Atomic Absorption Spectrometry (AAS)
With the contrAA 800 platform we offer the most advanced atomic absorption spectrometry(AAS) systems worldwide. The sensitivity and robustness of the contrAA 800 series rivals even ICP systems thanky to its unique high-resolution continuum source atomic absorption technology (HR-CS-AAS).
Next to the high-end contrAA 800 systems we offer a variety of additional atomic absorption spectrometers for a wide range of analytical tasks and requirements: The entry-level novAA 800 AAS is an optimal spectrometer for routine applications and can operate even in harsh environemnts. For certain trace analyses our AAS of the ZEEnit series with Zeeman background correction of the third generation are ideal.
AAS
Atomic absorption spectroscopy (AAS) is a powerful analytic method for quantitative and qualitative measurement of metals and metalloids in samples. This method, long since proven in analytic chemistry, relies on the principle of radiation absorption: The atoms in the element being analyzed absorb electromagnetic radiation, then the resulting decrease in the intensity of certain parts of the spectrum is measured, giving clues about the composition of the sample. Atomic absorption spectroscopy has the advantage of high selectivity and a broad range of applications.
AAS is a mainstay method for process control and quality control in many industries. As a reliable chemical analysis method, atomic absorption spectroscopy can be used, for example, to check food for toxic metals, to analyze trace toxic elements in surface and drinking water, or to measure heavy metals in sewage sludge and soil in accordance with standards. Other applications have come to the fore in the chemical industry, the oil and gas sector, mining, metal processing and environmental analysis. In practice, this method is applied with either aqueous solutions or solids.
Basics of atomic absorption spectroscopy
Atomic absorption spectroscopy is one of the atomic-spectrometric methods that makes use of each element's characteristic line spectrum. The AAS method thus relies on the radiation emanating from a light source hitting the atomized parts of the sample, where the radiation is partially absorbed. The standard light source used is a hollow-cathode lamp in which the cathode consists of the element of the analyte. For certain elements such as arsenic, mercury or lead, which absorb in the UV wavelength range, higher-intensity lamps are also used to optimize detection limits.
The absorption caused by the element under analysis results in an attenuation of the radiation intensity at the element's characteristic wavelength. According to the Beer-Lambert law, there is a proportional relationship between the attenuation of intensity with respect to the starting intensity and the concentration of the element in the sample.
AAS can be broken down into three variants of the process. These vary in the type of sample atomization and are therefore suitable for different applications. Atomization converts the sample into free atoms in the gaseous state. Atomization should produce as few excited or ionized atoms as possible. Some modern analytic machines are even capable of combining all three atomization variants in one apparatus. The novAA 800 D model from Analytik Jena, for example, covers the entire spectrum of AAS applications and has impressively fast and reliable routine analysis.
The three method variants and the principles behind them are explained below.
Flame AAS
Flame atomization is the conventional method of atomization in atomic absorption spectroscopy and is therefore widely used. It is based on pneumatic atomization of the sample in a mixing chamber, after which the sample is mixed with fuel gas and an oxidizing agent. The fine droplets of the resulting aerosol come into contact with a flame, usually an acetylene-air or acetylene-oxygen flame.
Combustion atomizes the sample: First the solvent evaporates, then the parts of the sample melt, vaporize and dissociate to free atoms. Finally, the atomized sample is irradiated with monochromatic light of a specific wavelength in order to induce and then measure the desired attenuation in the radiation intensity.
Graphite furnace AAS
Graphite tube (or graphite furnace) atomic absorption spectroscopy is based on the atomization of the sample in a heated graphite tube. Graphite heats when subjected to a voltage, allowing for a more precise and stepwise drying, heating and atomization process. In this method, the sample is first dried at a low temperature and then organic components are driven off with pyrolysis.
This is followed by the actual atomization of the sample at high temperatures. The required temperature in this step depends on the atomization temperature of the element that we wish to analyze – typically in the range of 1,500 to 2,500 °C. As with the flame technique, the atomized sample is then irradiated using a light source and the resulting radiation intensity is detected.
The graphite furnace atomic absorption spectroscopy technique offers a better detection limit compared to the flame technique, which makes it particularly suitable for samples with low concentrations. In addition, differing evaporation temperatures make it possible to largely remove interfering matrix components.
With the ZEEnit Series, Analytik Jena offers a powerful solution for analyzing challenging samples. The analysis machines in this series are equipped with a high-performance cross-heated graphite furnace.
Hydride systems
The hydride technique is based on the property of certain elements to react with atomic hydrogen and thus form hydride compounds. These compounds can easily be brought out of solution with an inert gas. During subsequent heating in a glass cuvette, the hydrides decompose into hydrogen and the now atomized element. Then, like with the other techniques, the element is irradiated and the attenuation in the radiation intensity is measured. The hydride technique is suitable for analyzing samples with certain elements such as tin, arsenic, bismuth, antimony or tellurium. For example, arsenic forms the hydride compound AsH3 with atomic hydrogen and can be drawn out by the inert gas argon and then atomized.
Cold vapor spectrometry (CV-AAS) represents a subform of the hydride technique. It is specially suited for analyzing the mercury content of a sample. Here, a reducing agent such as tin(II) chloride or sodium borohydride is used to reduce mercury ions to their atomic form; the mercury is then removed from the sample at room temperature using inert gas. The advantage of this technique is that the sample does not have to be heated. For practical reasons, the cuvette is simply heated to prevent the buildup of water vapor.
Analytik Jena leverages cold vapor spectrometry in the mercur DUO plus mercury analyzer model: This analyzer is a master of the proven cold vapor technology with combined atomic absorption and atomic fluorescence spectroscopy (CV-AAS/AFS). It boasts sensitive and standards-compliant mercury analysis.
How an AAS analyzer works
The setup of an AAS analyzer shown schematically in Figure 1 applies for basically all AAS methods, although there are individual differences, especially in how different elements are atomized.
- Hollow cathode lamp
- Beam chopper
- Atomizer
- Monochromator
- Detector
- Amplifier
- Display
Figure 1 Schematic structure of an AAS analyzer (source: PowerPoint presentation courtesy of tu-dresden.de)
Here is an overview of the key components of AAS analyzers:
- Light source: The choice of light source in atomic absorption spectroscopy depends on the element. Usually hollow-cathode lamps are used, which are equipped with a cathode made of the element that we want to analyze. Alternatively, depending on the objective, super lamps (with an additional cathode) or electrodeless discharge lamps may also be suitable. These promise better detection limits for UV-absorbing elements due to their higher light intensity.
- Atomizer: The atomizer unit's function is to drive off solvents and volatile components from the sample and dissociate them into free atoms. For quality measurement results, it is important that as few atoms as possible are brought to the excited or ionized state. Besides the conventional flame technique, both the graphite furnace technique and the hydride technique may be employed to atomize the sample. When analyzing for mercury, cold vapor spectrometry is an especially suitable method.
- Detector: The radiation-sensitive detector measures the intensity of the radiation emitted from the light source after it has been absorbed by (attenuated through) the atomized sample. The attenuation measured is proportional to the element's concentration. In atomic absorption spectroscopy, the detector is usually a photomultiplier (PMT). A photomultiplier is an electron tube that can precisely detect even weak light signals thanks to an amplification mechanism.
Modern AAS analyzers typically use background correction methods. Their purpose is to minimize interference from matrix effects and other interference sources. The reliable deuterium and Zeeman background correction methods, for example, have proven themselves with tough sample matrices.
With high resolution continuous source AAS (HR-CS AAS), analytic chemistry has succeeded in achieving an extreme level of resolution and sensitivity. This method's primary advantage is that the light source employed covers the entire wavelength range of AAS, thus allowing analysis of many different trace elements. The contrAA 800 model series from Analytik Jena, for example, works with a xenon short arc lamp and combines all atomization techniques in one machine.
Important application areas for atomic absorption spectroscopy
Due to its far-reaching advantages and numerous possible applications, the AAS method has become firmly established in quality control and process control for numerous industries. The following are examples of some typical applications of atomic absorption spectroscopy:
- Materials science: In the building materials industry, AAS methods are used to determine the main components of building materials using the flame technique. A typical application is the analysis of cement composition for elements such as calcium, iron, magnesium, sodium, potassium, aluminum and titanium.
- Mining: In the mining and metals industry, atomic absorption spectroscopy has proven its potential for quantifying the copper content in geological samples, for example. The main technique used in this context is the flame technique.
- Foodstuffs industry: In food technology, the AAS method is important to quality control. AAS can be used not only to check food for its mineral content, but also to reliably identify undesired toxic metals.
- Environmental analysis: When monitoring soils or sewage sludge, AAS offers reliable results when it comes to determining harmful elements. These include toxic metals such as cadmium, lead or chromium as well as high concentrations of other potentially harmful elements such as nickel or copper.
- Oil and gas industry: AAS is used in the oil and gas industry to perform routine analyses of petrochemical products. These analyses are often governed by standards. As one example, such analyses enable kerosene to be checked for undesirable traces of copper.
In addition to the examples mentioned here, there are numerous other applications where atomic absorption spectroscopy is used. Its precision, sensitivity and versatility make this method an indispensable analytic technology in quality control, environmental analysis and materials analysis.
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