Cold-vapor technique

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. 

Determination of Mercury, Fluorine, Boron, and Selenium. The Determination of Mercury. A coal sample is decomposed by igniting a combustion bomb containing a dilute nitric acid solution under 24 atm of oxygen. After combustion, the bomb washings are diluted to a known volume, and mercury is determined by atomic absorption spectrophotometry using a flameless cold vapor technique. 

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. 

Aquatic systems (MeHg) (Isotope dilution analysis) ICP-SEMS (Element) cold vapor technique Hg, Hg, 20 Hg 202 Hg, 99Hg/202Hg, 200Hg/202Hg 20 Hg/202Hg 0.6-1.4% LOD 0.2 ngk Sturup et al. ... 

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. 

Another AA method applicable to volatile elements tmd compounds is the cold-vapor technique. Mercury is a volatile metal and can be determined by the method described in Feature 28-1. Other metals form volatile metal hydrides that can also be determined by the cold-vapor technique. 

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. 

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. 

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. 

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. 

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

Nakahara, T., Tanaka, T., and Musha, S. (1978) Flameless atomic fluorescence spectrometry of mercury by dispersive and nondispersive systems in combination with cold-vapor technique. Bull. Chem. Soc. Jpn., 51, 2020-2024. 

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. 

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). 

Although the determination of mercury in air by absorption spectroscopy was practiced before the advent of AAS, significant utilization of the cold-vapor technique arose during the 1960s (following the work of Hatch and Ott) and has continued, essentially unaltered in procedure, to the present. 

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. 

No spectral interferences occur with the cold-vapor technique water vapor does not absorb at the 253.7nm line but water droplets carried into the cell may result in source attenuation. As a consequence, the cell is often operated at 200°C to prevent condensation. 

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. 

These are among the most harmful pollutants in sewage. Essential elements (e.g., Fe) as well as toxic metals such as Cd, Hg, and Pb are included. Main sources of heavy metals are industrial wastes, mining, fuels, coal, metal plating, etc. Metal determinations in sewage are preferably carried out by atomic spectrometry (flame and electrothermal atomization), atomic emission spectrometry, inductively coupled plasma-mass spectrometry, stripping voltammetry, spectrophotometry, and kinetic methods. Hg is advantageously determined by the cold vapor technique and As by the hydride technique. 

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