Technology of Atomic Absorption Spectrophotometry (AAS)

When using atomic absorption spectrophotometry (AAS) as an analytical technique the absorption of light of free atoms is measured. Therefore it is one of the branches of atomic spectroscopy, together with flame photometry (see Standardbase techniques: “Flame Photometry” that measures the intensity of light emitted by free atoms when their electrons return to ground state after the excitation by light). However - unlike flame photometry - AAS is based on the “first half” of the excitation process, while atoms absorb light getting their
electrons from the ground state to a higher energy level.

Figure 1: Photograph of an atomic absorption spectrophotometer

Although the atomic absorption spectrophotometer (fig. 1.) is quite expensive, the technique is very wide-spread, thank to the fact that by AAS it is possible to determine about 70 elements (mainly metals) at very low concentrations. The sample is atomised at a very high temperature (2500-3000 ºC) and the free
atoms have line spectrum. It means that they can only absorb the energy of light at discrete energy levels according to the excitations of electrons. Excitation energies in this case are determined by the difference between the energy level of the ground state and one of the excitation states of their electrons. Only a
light with a concrete wavelength belongs to each of these excitation energies and when this light is absorbed it is missing from the continuous spectra of the electromagnetic radiation: a black line appears in the absorption spectrum of the atom. There are no vibration or rotation energy levels that would widen the lines to brands in the spectrum (like it happens in the case of UV-Vis spectrophotometry, when molecules and ions are measured, see Standardbase technique: “UV-Vis Spectrophotometry”). Using AAS free atoms are “lit” by
monochromatic light (called “resonance radiation” that has got a special wavelength) that belongs to one line of their spectrum and therefore it has the suitable excitation energy mentioned above. Only the examined atoms can absorb it. As a result of absorption, the intensity of light decreases, which is proportional to the number of the examined atoms being present. That makes very sensitive quantitative measurements possible.

 

 

Fig. 2: A schematic diagram of atomic absorption spectrometer

To produce the proper monochromatic light necessary for the AAS, so called “hollow cathode lamps” are used. The cathode of this sort of lamp is made of the metal under investigation (or its alloy). It means that different lamps are used for the determination of each element. It is named after the cylindrical shape of
the cathode that gives direction to emerging beam, and helps re-deposit sputtered atoms back on cathode. The anode is made of tungsten and the electrodes are surrounded by noble gases. At high voltage the cathode produces electrons that speeding up in the electric filed cause the ionisation of noble gas atoms. These high-speed noble gas ions bombard the cathode and therefore sputtering occurs, dislodging atoms from the surface of cathode. These free atoms are excited by the high-speed electrons and then emit the line spectrum
characteristic of the particular element that is the cathode made of.
AAS called a “destructive technique”, because only solutions containing the investigated element can be used. Solid samples should be accurately weighed and then dissolved, often using strong acids (e.g. in cases when soil samples contaminated with heavy metal ions are measured). However only a very small amount of sample is enough, because of the high sensitivity of the technique.
The solvent of the solution is evaporated and all materials present in the sample are vaporised and dissociated to atoms at the very high temperature. (The process in the reality is a bit more complicated, since ions and oxides are also produced, decomposition and association reactions take place too.) The following
atomisation methods are known:

• Flame atomisation
• Graphite furnace atomisation
• Mercury hydride atomisation (this is only mentioned here, but not used while doing Standardbase experiments).

The source of atoms is usually flame (“flame atomisation”). Metals could be measured at ppm concentration (part per million, that is mg kg-1 or mg dm-3 in case of dilute solutions). The sensitivity could be increased when the light travels for longer in the flame. Therefore most of the burners are about 5-10 cm long.
The accuracy is very good, about 1-2%. The sample solution is sprayed (“nebulized”) continuously into the flame (similarly to the flame photometer).
The graphite furnace AAS (GFAAS), a more recent technique is even more sensitive than the traditional, cheaper AAS using flame. Measuremnts could be done at ppb level (part per billion, ppb = 10-3 ppm, that is μg kg-1 or μg dm-3 in case of dilute solutions!). The accuracy is still about 20% in this latter case. The
drying, combustion, vaporisation and atomisation of sample happen in a heated graphite tube that is placed in the way of light. This “graphite furnace” is protected against oxidation by an inert gas (e.g. argon).
The other parts of the atomic absorption spectrophotometer are similar to the one used in the case of UV-Vis spectrophotometer. The monochromator that selects the proper wavelength of the emitted spectra is the usual prism or optical filter. The detector is a photo-electron multiplier that produces an electric sign proportional to the intensity of emitted light. (It contains diodes /electrodes with increasing potential/ between the photocathode and anode, where multiplied electron emission is caused by the electrons bumping in them.) The electric sign is converted and appears as absorbance on the read-out (fig. 2.)

Reference textbooks:

• D. A. Skoog - D. M. West - F. J. Holler: Fudamentals of Analytical Chemistry (Saunders College Publishing, Fort Worth, US 1992.)
• J. Kenkel: Analytical Chemistry for Technicians (Lewis Publishers, Boca Raton, US 1994.)