Cold Vapor Mercury Technique

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

Similar to hydride generation, the mercury cold vapor technique can be applied. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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]. 

The cold-vapor technique is an atomization method applicable only to the determination of mercury because it is the only metallic element that has an appreciable... 

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 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). 

Techniques for analysis of different mercury species in biological samples and abiotic materials include atomic absorption, cold vapor atomic fluorescence spectrometry, gas-liquid chromatography with electron capture detection, and inductively coupled plasma mass spectrometry (Lansens etal. 1991 Schintu etal. 1992 Porcella etal. 1995). Methylmercury concentrations in marine biological tissues are detected at concentrations as low as 10 pg Hg/kg tissue using graphite furnace sample preparation techniques and atomic absorption spectrometry (Schintu et al. 1992). 

Cold vapor mercury Light absorbed by atoms of mercury generated by chemical reaction at room temperature is measured An excellent technique for mercury analysis... 

Why is the cold vapor mercury technique good only for mercury ... 

Mercury and its compounds may also be determined by ICP/AES. The method, however, is less sensitive than the cold vapor-AA technique. The metal also can be measured at low ppb level by colorimetry. Mercury ions react with dithizone in chloroform to show an orange color. Absorbance is measured at 492 nm using a spectrophotometer. 

Elemental composition Hg 61.80%, N 8.63%, O 29.57%. The compound dissolved in dilute hydrochloric acid and the solution diluted appropriately and analyzed for mercury by cold vapor—AA technique. The aqueous solution is analyzed for nitrate ion by nitrate ion-specific electrode or by ion chromatography. 

---