Article written by Sherri
McManaway, Production Chemist, Analytical Products Group
Beginning on June 1, 2005 with the
implementation of the new NELAC Field of Proficiency Testing the
reporting and the evaluation of THM’s and HAA’s has not only changed
for the reporting laboratory but also in how the PT provider
evaluates the laboratories’ data.
Prior to the Field of Proficiency
Testing (FOT) changes Trihalomethanes consisted of 4 individual
analytes (Bromodichloromethane, Bromoform, Chlorodibromomethane, and
Chloroform), in addition to the Total THM parameter. In the new
evaluation, laboratories are only asked to report values for the
four individual compounds. However, laboratories are being evaluated
for the Total THM parameter. The Proficiency Testing provider must
return an evaluation for this parameter based solely on the
laboratories performance of the four individual analytes. If one or
more of the four individual compounds receives a ‘not acceptable’
evaluation, the evaluation for Total THM must also be ‘not
acceptable’. If one or more of the four individual compounds is not
reported by the laboratory, the evaluation for Total THM’s will also
be ‘not reported’.
Haloacetic Acids (formerly known as Organic
Disinfection Byproducts) has a similar change, however even more
complicated. Haloacetic Acids samples are comprised of 6 compounds
(Bromochloroacetic Acid, Dibromoacetic Acid, Dichloroacetic Acid,
Monobromoacetic Acid, Monochloroacetic Acid, and Trichloroacetic
Acid). Laboratories are asked to report each compound on an
individual basis; they will receive evaluations for all six
compounds in addition to a Total HAA 5 parameter. The Total HAA 5
consists of five of the six compounds in the sample
(Bromochloroacetic Acid is not used for determination of Total HAA
5). Total HAA 5 will receive evaluation using the same criteria as
the Total THM. All compounds must be reported and the laboratory
must receive ‘acceptable’ results on four of the five to pass the
Total HAA 5. Thus, if one compound is not reported then the
evalution for Total HAA 5 will be ‘not reported’.
Not only does this
affect routine laboratory proficiency testing, but will affect any
remediation necessary for the laboratory. The laboratory will have
to retest for all four THMs and pass the second time. For example, a
laboratory receives a ‘not acceptable’ evaluation for Bromoform in
their routine PT testing sample; they would also receive a ‘not
acceptable’ for Total THM’s. During their remediation, the
laboratory must pass all four THMs to receive an ‘acceptable’
evaluation for the Total THMs.
Another change is in how the actual
mathematical determination of pass or fail on each analyte is
decided. In the past the parameters were ruled against either a
regression constant calculation (for HAA’s) or against fix
percentage acceptance limits (for the THMs). In the new calculations
only Bromochloroacetic acid has a fixed percentage acceptance range.
The acceptance range for the other five individual HAAs is based on
the study mean plus/minus 2 standard deviations. Limits can further
be changed if the 95% Confidence Interval is greater than 50% of the
study mean, then the limit will be set at 50%. Also if the 95% Confidence
Interval is less than 15% of the study mean the limit will be set at 15%.
These and many other changes occurred when
the new FOT became effective in June, to review the new tables
please see: http://www.epa.gov/nelac/pttables.html
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