|Date:||2 Aug 2002 04:38:24 -0000|
|Subject:||[CPEO-MEF] perchlorate detection|
This is in response to the message of 31 July from Charles G. Johnson
regarding detection limits for perchlorate (see below).
I believe that one source of misunderstanding is the distinction between Method Detection Limits (MDL) and Reporting Limits (RL). Perhaps the following will help clarify the matter.
The National Guard Bureau at the Massachusetts Military Reservation (MMR) is using EPA Method 314.0 for the detection of perchlorate. Method 314.0 was published in early 2000, and as published was designed to achieve a Reporting Limit of 4.0 ug/L (ppb) and a Method Detection Limit of 0.53 ug/L (ppb).
Through a few minor modifications, the National Guard Bureau at theMMR is using Method 314.0 to achieve a Reporting Limit of 1.0 ppb and a Method Detection Limit of 0.35 ppb. These minor modifications to the method were developed and agreed to between EPA chemists and the laboratory chemists, and include steps suchas utilizing slightly lower standard values to calibrate the instrument.
Any detection results between the MDL and RL are reported as "J", or estimated, values. An estimated or "J" value means the compound isdefinitely present; however, the concentration may be slightly higher or lower than reported. Standard practice is to simply utilized the value as reported (for example, "...perchlorate was detected at 0.8J ppb...").
Even though the Reporting Limit has been lowered from 4.0 ppb to 1.0 ppb, the quality of the data remains the same.EPA Region 1 has circulated a memo on these practices within the national agency.
Joel Feigenbaum -- Joel Feigenbaum 24 Pond View Drive E. Sandwich MA 02537 (508)-833-0144 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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