Cold-vapor technique for

The cold-vapor technique for Hg allows detection limits of <1 ng to be obtained when using 50 mL of sample and they can be improved still further by trapping. With the hydride technique detection limits below the ng/mL level can be achieved for As, Se, Sb, Bi, Ge, Sn, etc. Accordingly, the levels required for analyses used to control the quality of drinking water can be reached. 

Bourcier, D.R. and Sharma, R.P. (1981) A stationary cold-vapor technique for the determi-natio of submicrogram amounts of mercury in biological tissues by flameless atomic absorption spectrophotometry. J. Anal. Toxicol., 5, 65-68. 

The introduction of a gas phase sample into an atomizer has significant advantages over the introduction of solids or solutions. The transport efficiency may be close to 100%, compared to the 5-15% efficiency of a solution nebulizer. In addition, the gas phase sample is homogeneous, unlike many solids. There are two commercial analysis systems with unique atomizers that introduce gas phase sample into the atomizer. They are the cold vapor technique for mercury and the hydride generation technique. Both are used extensively in environmental and clinical chemistry laboratories. 

The cold vapor technique for determining mercury is the most widely accepted method for achieving sub ppm concentrations. The sample is mildly digested with sulfiuric and nitric acid in the presence of potassium permanganate and potassium persulfate in order to convert all forms of mercury to the ionic form. 

The cold-vapor technique for Hg allows detection limits of <1 ng to be obtained when using 50 mL of sample and they can be improved still further by trapping. 

Elemental composition Hg 79.40%, C 9.51%, N 11.09%. Aqueous solution is analyzed for mercury metal by AA-cold vapor techniques or by ICP/AES (see Mercury). The cyanide ion may be measured by cyanide ion-specific electrode or by ion chromatography after appropriate dilution. 

The 253.7 nm analytical line is routinely used for AAS, although the 184.9 nm line is an estimated 50 times more sensitive. This line is beyond the wavelength where flame and atmospheric absorption are prohibitive. Using the cold vapor technique with a nitrogen-purged monochromator would permit greater sensitivity. 

Whatever instrument is used, provision must be made for using both air and nitrous oxide-supported flames. A fume exhaust must be provided. If arsenic, selenium, or mercury are to be determined, an apparatus for vapor generation should be used. Such apparatus is usually available from the instrument manufacturer. Mercury is usually determined by a flameless or cold vapor technique. 

The cold vapor technique is used for mercury. This technique involves reducing the mercury to the zero valence state with either sodium borohydride or stannous chloride. The mercury is then swept into a gas cell aligned in the light path of the spectrophotometer, using a stream of nitrogen or air. Fig. 4 shows a diagram of a typical unit. 

EPA. 1994g. Method 7471A. Mercury in Solid or Semisolid Waste (Manual Cold-Vapor Technique) Test Methods for Evaluating Solid Waste. Office of Solid Waste, U. S. Environmental Protection Agency. 

Engel U., Bilgic A. M., Haase O., Voces E. and Broekaert J. A. C. (2000) A microwave-induced plasma based on microstrip technology and its use for the atomic emission spectrometric determination of mercury with the aid of the cold-vapor technique, Analytical Chemistry 72 193-197. 

Atomic absorption spectrometry, belonging to a class of techniques also defined as optical atomic spectrometry, has been for some four decades - and continues to be - one of the most important, dominant determinative techniques. It includes flame atomic absorption spectrometry (FAAS), electrothermal atomization atomic absorption spectrometry (ETAAS) (including graphite furnace AAS (GFAAS), carbon rod AAS, tantalum strip AAS), and gaseous generation (cold vapor AAS for Hg, hydride gener-... 

Chemical vapor generation is another important variant of AAS suitable for the determination of several elements forming elemental vapors (Hg) or volatile hydrides (As, Se, Bi, Sn, Ge, Te, Pd). The cold vapor technique generating the volatile element is almost exclusive to Hg, although there is one report of Cd. There is a voluminous literature on the determination of Hg by atomic absorption of Hg atoms in the gaseous phase beginning from the early days after development and continuing presently. 

Mercury is best determined by the cold vapor atomic absorption method. The instrumental conditions for this determination have been discussed by Welz (1985). The graphite furnace can be used to determine Hg but, because a small sample is taken, the sensitivity is not as favorable as the cold vapor technique. 

Some interferences are removed by the hydride technique because the analyte is physically separated from the remainder of the matrix. But there are interferences in the process of generating the hydride and the variability in the rate of generation. There appears to be some black art in the quartz decomposition cell, and sometimes simple electrical furnaces are used to convert the hydride to the atomic vapor. In the opinion of this author, the furnace technique has fewer interference problems for the P block elements than the hydride technique but the furnace equipment is more expensive and, if the quantity of sample is not limited, the furnace is less sensitive. There is a great deal of historical and practical information on both the cold vapor method for Hg and the hydride method in the AAS book by Welz (1985), who is an experienced authority in these methods. 

Kothandaraman, P. and Dallmeyer, J.F. (1976) Improved method for mercury cold vapor technique. At. Abs. Newsl., 15,120-121. 

Vapor generation techniques The generation of gaseous analytes from the sample and their introduction into atomisation cells for subsequent absorption spectro-metric determination offers a number of advantages over the conventional sample introduction by pneumatic nebulisation of the sample solution. These include the elimination of the nebuliser, the enhancement of the transport efficiency, which approaches 100 %, and the presentation of a homogenous sample vapor to the atomiser. The most common and versatile techniques for the formation of volatile compounds are the hydride generation technique and the cold vapor technique. 

Since mercury is present already in the atomic state in the cold vapor technique, there is no need for an atomiser as such. The sample vapor is swept directly from the reduction cell or the amalgamation trap in the carrier gas stream to a 10 cm length T-shaped quartz tube that is moderately heated (to ca. 200 °C to prevent condensation of mercury). This quartz cell is located in the light path of a conventional AA spectrometer where the attenuation of a characteristic Hg line source is measured. Dedicated AA spectrometers (which, in this case, often have a continuum light source) may also be used with longer absorption cells (300 mm pathlength) to increase the sensitivity. 

Fiydride generation (and cold-vapor) techniques significantly improve atomic absorption spectrometry (AAS) concentration detection limits while offering several advantages (1) separation of the analyte from the matrix is achieved which invariably leads to improved accuracy of determination (2) preconcentration is easily implemented (3) simple chemical speciation may be discerned in many cases and (4) the procedures are amenable to automation. Disadvantages with the approach that are frequently cited include interferences from concomitant elements (notably transition metals), pH effects, oxidation state influences (which may be advantageously used for speciation) and gas-phase atomization interferences (mutual effect from other hydrides). 

Hydride generation and cold-vapor techniques may be conveniently characterized by three steps (1) generation of the volatile analyte (2) its collection (if necessary) and transfer to the atomizer and (3) decomposition to the gaseous metal atoms (unnecessary for mercury) with measurement of the AA response. Each of these steps will be briefly reviewed prior to considering the analytical performance of these techniques. 

Most commonly used instruments use a flame (flame AAS (FAAS)) produced by combustion of an air/acetylene or dinitrogen oxide/acetylene mixture. The few interferences are easy to avoid, and the sensitivities that are reached are adequate for the metals of greatest interest to the food industry. Variants of this technique, such as the coupling of hydride generation (HG) systems (HG-AAS), increase its scope to higher-sensitivity determination of elements like selenium, arsenic, tin, and other elements that form hydrides. In a similar vein, the determination of mercury using the cold vapor technique should be highlighted. 

By combining the cold-vapor technique with HPLC, a very sensitive method for the determination of Hg spedes at the sub-ng level becomes possible [322]. When applying cold-vapor AAS to the detection of mercury subsequent to the separation of the species by HPLC, which also enables thermally labile compounds to be separated, the organomercury compounds have to be destroyed to allow for the AAS determination. They can be destroyed by wet chemical oxidation with H2S04-Ct207 or by photochemical oxidation. It is then possible to perform spe-ciation of mercury in gas condensates easily, where the species can be separated by reversed phase HPLC [323]. 

It means they can now detennine the vast majority of the environmentally significant elements/species by one technique. This capability is very attractive because it means they can typically analyze 5-10 times more samples per day, for a full suite of elements, compared to other approaches that use a combination of flame atomic absorption (FAA), GFAA, CVAA (cold vapor AA for Hg), and ICP-OES. This productivity improvement is exemplified in Table 19.2, which compares the productivity of a drinking water analysis for 12 primary contaminants using three different analytical scenarios. " ... 

A microwave-induced plasma based on microstrip technolc and its use for tile atomic emission spectrometric determination of mercury with the aid of the cold-vapor technique,... 

Atomic absorption (AA) spectrometry is a generally accepted method for the analysis of many metals [74]. In a typical A A method, a liquid sample is aspirated into a flame, where ions within the liquid are reduced to the atomic state. The metals in the atomic state can then quantitatively absorb light at the wavelengths characteristic of their resonance frequencies, 217.0 and 283.3 nm for lead. Alternately, the ions may either be chemically reduced by a cold vapor technique [32] or be thermally reduced in a graphite furnace before analysis, which usually gives better sensitivities than flame AA (as low as 50 pg/kg). 

Several hundred samples can be analyzed in a work day if the samples are already prepared. In flame techniques and cold vapor techniques, the sample must be in solution. Procedures for solution of some samples may be time consuming. Use of a graphite furnace for sample reduction eliminates many problems in sample preparation. 

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

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