Article written by
Wade DeLong, Lab Manager, Analytical Products Group
The implementation of the new NELAC Fields of Proficiency Testing (FOT) Tables on June 1, 2005
brought some confusion with the evaluation of Total Trihalomethanes (TTHMs). The NELAC FOT did not
fully reflect the requirements of the Disinfection Byproduct Rule (40 CFR 131 (b) (2)) for
evaluation of PT results. The NELAC PT Board and US EPA Office of Ground Water and Drinking Water
have reached an agreement on the evaluation of TTHMS PT results.
The “Total Trihalomethanes” parameter name will be removed from the FOT. A laboratory must
successfully analyze all four individual trihalomethane compounds to maintain accreditation for
TTHMS analysis. Laboratories are not required to report a numerical result for Total Trihalomethanes.
For a more in depth explanation of the Disinfection Byproduct Rule PT requirements, refer to the October
2005 APG newsletter article Changes in the reporting and evaluation of Trihalomethanes (THM’s) and
Haloacetic Acids (HAA’s).
The maximum contaminant level (MCL) for arsenic is being reduced to 10 ug/L on January 23, 2006,
along with this new MCL comes a new PT requirement. To obtain or maintain accreditation for arsenic,
federal regulations (40 CFR 141.23) require a laboratory to successfully analyze a PT study having a
concentration greater than 5 ug/L and an acceptable range of ± 30% of the true value.
The NELAC FOT has
been revised to include this revision to arsenic PT requirements.
PT providers are required to implement these changes by January 23, 2006; therefore the January 2006 WS
study will incorporate these changes.
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