Cold vapour technique

The determination of mercury is possible by the cold vapour technique, since it is based on the unique properties of this element. Mercury has an appreciable vapour pressure at ambient temperatures (0.16 Pa at 293 K) and the vapour is stable and monoatomic. Mercury can easily be reduced to metal from its compounds. By selective reductions of inorganic mercury(ii) compounds and organomercury compounds, it is possible to determine successively the inorganic and organic mercury fraction in the same sample. 

The mercury vapour may be entrained in a stream of an inert gas or in air and measured by the atomic absorption of the cold vapour without the need of either flame or flame atomizers. 

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A) Cold vapour technique. This procedure is strictly confined to the determination of mercury,45 which in the elemental state has an appreciable vapour pressure at room temperature so that gaseous atoms exist without the need for any special treatment. As a method for determining mercury compounds the procedure consists in the reduction of a mercury(II) compound with either... 

The relative advantages and disadvantages ofvoltammetric and atomic absorption methodologies are listed below. It is concluded that for laboratories concerned with aquatic chemistry of metals (which includes seawater analysis), instrumentation for both AAS (including potentialities for graphite furnace AAS as well as hydride and cold vapour techniques) and voltammetry should be available. This offers a much better basis for a problem-orientated application of both methods, and provides the important potentiality to compare the data obtained by one method with that obtained in an independent manner by the other, an approach that is now common for the establishment of accuracy in high-quality trace analysis. 

Jurka and Carter [50] have described an automated determination of down to O.lpg L 1 mercury in river sediment samples. This method is based on the automated procedure of El-Awady [51] for the determination of total mercury in waters and waste waters in which potassium persulphate and sulphuric acid were used to digest samples for analysis by the cold vapour technique. These workers proved that the use of potassium permanganate as an additional oxidizing agent was unnecessary. 

Hydride Generation and Cold Vapour Technique Coupled to an Inductively Coupled Plasma Source... 

In analogy to sample introduction by hydride generation, mercury trace analysis is possible by reducing Hg compounds to the metal using the cold vapour technique or the determination of iodine at the ultratrace level (after oxidation with 70 % perchloric acid of iodide to iodine) via the gas phase. 

Hydride and cold-vapour techniques represent a special combination of chemical separation and pre-enrichment with AAS determination, resulting in higher powers of detection for elements with volatile hydrides, eg, As, Bi, Se, Sb, Hg. Recent literature on vapour generation has been reviewed by Hill et al. (1991). Some examples of the use of hydride generation for the analysis of plant material are given by Muse et al. (1989), Leuka et al. (1990) and Ainsworth and Cooke (1990). Hydride generation can also be used with ICP-EAS (see below) and applications have been reviewed (Nakahara, 1991). 

Usual flame techniques are often insufficiently detective to measure the low levels of As, Se and Hg present in foodstuffs. Mercury is commonly determined via the flameless cold-vapour technique, whereas there is much current activity in respect of the measurement of As and Se via their conversion to hydrides with subsequent decomposition in cool argon—hydrogen-entrained air flames or electrically-heated cells. Table 4 contains information on these techniques. 

Elements such as B, Cd, Co, Cr, Mo, Ni, Pb and V are also often found in foodstuffs at concentrations too low to be amenable to direct flame absorption. Resort must be made to concentration via, e.g., solvent extraction prior to introduction into the flame (SEFAAS) or to the newer technique of EAAS. The elements Na, K, Mg, Ca and Hg, although amenable to determination by EAAS are not listed in Table 5 as the flame provides adequate detectivity for the four macroelements, whereas the cold-vapour technique is clearly the one of choice for mercury. A good number of other elements in Table 5 can usually be determined in foods by flame spectrometry EAAS information for them is included, however, to cover the occasional sample with extremely low trace-element content. 

At the present time there are no ETA—AAS methods that can compete with the cold vapour technique for Hg or with hydride generation methods for Sb and Te. Another attractive method for Sb and Te is low pressure microwave induced plasma (MIP) emission spectroscopy [138]. Using low-temperature ashing and solvent extraction as preparation, physiological concentrations of both elements ([Pg.376]

Flame atomisation is not necessary for the atomic absorption spectrophotomehy of mercury. The cold vapour technique described here employs a reduction vessel (which may be purchased) to produce mercury vapour the vapour is led to a quartz absorption cell within the atomic absorption inshument. The method is applicable to inorganic and organic mercurial compounds in urine. 

Mateo MD, Forteza R, Cerda V, et al. 1988. Comparative study of a kinetic - thermometric method and the atomic-absorption cold-vapour technique for determination of mercury traces and ultra-traces. Thermochimica Acta 128 21-30. 

The Cold-Vapour technique is required for determination of mercury because mercury is insensitive to the other AAS techniques. This method utilizes the unique characteristic of mercury to exist in the atomic form at room temperature. This method can only be used for determination of mercury because of unspecific interferences for other elements [2],... 

A.M. Serra, J.M. Estela, V. Cerda, MSFIA system for mercury determination by cold vapour technique with atomic fluorescence detection, Talanta 77 (2008) 556. 

To summarise, FAAS is very easy to use. Interferences are known and can be controlled. Extensive application information is also readily available. Its precision makes it an excellent technique for the determination of a number of commonly analysed elements at higher concentration in polluted soil samples. Its main drawback is its speed in relation to multi-element techniques such as ICP-AES and ICP-MS. Where direct-aspiration flame atomic absorption technique does not provide adequate sensitivity, reference is made to specialised techniques (in addition to graphite furnace procedure) such as the gaseous-hydride method for arsenic, antimony and selenium and the cold-vapour technique for mercury. 

Table 4.5 Performance characteristics for mercury by cold vapour techniques obtained with the M6000A for CV-AAS and the Millennium PSA for CV-AFS...

Thompson, K.C. and Godden, R.G. (1975) Improvments in the atomic-fluorescence determination of mercury by the cold vapour technique. Analyst, 100, 544-548. 

Cold vapour technique. An analytical technique for the determination of mercury. Mercury is first reduced to the metallic element, vaporized, and introduced into the light path of the instrument. 

The cold vapour technique may be fully automated by using a FIA system, which is discussed in section 6.4,2. 

In AAS, FIA has been applied to hydride generation and cold vapour techniques, microsampling for flame atomic absorption, analysis of concentrated solutions, addition of buffers and matrix modifiers, dilution by mixing or dispersion, calibration methods, online separation of the matrix and analyte enrichment, and indirect AAS determinations. 

Figure 95 A FIA system adapted for hydride generation and cold vapour techniques (Perkin Elmer Corp.)...

Initially hydride generation and cold vapour techniques were developed for the quantitative determination of the hydride-forming elements and mercury by atomic absorption spectrometry (Chapters, Sections 6.2 and 6.3), but nowadays these methods are also widely used in plasma atomic emission spectrometry. In the hydride generation technique, hydride-forming elements are more efficiently transported to the plasma than by conventional solution nebulization, and the production and excitation of free atoms and ions in the hot plasma is therefore more efficient. Spectral interferences are also reduced when the analyte is separated from the elements in the sample matrix. Both continuous (FIA) and batch approaches have been used for hydride generation. The continuous method is more frequently used in plasma AES than in AAS. Commercial hydride generation systems are available for various plasma spectrometers. 

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. 

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