Atomic Absorption Spectroscopy

How much lead is in a child’s blood? How much copper is in a sample of tap water? Is the zinc content of a food supplement what the label claims? For decades, the standard answer to questions like these has been atomic absorption spectroscopy (AAS): a technique that can measure a specific metal in a sample at concentrations of parts per million, or even parts per billion.

The principle

Every element’s atoms have their own set of electron energy levels. A free atom in its ground state can absorb light only at the specific wavelengths that match the gaps between those levels. These are the same wavelengths the element emits when heated, which is why each element gives a characteristic flame colour. See flame test colours.

In AAS:

1. The sample is turned into a cloud of free, ground-state atoms (atomisation).
2. Light of exactly the wavelength that the target element absorbs is shone through the cloud.
3. The atoms absorb some of it.
4. The more atoms of that element are present, the more light is absorbed.

The absorbance is proportional to concentration, just as in the Beer–Lambert law, so a calibration curve turns the absorbance into a concentration.

Why measure absorption, not emission?

In a flame at 2000–3000 °C, only a tiny fraction of atoms (often far less than 1%) are in excited states at any moment; almost all are in the ground state. Measuring absorption by the huge population of ground-state atoms is therefore more sensitive and less affected by small changes in flame temperature than measuring the light emitted by the few excited atoms.

The instrument

1. Light source: the hollow cathode lamp

The key to AAS is a light source that emits exactly the right wavelengths for the element being measured. A hollow cathode lamp has a cathode made of (or coated with) the target element. When a current flows, atoms of that element are knocked off the cathode and excited, and they emit the element’s own characteristic spectral lines.

Because the lamp’s lines are exactly those the sample atoms absorb, only the target element is measured. To analyse a different metal, you change the lamp (some lamps contain several elements).

2. Atomiser

The sample must be converted into free atoms:

• Flame AAS: the sample solution is sucked up (aspirated) and sprayed as a fine mist into a flame, typically air–acetylene (about 2300 °C) or, for elements that form stable oxides, nitrous oxide–acetylene (about 2800 °C). In the flame, the solvent evaporates, salts vaporise and molecules break apart into atoms. The light passes along a long, narrow flame (about 10 cm) to maximise absorption.
• Graphite furnace AAS (GFAAS): a few microlitres of sample are placed in a small graphite tube, which is heated electrically in stages: drying, charring (to remove the matrix) and then rapid atomisation at up to about 3000 °C. All the atoms are held briefly in the light path, making this method roughly 100–1000 times more sensitive than flame AAS.
• Hydride generation and cold vapour: special methods for elements such as arsenic, selenium and mercury, which are converted to volatile compounds (or, for mercury, free atoms at room temperature).

3. Monochromator

Selects the specific wavelength line of interest and removes other light (for example, from the flame itself).

4. Detector

A photomultiplier measures how much light passes through, and the instrument calculates absorbance.

Characteristic wavelengths

Element	Typical AAS wavelength (nm)
Sodium	589.0
Potassium	766.5
Calcium	422.7
Magnesium	285.2
Iron	248.3
Copper	324.8
Zinc	213.9
Lead	283.3 or 217.0
Cadmium	228.8
Mercury	253.7

Many are in the ultraviolet.

Measuring a sample

1. Prepare standards of the target metal at known concentrations, covering the expected range, plus a blank.
2. Prepare the sample: most samples must be dissolved. Solids like soil, food or tissue are digested with acids (such as nitric acid) to release the metals into solution.
3. Zero the instrument with the blank.
4. Measure the standards and plot absorbance against concentration.
5. Measure the sample and read its concentration from the calibration curve.
6. Account for dilution to find the concentration in the original sample.

Worked example

A calibration for copper gives absorbance = 0.0850 × concentration (mg/dm³). A tap water sample, used undiluted, gives an absorbance of 0.051. What is its copper concentration?

c = 0.051 ÷ 0.0850 = 0.60 mg/dm³

Many drinking water standards set a limit for copper of about 1–2 mg/dm³, so this sample would be within typical limits.

Interferences and how they’re handled

• Chemical interference: some elements form stable compounds in the flame that don’t break into atoms. Phosphate, for example, reduces calcium absorption by forming heat-stable calcium phosphate. Adding a releasing agent such as lanthanum chloride, which binds the phosphate preferentially, frees the calcium.
• Ionisation interference: easily ionised elements (such as potassium and sodium) can lose electrons in a hot flame, reducing the number of neutral atoms. Adding a large excess of another easily ionised element (an ionisation suppressor) reduces this effect.
• Background absorption: smoke, particles or molecules in the flame can absorb or scatter light. Instruments use background correction (for example, with a deuterium lamp) to subtract it.
• Matrix effects: differences in viscosity or salt content between samples and standards. These are reduced by matrix-matching standards or using the standard addition method.

Uses of AAS

• Drinking water and environmental monitoring: lead, copper, cadmium, chromium, arsenic, mercury.
• Clinical chemistry: blood lead in children; calcium, magnesium, zinc and copper in serum.
• Food and agriculture: metal content of foods, fertilisers and soils.
• Mining and metallurgy: metal content of ores and alloys.
• Pharmaceuticals: checking for toxic metal impurities.
• Forensics: poisoning cases involving arsenic, thallium or other metals.

AAS compared with other techniques

	Flame AAS	Graphite furnace AAS	ICP-MS
Elements per run	one at a time	one at a time	dozens at once
Typical detection limits	parts per million	parts per billion	parts per trillion
Sample volume	millilitres	microlitres	millilitres
Cost	lower	moderate	high
Speed per element	fast	slower	very fast for many elements

For many elements at once and at very low levels, laboratories increasingly use ICP-MS. AAS remains widely used because it’s robust, relatively inexpensive and very reliable for specific elements.

A brief history

AAS as a practical analytical method was developed in the 1950s by Alan Walsh at the CSIRO in Melbourne, Australia. His key insight was to use a light source emitting the element’s own sharp spectral lines (the hollow cathode lamp), which made the technique sensitive and selective. It quickly became one of the most widely used analytical techniques in the world.

Key takeaways

• AAS measures how much light of an element’s characteristic wavelength is absorbed by free ground-state atoms.
• A hollow cathode lamp made of the target element provides exactly the right wavelengths, making the method highly selective.
• Samples are atomised in a flame (ppm levels) or a graphite furnace (ppb levels).
• Concentrations are found using a calibration curve; interferences are handled with releasing agents, background correction and matrix matching.
• AAS is used for metals in water, blood, food, soil and ores.