Cyanide Measurements: Questions and Answers
Between 2007 and 2017, the EPA made several sets of revisions to the entries for required containers and preservation techniques in Table II at 40 CFR Part 136 regarding samples to be analyzed for cyanide. Samplers and laboratories are advised to consult the recent most version of Table II of 40 CFR 136.3.
- The ASTM D19 standard, "Standard Practice for Sampling, Preservation, and Mitigating Interferences in Water Samples for Analysis of Cyanide" appears to conflict with the footnotes in the regulation.
The version of the ASTM D19 Standard cited in Footnotes 5 and 6 of Table II since 2012 is ASTM D7365-09a. The earlier lengthy version of Footnote 6 was removed from the table in 2012 after ASTM revised D7365 to more thoroughly address cyanide sampling, preservation, and interference mitigation. Footnote 6 continues to note that the preservation steps may not work in every circumstance and concludes by stating:"Any technique for removal or suppression of interference may be employed, provided the laboratory demonstrates that it more accurately measures cyanide through quality control measures described in the analytical test method."
Thus, the footnote allows analysts the flexibility to develop more useful procedures and regulatory authorities may allow use of updated interference mitigation procedures, including the ASTM procedures, on a case-by-case basis. The “quality control” measures cited in Footnote 6 would include blanks and matrix spike samples. However, if matrix spike samples are prepared in the laboratory from samples treated in the field to remove interferences, the effects (adverse or otherwise) of the sample treatment may not be evident. Therefore, field quality control samples such as field-prepared spikes and field dilutions may be helpful in demonstrating performance of any techniques not covered in ASTM 7365-09a. - The ASTM practice specifies that samples are collected in glass or high density polyethylene, whereas the final rule allows polyethylene, fluoropolymer, and glass.
The ASTM standard states that glass or high density polyethylene (HDPE) containers are suitable. It does not prohibit the use of the fluoropolymer containers cited in Table II. Footnote 1 of Table II distinguishes between “P” for polyethylene and “LDPE” for low density polyethylene, and therefore, “P” is synonymous with HDPE. - The ASTM practice requires the sample to be held at room temperature for 4 hours prior to analysis. Is this addressed in the final rule or in some of the approved methods?
That specification applies to samples that are known or suspected of containing particulate cyanide, for example ferric ferrocyanide or Prussian blue. For samples that are not known or suspected to contain particulate cyanide, that 4-hour holding period is not required. The other interference removal techniques described in ASTM D7365-09a do not prescribe holding the sample in that same fashion. However, the 14-day holding time in Table II applies in general. Although this holding period is not specifically addressed in the approved methods, as stated at 40 CFR 136.3(e), the information in Table II (and the accompanying footnotes) takes precedence over information in specific methods or elsewhere.