As you can see, the ICP-OES technique using either the 200.5 or the 200.7 method is not approved for mercury analysis. Although mercury is listed in the analytes table in EPA Method 200.7 (downloadable PDF), mercury has a low MCL that is close to the detection limit for ICP-OES. In addition, sample preparation using above methods may evaporate mercury and lead to underestimate of the detection results. Because mercury has highly volatile, EPA Method 245.1 (downloadable PDF) using cold vapor absorption technique (downloadable application note for mercury analysis) is a preferred choice.
In addition, EPA Method 200.7 is approved for use only with cadmium testing when the preconcentration method may have to be used to get the lower detection limit to meet the MCL requirement. The question then becomes, why Method 200.5 is approved for these metals (except for mercury) but not Method 200.7? When the currently recommended version 4.4 of Method 200.7 was developed in 1994, the plasma axial view technology of ICP-OES had not been developed. When Method 200.5 was developed in 2003, the axial view technology was applied to detect trace metals with low MCL, such as arsenic, lead, and cadmium.
With addition of the axial view, Method 200.7 can also be used to analyze water samples based on the EPA’s Guidelines Establishing Test Procedures for the Analysis of Pollutants Under the Clean Water Act (link to the document, page 25). However, because the low MCL of these metals is close to the detection limit of ICP-OES, using Method 200.8 (downloadable PDF) is preferred. In Method 200.8, Collision Cell Technology (CCT) applied to ICP-MS has commonly been used to remove polyatomic interferences and lower the detection limit for different type of waters. But CCT has not been approved for drinking water analysis.
As mentioned above, for arsenic and mercury, only certain species are toxic (inorganic arsenic, elemental mercury, and methylmercury). Speciation analysis should be done to make sure we know if the toxic species are present in our water or food. One effective speciation technique is to couple ion chromatography with ICP-MS.
Resources Supporting Methods for Heavy Metal Analysis
We have several EPA-method specific applications that can be used for the analysis of heavy metals in water: