Cold Vapour Atomic Absorption Spectrometry (CV-AAS/AFS)

Cold vapor spectrometry with AAS/AFS detection

Cold vapor spectrometry with AAS/AFS detection is an analytic method used for measuring mercury in the trace and ultratrace range. This method is based on cold vapor spectrometry, by which mercury is turned into a gaseous state. The mercury content is measured by combining the methods of atomic fluorescence spectroscopy (AFS) and atomic absorption spectroscopy (AAS). Thanks to this hybrid approach, it is possible to meld the advantages of each detection method in one machine.

Cold vapor spectrometry with AAS/AFS detection is favored in analytic chemistry thanks to high selectivity and sensitivity at extreme detection limits. This method is routinely used in many applications, for example in industry, environmental analysis, medicine and research.

The element mercury and its significance

The chemical element mercury (Hg) is a transition metal and belongs to the group of precious metals. Mercury is the only pure metal that is liquid at room temperature and also gaseous due to its low vapor pressure. Due to its special chemical and electrical properties, the metal was and is used in numerous applications, the best known of which include its use in thermometers, pressure gauges, switches and vapor lamps as well as in electrolysis and gold washing.

Mercury is released in large quantities during various human activities such as gold mining, coal burning and steel production, either as gaseous mercury in the atmosphere or in dissolved form in soil and water. Due to its toxic effect, mercury can cause considerable damage to health: This heavy metal is comparatively harmless when ingested via the digestive tract, but gives off highly toxic vapors even at room temperature. However, the hazardous potential of mercury increases significantly as soon as it is present as an organic compound.

In response to the high toxicity, numerous laws and regulations have been passed to set binding maximum limits for mercury. In the EU, for example, maximum levels of mercury in food are defined in Regulation (EC) No. 1881/2006. There are also limits for environmental parameters (such as drinking water and soil) and for industry. 

Basics of AAS/AFS detection with cold vapor spectrometry

Cold vapor spectrometry with AAS/AFS detection utilizes two methods from atomic spectroscopy: Atomic fluorescence spectroscopy (AFS) and atomic absorption spectroscopy (AAS).

Atomic spectroscopic methods are used for the quantitative and qualitative measurement of chemical elements and are widely used in analytic chemistry. Each method relies on analyzing radiation spectra, where the measured intensity distribution allows conclusions to be drawn about the chemical composition and concentration of the sample being tested.

Atomic absorption spectroscopy (AAS)

Atomic absorption spectroscopy is based on the resonance absorption effect Here, radiation interacts with free atoms in a specific way. The attenuation of element-specific wavelengths is used to measure concentrations with AAS. Incident light is selectively absorbed at certain wavelengths by the analytes in the sample. The level of attenuation in this case is proportional to the concentration in the sample. The concentration in the sample can by ascertained by comparing against a known series of calibration specimens. To measure the analytes in atomic absorption spectroscopy, the sample is first atomized. Atomization is often done at high temperatures, for example with an acetylene-air or acetylene-nitrous oxide flame, or in an electrothermally heated graphite tube.

When analyzing certain chemical elements, the hydride technique can also be used as an alternative. First, the elements to be measured, which are present in a highly volatile form, are pulled from the sample using inert gas. The gaseous analytes are then transferred to a special detection cell and atomized if necessary. The cold vapor technique, which can be regarded as a sub-form of the hydride technique, is generally used for mercury in particular. Cold vapor atomic absorption spectroscopy (CV-AAS) is based on the reduction of mercury ions to their atomic form. Tin(II) chloride or sodium borohydride is usually used as a reducing agent. The elemental mercury can then be driven out of the sample at room temperature using inert gas (argon) and transferred to the detection unit (e.g. quartz cell) in its atomized form.

Compared to other analytes of the hydride technique such as arsenic, selenium or antimony, no heating of the quartz cell is necessary for mercury analysis using the cold vapor technique. Even so, the atomizer is usually heated slightly in order to minimize the formation of unwanted water vapor in the cuvette.

Atomic absorption spectrometry is a robust and cost-effective technique that has been used routinely for decades. Thanks to ample documentation of irregularities, many possible interferences are already known and can be compensated or circumvented with established methods as needed. This means that analytes in samples with challenging matrices can also be reliably quantified.

Atomic fluorescence spectroscopy (AFS)

Unlike atomic absorption spectroscopy, atomic fluorescence spectroscopy does not measure the absorption of element-specific radiation; rather, it measures the specific radiation from the analytes at a 90° angle to the source of excitation. This method is used primarily for quantifying mercury levels. In contrast to AAS, AFS has a long linear measurement range and, thanks to its superior signal-to-noise ratio, excellent detection limits.

The atomic fluorescence spectroscopy method is based on the excitation of mercury atoms using electromagnetic radiation. With this method, the free atoms of mercury in the observation cell are initially in the ground state and are moved to an excited state by mercury-specific radiation. The atoms then return to the ground state and re-emit mercury-specific fluorescent radiation, which is converted to an electric signal in a detector. The measured intensity makes it possible to calculate the mercury concentration in the sample.

Due to the high detection limit of the method, and in combination with the enrichment and subsequent thermal resorption of the mercury on a gold surface (amalgamation), it is possible to achieve detection and quantitation limits in the sub-ng/L range. In many cases, the quantifiability of mercury in this technique is not limited by the method itself, but rather by the purity of the reagents used, the reaction vessels and the laboratory environment.

Modern hybrid systems with CV-AAS/AFS

Modern mercury analyzers rely on proven cold vapor technology with combined atomic absorption and atomic fluorescence spectrometry (CV-AAS/AFS). This approach makes it possible to combine the advantages of both spectrometry methods and enable the widest possible portfolio of applications.

The design of modern mercury analyzers such as the mercur DUO plus is based on the premise of ensuring simple, cost-effective and extremely powerful analytic equipment. The light source is a high-intensity low-pressure mercury lamp. The swiveling detector (secondary electron multiplier, SEM) makes it possible to switch automatically between AA and AF technology without retrofits.

The mercur DUO plus uses tin(II) chloride or sodium borohydride to reduce mercury compounds to atomic mercury. Thanks to the flow injection technique in combination with an autosampler, samples can be analyzed fully automatically. When detecting mercury levels, both the robust AAS method as well as the sensitive AFS method are available.

In addition to the AA and AF methods, the mercur DUO plus also offers mercury enrichment using a gold collector. Assisted by software, the user can select between the options of No enrichment, Single enrichment or Double enrichment (requirements of EPA Method 1631).

The mercur DUO plus mercury analyzer enables you to meet relevant standards for mercury quantitation. The following methods and specifications are particularly relevant here:

  • EPA 245.1, 245.2 and 245.7
  • EN 13806

In cold vapor spectrometry with AAS/AFS detection, special attention is paid to the handling of complex samples. If samples with very high mercury contents are measured in the analyzer, carry-over effects may occur. The resulting mercury impurities can be reduced and removed by thoroughly purging the system and/or by cleaning the reactor and other components. Foaming samples should also be treated beforehand with an appropriate reagent (e.g. amyl alcohol) to prevent carry-over effects. As an additional safety function, the mercur DUO plus Hg analyzer has a so-called "bubble sensor" that monitors the gaseous sample output after the dry hose and, if necessary, automatically closes the valves.

Powerful, sensitive, versatile: Advantages of CV-AAS/AFS

Cold vapor spectrometry with AAS/AFS detection has proven itself in numerous applications thanks to its many advantages and the requirement for analysis that complies with current standards and regulations. The main arguments for this method's regular use in laboratories are:

  • High sensitivity: The indirect method using the cold vapor technique allows mercury to be detected very sensitively with minimal background.
  • Excellent detection limits: The superior signal-to-noise ratio of the atomic fluorescence method allows for detection and quantitation limits in the sub ng/l range even without enrichment with a gold collector.
  • Standards compliance: The requirements of EPA method 1631 can be met when using double amalgamation with two gold collectors in the integrated enrichment module.
  • Automation: The mercur DUO plus can analyze samples fully automatically with an autosampler. Even with complex samples, the method guarantees reliable and low-maintenance routine analyses.

The aforementioned advantages of cold vapor technique with AAS/AFS detection have led, among other things, to this method becoming firmly established in a wide range of industries and applications for measuring trace and ultra-trace amounts of the toxic element mercury.

Environmental Analysis

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Pharma & Life Science

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Food & Agriculture

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From the environmental industry to the food industry: Applications of CV-AAS/AFS

The areas of application of cold vapor spectrometry with AAS/AFS detection range from drinking water quality control in the environmental sector to the identification of mercury poisoning in medicine and the analysis of foodstuffs for mercury content. Due to its highly toxic properties, laws contain strict regulations and limit values for mercury concentrations for many different areas of life – in all these areas, regular monitoring of these values using a suitable analytic method is essential. Since the limit values can vary greatly between the different areas, the cold vapor technique with AAS/AFS detection is a shoe-in thanks to its wide quantitation limits.

Mercury analyses featuring cold vapor spectrometry with AAS/AFS detection feature in the following industries:

  • Conservation: In environmental engineering, this analytic method serves to ensure mercury limits are met in drinking water, surface water, wastewater, soils and other samples that are relevant to public health and sound environmental practice.
  • Medicine: In the field of medicine, mercury analysis makes it possible to identify the heavy metal safely and reliably whenever it may occur in blood, urine, serum or saliva, for example.
  • Foodstuffs industry: The foodstuffs industry is subject to stringent limits for mercury concentration. Mercury can accumulate in various foods such as fish, drinks or cereals.
  • Geology: Anthropogenic and other natural processes cause mercury to accumulate in stone, minerals or ashes.
  • Industry: In industry, the cold vapor technique with AAS/AFS detection is used, for example, in emission control.  Various processing industries can release mercury concentrations that are harmful to the environment and human health.