An introduction to the Atomic Absorption Spectrophotometer

The Atomic Absorption Spectrophotometer (AAS) is a high quality analytical instrument that provides low cost trace elemental analysis. With low service and running costs, it offers an extremely economical alternative to ICP systems for the detection of elemental concentrations at the parts per million (ppm) and parts per billion (ppb) level. The AAS is used in in research, testing and quality control applications in many different industries including environmental, food and medical. A flame or furnace arrangement is used to atomise a sample and the resulting atoms are irradiated with light. An optical detection system is used to measure the amount of light absorbed by these atoms in order to allow the concentration of the element in the sample to be calculated. Different elements absorb light at specific wavelengths.

Atomisation methods in AAS

In the flame atomisation method, small droplets of sample are introduced into a flame, usually generated using an air/acetylene mixture. The sample must be in solution. Typical elemental detection limits for the flame system are at the sub ppm level. In the furnace technique, the sample is placed inside a graphite tube which is heated electrically. This method offers sophisticated temperature control, allowing the sample to be dried, ashed and vaporised at different temperatures. This method can accommodate small amounts of sample and can also be used for solid samples without pre-treatment. It also offers lower detection limits than the flame method, with detection possible at the ppb level. Instruments are available as flame-only atomic absorption spectrophotometers, furnace-only atomic absorption spectrophotometers and tandem flame-furnace models, where the heating systems can be interchanged.

Atomic Absorption Spectrophotometer optics

The traditional optical configuration consists of a monochromatic light source and collimator to generate a beam which then passes through the atomised sample. The atoms of the element then absorb some of the light, and the beam then passes through a monochromator (for wavelength selection) and on to a detector (usually a photomultiplier). Since AAS techniques can detect elements to sub ppb levels, only a tiny proportion of light is absorbed by the sample. The absorption signal is measured by subtracting the transmitted intensity signal from the original light intensity signal. The result is a very small difference between two large signals. Anything that compromises this small difference signal, such background interference, must be removed to avoid errors in measurement. The traditional light sources used in AAS are hollow cathode lamps, chosen to emit light at a wavelength that is characteristic to the element under study. For multi-element analyses, different lamps can be mounted on a rotating turret so that the individual elements can be analysed sequentially.

Background correction

As noted above, AAS analysers can be hindered by the presence of background interferences, or non-specific absorptions. These can arise from absorption by free molecules from matrix components which have not completely dissociated and scattering by solid or liquid particles of matrix substances which have not completely evaporated. Correction for these effects is a critical requirement. Three different techniques are commonly used in the atomic absorption spectrophotometer to correct for background interferences. These are: Deuterium background correction, Zeeman background correction and the Smith-Hieftje (or self-reversal) background correction. These will be discussed in detail in future blog articles. If you have questions in the meantime, why not contact a member of the team?