1-11
Plot D illustrates a problem for the analyst. Increments found are
uniform and the recovery of the standard was complete. The result of
the first analysis was 0 mg/L and the graph plots back through 0 mg/L.
If interferences are known to be present, the interference may be present
in an amount equal to the substance in question, thereby preventing the
analyst from finding the substance. This would be an uncommon
situation.
Branch J
If the standard addition gives the correct result, the analyst must then
determine if interfering substances are present. If interfering substances
are not present, the result of the analysis prior to the standard addition is
correct. If interfering substances are present, proceed to Branch C.
One of the greatest aids to the analyst is knowledge of the sample's
composition. An analyst need not know the exact composition of each
sample but should be aware of potential interferences in the method of
analysis to be used. When performing a particular method, the analyst
should know if those interferences are present or not in order to have
confidence in the accuracy of the results.
USEPA Approved
The United States Environmental Protection Agency (USEPA)
establishes limits for maximum contamination levels for certain
constituents in water. They also require that specific methodology be
used to analyze for these constituents. These methods originate from
several sources. The USEPA has developed some of these methods. In
other cases the USEPA has evaluated and approved methods developed
by manufacturers, professional groups, and public agencies such as
APHA
1
, AWWA
2
and WCPF
3
(Standard Methods for the Examination
of Water and Wastewater), ASTM
4
, USGS
5
and AOAC
6
. All USEPA-
approved methods are cited in the Federal Register and compiled in the
Code of Federal Regulations.
USEPA Accepted
Hach has developed analytical methods that are equivalent to USEPA-
approved methods. Even though minor modifications may exist, the
USEPA has reviewed and accepted certain methods for reporting
purposes. These methods are not published in the Federal Register, but
are referenced to the equivalent USEPA method.
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