Atomic Absorption Spectroscopy (AAS) Guide for Students

30 September 2026Chemistry Try the interactive 3D model →

Atomic Absorption Spectroscopy (AAS) measures how much of a specific metal is in a sample — for example, how many micrograms of lead are in a litre of drinking water. The sample is converted into free atoms, light is shone through them, and the atoms absorb only their own characteristic wavelengths; the amount absorbed reveals the concentration.

AAS is valued for its selectivity and sensitivity, detecting metals down to parts per billion (ppb). Water-testing, food-safety and mining labs use it daily, and its principle — the Beer-Lambert law — is a classic exam favourite.

Key Concepts

Hollow cathode lamp: the light source, whose cathode is made of the SAME element being measured — a lead lamp for lead. It emits exactly the wavelengths that element's atoms absorb.
Atomization: converting the sample into free ground-state atoms, the form that absorbs light. Flame AAS uses a hot air–acetylene flame (~2300 °C); graphite furnace AAS uses an electrically heated graphite tube (~2500–2700 °C) on a tiny sample drop, reaching far higher sensitivity.
Beer-Lambert law: absorbance is directly proportional to concentration, which makes AAS quantitative.
Calibration curve: standards of known concentration are measured, absorbance is plotted against concentration, and the unknown's concentration is read from the graph.
Detection limits: flame AAS works in the ppm (mg/L) range; graphite furnace AAS reaches ppb (μg/L).
Interferences: chemical (stable compounds resisting atomization, e.g. phosphates tying up calcium — fixed with a releasing agent like lanthanum), ionization (easily ionized elements form ions instead of atoms), spectral (overlapping lines from other elements).

Formulas

Beer-Lambert law: A = εbc. A = absorbance (unitless), ε = molar absorptivity (L mol−1 cm−1), b = path length (cm), c = concentration (mol/L). In one setup ε and b are fixed, so A is directly proportional to c.
Calibration line: A = m × c + intercept; unknown concentration c = (A unknown − intercept) / m. Valid only in the linear range — high absorbances curve off and samples must be diluted.
Transmittance: A = −log10(I/I0), where I0 is incident and I transmitted light intensity.
Units: mg/L (ppm) or μg/L (ppb); 1 mg/L = 1000 μg/L.

Worked Example

Lead standards at 1.0, 2.0 and 4.0 mg/L give absorbances 0.12, 0.24 and 0.48. A water sample gives 0.30. Find the lead concentration.

Step 1: Slope m = 0.12 absorbance per (mg/L); the line passes through the origin.
Step 2: c = 0.30 / 0.12 = 2.5 mg/L.
Step 3: 2.5 mg/L = 2500 ppb — far above safe drinking-water lead limits (low ppb range), so this water is unsafe.

This is the whole AAS workflow in miniature: standards make the line, the unknown's absorbance lands on it, and simple division gives the concentration.

Common Misconceptions

"One lamp measures all metals." Wrong. The lamp must match the element — a copper lamp to measure copper.
"AAS measures everything in the sample." Wrong. It sees only elements that atomize and absorb at the lamp's wavelength; everything else is invisible to it.
"Hotter is always better." Wrong. An over-hot flame ionizes easily ionized elements like sodium, removing the ground-state atoms AAS needs and lowering the signal.

Real-World Applications

Drinking-water labs test for lead, arsenic, cadmium and mercury at ppb levels.
Food labs check infant formula and spices for toxic metals.
Mining labs assay ores for gold, silver and copper content.
Clinical labs measure blood lead levels in children and monitor metal poisoning.

AAS turns a sample into free atoms, shines element-specific light from a matching hollow cathode lamp through them, and converts absorbed light into concentration via the Beer-Lambert law. Flame AAS handles ppm; graphite furnace reaches ppb — precision that keeps water, food and medicine safe.