2.3 Specialized Sample Introduction Techniques and Analysis
2.3.2 Mercury Cold Vapor
Inorganic and organic forms of mercury are ubiquitous in the environment, including water and food, and comes primarily from the burning of coal. As a result, it is necessary to detect the concentration of mercury to assess the danger caused by this toxin. Mercury is a neurotoxin and extremely small concentrations (part per billion or part per trillion) and can have detrimental effects due to bioaccumulation in the food chain (increases in concentration as one goes from one tropic level to the next). Several fish species, located in streams downwind from coal burning regions contain significant, and in some cases, dangerous concentrations of Hg. Flame AAS techniques only yield detection limits of approximately one part per million which is inadequate for environmental and food monitoring. The cold vapor technique described below yields detection limits in the parts per trillion range. Equal or even lower detection limits can be obtained by ICP-MS (the subject of Chapter 3 and 4) and atomic fluorescence techniques (a more advanced technique not included in this textbook).
An overview of the cold vapor system is shown in Animation 2.5 where an external glass vessel is used to generate elemental (and volatile) mercury that is passed through a Pyrex/quartz cell placed on the standard burner head. No flame is needed to atomize the mercury, hence the name “cold” vapor technique.
A sample containing digested water, sediment, or tissue that contains cationic mercury (Hg2+) is added to the external glass vessel, which is then closed and purged with argon to remove any oxygen. Next, SnCl2 is added via a syringe to reduce Hg2+ to elemental Hg. The elemental Hg is stripped from the water solution and passes as a pulse of vapor through the sample cell. The instrument is operated in the absorption mode; a hollow cathode Hg source lamp provides a specific wavelength to be absorbed by Hg(g) in the cell. After detection, the mercury vapor passes through a potassium permanganate solution to convert the mercury vapor back to ionic mercury so that no mercury is released into the laboratory environment or into the natural atmosphere. All blanks, external standards, and samples must be analyzed in the same manner. A drawback to this method is that samples must be processed individually, without automation. For each new sample, the argon stream must be interrupted to allow addition of a new sample to the glass container, which must then be purged with argon.
The advantage of the technique is a three order of magnitude improvement in the detection limit. Disadvantages are labor costs associated with digestion and manual instrumental analysis. When numerous samples are routinely analyzed this technique has been replaced with ICP-AES (with a detection limit of 1 part per billion) and ICP-MS (with a detection limit of less than 10 parts per trillion). Cold vapor mercury analysis is still commonly used when mercury is the only metal of interest and economics does not support the purchase and maintenance of an ICP-MS system. The reader should review Animation 2.4 at this time.
Animation 2.4 Illustration of a Cold Vapor Mercury Sample Introduction System.
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