | A graph of absorbance versus concentration constructed from standards of known concentration; used to determine the concentration of an unknown sample. | |
| 1.5 | The symbol ε; a constant (L mol-1 cm-1) that reflects how strongly a substance absorbs at a chosen wavelength. | |
| 1.6 | The distance (cm) the light beam travels through the sample, represented by l in Beer-Lambert law. | |
| 1.7 | The process of converting dissolved ionic species into free ground-state atoms using a flame or graphite furnace; the critical first step in AAS. | |
| 1.8 | The light source in AAS; it emits light at the characteristic wavelengths of a single target element, ensuring element-specific detection. | |
| 1.9 | An analytical technique that determines the concentration of a specific metal by measuring how much of the element’s characteristic light is absorbed by free ground-state atoms. | |
| 1.10 | Interference from other substances in a sample that changes the measured absorbance and can lead to systematic errors; minimised by matrix-matching standards. |
2. True or false — with correction
Circle T or F. If the statement is false, write the corrected version on the line provided. 10 marks (1 for T/F, 1 for the correction where needed)
2.1 In Beer-Lambert law the full expression is A = εc, because path length is always 1.00 cm. T / F
2.2 A calibration curve is built by measuring the absorbance of standard solutions with known concentrations. T / F
2.3 AAS detects metal ions directly in solution without changing the physical state of the sample. T / F
2.4 The hollow cathode lamp in AAS emits light at wavelengths characteristic of the element being analysed. T / F
2.5 UV-Vis spectroscopy and AAS are equally well-suited for detecting trace non-metallic elements such as fluorine. T / F
3. Function recall
Answer each in 1–2 sentences using precise lesson terms. 10 marks (2 each)
3.1 What is the role of the calibration curve in UV-Vis spectroscopy?
3.2 What is the role of the hollow cathode lamp in AAS?
3.3 What is the role of atomisation in AAS?
3.4 What is the role of the molar absorptivity (ε) in Beer-Lambert law?
3.5 Why is AAS described as element-specific but UV-Vis is not?
4. Fill in the blanks
Complete the paragraph using the word bank below. Each term is used once. 8 marks
In UV-Vis spectroscopy, the law states that absorbance is directly proportional to . An analyst prepares a set of standards and plots their absorbances against concentration to create a , from which the concentration of an unknown sample can be read.
In AAS, a provides light at the characteristic of the target element. The sample is converted to free by in a or graphite furnace. These atoms then absorb the lamp’s light, and the reduction in transmitted intensity is used to determine concentration.
5. Build a concept map
Draw labelled arrows connecting the six terms below to show how they relate. Each arrow must carry a linking phrase. Aim for at least 6 labelled arrows. 6 marks
Supplied terms: Beer-Lambert law · absorbance · calibration curve · unknown concentration · standard solutions · molar absorptivity.
Q1 — Term–definition matches
1.1 UV-Vis spectroscopy • 1.2 absorbance • 1.3 Beer-Lambert law • 1.4 calibration curve • 1.5 molar absorptivity • 1.6 path length • 1.7 atomisation • 1.8 hollow cathode lamp • 1.9 AAS • 1.10 matrix effects.
Q2 — True / false with corrections
2.1 False. The full Beer-Lambert expression is A = εcl; path length l must be included. While a standard 1.00 cm cuvette is common, the law still requires l to be stated.
2.2 True.
2.3 False. AAS measures ground-state atoms produced after the sample has been atomised in a flame or furnace, not ions directly in solution.
2.4 True.
2.5 False. Neither technique is well-suited to direct analysis of non-metallic elements such as fluorine; AAS in particular is limited to metals and metalloids that can be atomised and have characteristic absorption wavelengths in the accessible spectral range.
Q3 — Function recall
3.1 A calibration curve relates instrument absorbance readings to known concentrations of the analyte; it allows the concentration of an unknown sample to be determined by reading off the graph at the measured absorbance value.
3.2 The hollow cathode lamp emits light at the characteristic wavelengths specific to the target element, so only atoms of that element in the flame absorb it, giving AAS its element-specific selectivity.
3.3 Atomisation converts dissolved species in the sample into free ground-state atoms in the flame or furnace; only these free atoms absorb the characteristic lamp light, so atomisation is essential for the analytical measurement to occur.
3.4 Molar absorptivity (ε) is a proportionality constant that describes how strongly a given substance absorbs light at the chosen wavelength; a larger ε means the substance absorbs more strongly and the method is more sensitive to low concentrations.
3.5 AAS is element-specific because it uses a hollow cathode lamp matched to a single target element, which emits only that element’s characteristic wavelengths; UV-Vis uses a broad-spectrum light source and selects a wavelength by a monochromator, so it measures the total absorbance of all species in solution at that wavelength, not a single element.
Q4 — Cloze answers (in order)
Beer-Lambert · concentration · calibration curve · hollow cathode lamp · wavelength · ground-state atoms · atomisation · flame
Q5 — Sample concept map
Acceptable arrows include: Beer-Lambert law —relates absorbance to→ absorbance; Beer-Lambert law —includes as a factor→ molar absorptivity; standard solutions —provide data to build→ calibration curve; calibration curve —is used to determine→ unknown concentration; absorbance —is read from instrument to find→ unknown concentration; molar absorptivity —determines the slope of→ calibration curve. Award 1 mark per correctly labelled, causally valid arrow up to 6.