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Metals Questions

Test Methods:  Frequently Asked Questions

Duplicates or Spike Duplicates for Method 6010B

Question: Should sample duplicates or sample spike duplicates be performed when using ICP AES Method 6010B?

Answer: Sec. 8.4 of Method 6010B details the frequency of sample duplicates and matrix spikes. It is generally recognized that either a sample/sample duplicate or a matrix spike/matrix spike duplicate (MS/MSD) may be performed for each analytical batch or every 20 samples, whichever is more frequent. The MS/MSD provides more information on the precision of the method than does the sample/sample duplicate when the target analytes are not present in the samples. While one interpretation of Sec. 8.4 may be that the sample, sample duplicate, matrix spike, and matrix spike duplicate must be analyzed, EPA's intent is that either the sample and the sample are analyzed, or the sample and the MS/MSD are analyzed, but not both. With respect to Chapter One (Secs. 3.4.1 and 4.4.3) and other determinative methods for metals, the analysis of both a sample/sample duplicate and an MS/MSD is not generally required.

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Method 6020 and Calibration Verification

Question: The references to several of the solutions are very confusing in Method 6020. It appears the initial calibration verification (ICV) solution in Sec. 5.7 should be the solution referenced in Sec. 7.9. However, Sec. 7.9 refers to a calibration verification solution from Sec. 5.6, which is actually the interference check solution. Sec. 7.8 references the calibration verification solution from Sec. 5.7, which seems correct. Sections 8.8.1, 8.8.2 and 8.8.3 also seem to want to verify calibration with the interference check solution. How should these sections be read?

Answer: You are correct, the text in Sec. 7.9 should refer to the ICV solution as being in Sec. 5.7, not Sec. 5.6. This is a simple typographical error, and since the name of the solution is also given in the text, it is reasonably clear what is really intended.

Sec. 8.8.1 correctly refers to the ICV as being in Sec. 5.7.

Sec. 8.8.2 is incorrect in two ways. First, it refers to the solution used to verify the calibration as the "instrument check standard." This reference could be seen as meaning the interference check standard. The problem is compounded by a bad citation for the verification standard, again to Sec. 5.6, which is the location of interference check standard. Clearly, what is intended in Sec. 8.8.2 is that the analyst use the initial calibration verification (ICV) solution to verify the calibration.

Sec. 8.8.3 cites no section numbers, but refers to both the initial calibration verification solution and the instrument check standard. Clearly, the ICV is the solution in Sec. 5.7. However, in this case, the "instrument check standard" should be the interference check standard in Sec. 5.6.

We had hoped that all of this was cleared up in the latest version of the method, 6020A, in Update IVA. By and large it is. However, there are still two errors in that version. Because the newest version of the method uses the EPA EMMC format, there are changes to section numbers in general.

Sec. 11.6 of 6020A addresses the calibration verification. It refers to the calibration verification solution, but the citation is to Sec. 7.6, which is the interference check solution. (The old Sec 5.0 became Sec. 7.0 in the EMMC format and clearly, the bad citation to Sec. 5.6 was simply updated to 7.6, still in error).

Secondly, the QC section is now Sec. 9.0, and Sec. 9.8.2 (analogous to the old 8.8.2) refers to the "instrument check standard" in Sec. 7.6. This is also a holdover from the bad terminology in the old Sec. 8.8.2. The solution used is the ICV, and is now in Sec. 7.7, not 7.6.

EPA will correct these errors in the next revision of the method.

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Microwave Digestion for Mercury

Question: Mercury is listed as an analyte in both Draft Update IVA Method 3015A and Method 3051A. Can digests from those two methods be used directly on an automated mercury analyzer without any further sample treatment (elimination of the aqua-regia and permanganate digestions)?

Answer: Sec. 1.1 of Method 3015A lists mercury, as does Sec. 1.1 of Method 3051A and Sec. 1.1 of Method 3052. Samples containing mercury may be digested using the conditions described in these methods. Data from validation of the microwave procedures may be found in the methods as well as the references listed below.

Kingston, H. M. Skip, Walter, P. J., Chalk, Stuart, Lorentzen, Elke, Link, Dirk, "Environmental Microwave Sample Preparation: Fundamentals, Methods and Applications," Chapter 3, in Microwave Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications, ACS Professional Reference Book Series, American Chemical Society, Washington, DC, 1997.

Link, Dirk D., Peter J. Walter, and H. M. Skip Kingston, "Development and Validation of the New EPA Microwave-Assisted Leach Method 3051A," Environmental Science and Technology, Vol. 32, p. 3628-3632, 1998.

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Triplicate Sample Aliquots for Mercury Analysis by Method 7471A

Question: Sec. 7.1 of Method 7471A says to weigh out triplicate 0.2-g portions of the sample and place them in the bottom of a BOD bottle. Does this mean weighing 0.2 g into each of three bottles, or one bottle with three 0.2-g aliquots?

Answer: One bottle. The language in the method was revised for Method 7471B (in Update IVA), where Sec. 11.1 (in the EMMC format) directs the user to weigh out 0.5 to 0.6 g of a well-homogenized sample. We believe that the text in Method 7471A specified triplicate weighings in order to achieve a more homogeneous subsample of the original field sample. If you are not required to use the promulgated version of the method, 7471A, then we suggest that you use the latest version, 7471B. If you must use Method 7471A, then you need only add the three 0.2-g portions to a single BOD bottle.

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