Atomic absorption spectrometry (AAS) is an analytical technique that measures the concentration of an element by measuring the amount of light (intensity of light) that is absorbed - at a characteristic wavelength - when it passes through a cloud of atoms of this element.
As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way.
History of Atomic Absorption Spectrometry
The phenomenon of light absorption had already been investigated at the beginning of the 18th century. It was observed that the original radiation intensity is resolved into three components: into reflected, transmitted and absorbed radiation.
The two fundamental laws governing the fraction of incident radiation absorbed on passing through a sample are the Lambert’s law (1768) and the Beer’s law (1852).
The first law, was stated by Bouguer in 1729 and restated by Lambert in 1768 is called the Lambert’s law and predicts the effect of thickness of a sample medium upon the fraction of radiation which is absorbed.
Lambert reached the intuitively reasonable conclusion that each unit length of material through which radiation passes absorbs the same fraction of radiation. If for example a monochromatic beam of intensity Io passes through a thickness d of absorbing material the transmitted beam intensity It will be reduced and will be equal with:
It = Io * e-k.d (1)
The proportionality constant kis called absorption coefficient and depends on the wavelength and the temperature of a given substance.
Lambert’s law (1) is an exact law and applies to any homogeneous, nonscattering medium, regardless of whether it is a gas, liquid, solid or solution.
Lambert’s law underwent a thorough examination by Beer and he proposed a more useful law for chemistry:
A = log Io / It= α * c * d (2)
It states that the absorbance A is proportional to the concentration cof the absorbing substance and to the thickness d of the absorbing layer. The constant α is a new proportionality constant.
The Atomic Absorption Process
If light of just the right wavelength strikes on a free, ground state atom, the atom absorbs the light as it enters in an excited state. This process is called atomic absorption and is illustrated in Fig. 1.
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| Fig. 1: The atomic absorption process. The atom absorbs light of a specific wavelength (specific amount of energy) and it goes to an excited state. |
The capability of an atom to absorb very specific wavelengths of light is utilized in atomic absorption spectrometry.
Light of a specific wavelength and of initial intensity Io is focused on the flame cell containing ground state atoms. The initial light intensity is decreased by an amount determined by the atom concentration in the flame cell. The light is then directed to the detector where the reduced intensity, It, is measured. The amount of light absorbed is determined by comparing It to Io and according to Beer’s law:
A = log Io / It = α * c * d (2)
Atomic Absorption Spectrophotometer
Modern atomic absorption spectrometry was introduced in 1955 as a result of the independent work of A. Walsh and C.T.J Alkemade. Commercial spectrophotometers were introduced by the early 1960’s, and the importance of atomic absorption as an analytical technique was soon evident. PerkinElmer has been the undisputed global leader in AAS for over 50 years.
There are five basic components of an atomic absorption instrument: The light source, an “absorption cell”, a monochromator, a detector and a screen.
The light source that emits light of a specific frequency for the element under investigation. The light source is usually a hollow cathode lamp or an electrodeless discharge lamp. The light source is electronically modulated or mechanically chopped to differentiate between the light from the source and the emission from the sample cell.
An “absorption cell” in which atoms of the sample are produced (flame, graphite furnace, FIAS cell, FIMS cell, MHS cell). When flame atomization is used the sample is first converted into a fine mist consisting of small droplets of solution. This is accomplished by using a nebulizer assemply.
Subsequently, thermal energy volatizes the particles producing a vapor consisting of molecular species, ionic species and free atoms. Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Air-acetylene (temperature: 2100-2400 C) and nitrous oxide – acetylene flames (temperature: 2600-2800 C) are used most frequently.
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| Fig. 2: The basic components of an atomic absorption instrument |
A monochromator for light dispersion. The monochromator is used to select the specific wavelength of light – a spectral line – which is absorbed by the sample, and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others. The light selected by the monochromator is directed onto a detector that is a photomultiplier tube. This produces an electrical signal proportional to the light intensity.
A detector, which measures the light intensity and amplifies the signal.
A screen that shows the reading after it has been processed by the instrument.
Uses of Atomic Absorption Spectrometry
Atomic absorption spectrometry has many uses in different areas of chemistry such as:
Clinical analysis: Analyzing metals in biological fluids and tissues such as whole blood, plasma, urine, saliva, brain tissue, liver, muscle tissue, semen, umbilical cord.
Pharmaceuticals: In some pharmaceutical manufacturing processes, minute quantities of a catalyst that remain in the final drug product, active pharmaceutical ingredient.
Water analysis: Analyzing water for its metal content
Food Analysis:
Mining: By using AAS the amount of metals such as gold in rocks can be determined to see whether it is worth mining the rocks to extract precious metals
REFERENCES
- (a) A. Walsh, Anal. Chem., 63, 933A–941A, 1991 (b) S.R. Koirtyohann, Anal. Chem., 63, 1024A–1031A, 1991 (c) W. Slavin, Anal. Chem., 63, 1033A–1038A, 1991.
- D. Harvey, “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
- D.A. Skoog, F.J. Holler, T.A. Nieman, “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.
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