Can EPA Method 3005A be used for mercury analysis by ICP-MS (Method 6020A), if the lab demonstrates acceptable precision, accuracy, and sensitivity? Mercury is listed in the analyte list for Method 6020A, but is not listed in 3005A.
Acid digestion itself is not sufficient for mercury, so permanganate should be added to the final acid digest until a persistent purple color is obtained. Then the extract can be analyzed by CVAA as described in the Hg 7000 series. This is a key step if Method 3005A is used for preparing mercury samples for analysis by CVAA.
If mercury will be analyzed by ICP‐AES (method 6010) or ICP-MS (method 6020), addition of permanganate is not recommended. In such cases, we recommend another oxidizing agent that works at a lower concentration and does not interfere with any of the target analytes. We have successfully used 3005A and other metals digestion methods for mercury, and stabilized the mercury with 2 ppm gold chloride, followed by ICP‐AES or ICP‐MS detection. As long as the digest has a pH of 2 or less and has 2 ppm Au+3 ions, the mercury will remain oxidized and stay in solution over a long period (e.g., NIST trace‐level mercury in water reference materials prepared in this manner will still give the reference value, even though they were prepared in the 1970s). The gold also eliminates the long washout times mercury would require in an ICP system if gold were not used. Note, the gold must be at the same level in the calibration standards and samples. Also, the gold does not cause any interferences on any target analytes when using ICP‐AES or ICP‐MS.
The higher level of chlorides from a 3005A digestion can make analysis for arsenic and selenium by ICP‐MS more challenging. Use of collision cell or dynamic reaction cell technology to lower molecular ion interferences is recommended.
We recommend adding the gold under acidic conditions for any digestion, whether closed or open digestion when determining mercury by ICP-MS or ICP‐AES. It prevents deposition of mercury in the sample introduction system and prevents long wash‐out times between samples. Simply stated, gold does an excellent job as a strong oxidizer keeping mercury oxidized in its ionic state. As long as mercury is kept in its ionic state, it behaves like any other dissolved metal and will remain in solution over long periods and will not be lost due to heat or being open to the atmosphere. Without gold or another strong oxidizer like permanganate, mercury can reduce to elemental mercury at active sites on vessel walls (especially plastic) or soil particles and become vapor which may be lost through the vessel walls or in the headspace above the liquid. HCl does not totally stabilize mercury like gold does.
Other Category: Inorganic