Cold Vapour Method

Mercury can be determined in plasma AES by reducing it first to elemental mercury and then transporting the mercury vapour into the plasma. The same reduction methods may be used as for AAS. Commercial hydride generation systems can be adopted to the cold vapour method. The detection limit is about 0.02 mgP  

3 Plasma Emission Spectrometer as a Detector for Gas and Liquid Chromatographs 

In liquid chromatography the effluent can directly enter the nebulizer of the instrument, whereas in gas chromatography the nebulizer must be removed and an additional plasma gas flow must be employed. Liquid chromatographs cannot be connected to microwave plasmas, because the sample intake rate by HPLC is too slow for MWPs. In contrast, MWPs have been widely used for detectors in gas chromatography. I CPs and DCPs can be used both in gas and liquid chromatography. 

The analytes pass into the plasma through chromatographic separation as hydrides or other volatile compounds, metal chelates, anions, or organome-tallic compounds. When the exact composition of the metal complex studied is known, it is possible to calculate the amount of the organic ligands on the basis of the amount of metal in the sample. Detection limits obtained by HPLC-plasma-AES or GC-plasma-AES methods are generally somewhat higher than those for conventional plasma—AES methods, but the sample volume required for the sample introduction by HPLC or GC is much smaller than that required for the solution nebulization. 

Spectral interferences can be avoided by chromatographic sample introduction. For example, when rare earths are determined by IGP-AES with direct solution nebulization, spectral interferences are often caused by other co-existing rare earth elements. When a HPLC is connected to an IGP-AES, these spectral interferences can be excluded by separating the rare earths by HPLC before their introduction into the plasma. Sodium or potassium salts of or-isobutyric acid, EDTA, or citric acid generally used for the separation of rare earths are unsuitable elution reagents for the IGP-AES detection, as these compounds easily clog the nebulizer and reduce the nebulization efficiency because of their high viscosity. 

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With mercury, Hg° is formed instead of the hydride. A special cell, which does not need to be put into the flame is used. This is called the cold vapour method, and requires specialised instruments (reduction by SnCI2). 

Many other household products can be analysed in similar ways to those described above for chemicals. Household bleach is essentially an inorganic chemical. There has been concern expressed about mercury levels in hypochlorite bleach because of the way it is manufactured. The cold vapour reduction/aeration method referred to above is a good way of determining low mercury levels with minimal matrix problems [82]. In the past organo-mercurial compounds have been used (e.g. as bactericides) in some household products these may be selectively determined by extraction with an organic solvent (e.g. carbon tetrachloride or benzene), and then application of the cold-vapour method following the addition of cysteine acetate, or by coupled gas chromatography/atomic absorption [83],... 

The benefit of sample preparation techniques using microwave acid digestion and bomb combustion is that the sample is totally enclosed during the decomposition. These methods remove matrix interference and generate aqueous solutions, which can be analysed using ICP-OES. Sub-trace concentrations can be detected when hyphenated attachments are used, e.g. ultrasonic nebuliser, hydride generation or continuous cold vapour method. These methods are essential where trace levels of toxic elements are present that need to be identified and quantified. 

The metals As, Cd, Cr, Pb and Se are analysed against multi-element standards of 0.0, 0.1, 0.25, 0.5 and 1.0 ppm prepared in 0.1 M HN03 using ICP-OES with the CETAC 5000AT ultrasonic nebuliser attachment. The element Hg is measured using the continuous cold vapour method. 

The analysis of petroleum products for the presence of toxic metals is an environmental requirement and results of analysis must be made known to all concerned bodies including the public. In order to establish the true concentrations present, a reliable method must be employed in order to be certain of the results. A sensitive method must be available and the use of an ICP coupled with an ultrasonic nebuliser, hydride generation or cold vapour method for Hg offer the most sensitive and reliable methods available. If analysing for lower levels of these metals, a mass spectrograph with hyphenated attachments may be required. 

Figure 92 Principle of the dynamic cold vapour method...

Metallic mercury formed during the reaction is determined by the cold vapour method. The detection limit is about 5 ng SOs ". 

Normally samples are introduced as solutions into the plasma, but the direct introduction of solids and gases is also possible. Hydride and cold vapour methods are also applied to plasma atomic emission spectrometry. In addition, plasmas can be used as detectors for gas and liquid chromatographs. 

The hydride generation and cold vapour methods for the determination of the gaseous hydride forming elements and mercury, and gas chromatographic methods are discussed in Section 6. 

Cold Vapour Method. The capability of the cold vapour technique for the determination of mercury has been examined by using non-dispersive instruments. Mercury is first reduced with tin(ii) chloride to metallic mercury, and the mercury vapour is fed in an argon stream to the excitation cell. A mercury lamp is used for the excitation measurement, and the fluorescence signal is detected by a photomultiplier. A monochromator or filter is required in this technique. 

Mercury AAS with cold vapour method Principle of the method... 

Mercury ions are reduced to metallic mercury with tin(II) chloride. The metallic mercury is transferred into a quartz cuvette with the aid of a current of inert gas and the absorption of the atoms is measured in the beam of an atomic-absorption spectrometer (cold vapour method). 

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