[CPEO-MEF] perchlorate detection

[joelf@cape.com]

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 the
MMR 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 is
definitely 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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