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30 Mercury Analysis


Figure 1: An overview of cold vapour atomic absorption Figure 1b: Hydra II AACVAA Mercury Analyzer


that the total mercury in 1 ml of a sample that contains 5 ppt (ng/L) of mercury is only 0.005 ng - which is right at the detection limit for direct analysis.


In contrast 5ppt is a concentration that is trivial to measure by CVAFS. When dealing with solid samples however, the sensitivity difference is quite small since the digestion required to put the sample into solution typically introduces a significant dilution.


Figure 3 shows an overview of the direct analysis technique. With this approach, a weighed sample is introduced into the decomposition furnace with oxygen (or air) flowing over the sample. The furnace temperature is ramped in two stages; first to dry the sample and then to decompose it. As the evolved gases are released, they are carried into a catalyst where further decomposition occurs and elemental mercury is released. When the gas stream leaves the catalyst elemental mercury is captured on the surfaces of a gold amalgamation trap. After the sample’s mercury has been collected, the gold trap is heated and the accumulated mercury proceeds to an atomic absorption detector for quantitation.


ICP or ICP-MS


Although some analysts prefer to utilize inductively coupled plasma mass spectrometry (ICP-MS) for the determination of mercury, it does involve special sample handling including the addition of small amounts of gold to the sample to expedite baseline recovery. And, the cost of such equipment can be as much as 3 to 5 times higher than dedicated CVAAS or CVAAF systems. Note: Although inductively coupled plasma optical emission spectrometry (ICP-OES) based instruments can be used to measure mercury, trace level analysis is problematic due to poor sensitivity.


Which Technique is right for you?


Selecting the right technique really depends on your analytical needs. For some laboratories, the decision will be driven solely by the need to comply with a specific regulatory method. For example, if your lab is required to analyse samples using EPA method 245.1, then you will need to use the technique of CVAAS. If you are required to follow specific regulatory methods you may find Table 1 helpful as a reference.


If your lab is not constrained by a regulatory method, the driving force for the decision will more likely be criteria such as:


• the characteristics of your sample matrix (i.e. solid or a liquid),


• the detection limits you need to reach in that matrix • your preferences regarding digesting the sample or not • your budgetary constraints.


Figure 3: An overview of direct analysis using thermal decomposition. AET October/November 2011 www.envirotech-online.com Figure 2: An overview of cold vapour atomic fluorescence


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