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Chemical Analysis
3.4 Method Performance
3.4.1 Estimated Detection Limit (EDL)
3.4.2 Method Detection Limit (MDL)
Chemical Analysis
Page 42
8. Some purchased standards may be very acidic and will not work directly
with Hach procedures. Adjust the pH of these standards as described above.
Adjust the final concentration of the standard for the dilution. The Hach
standard solutions suggested in the procedures are formulated so that no pH
adjustment is necessary.
Ranges for chemical measurements have limits. The lower limit is important
because it determines whether a measurement is different from zero. Many
experts disagree about the definition of this detection limit, and determining it
can be difficult. The Code of Federal Regulations (40 CFR, Part 136, Appendix B)
provides a procedure to determine the "Method Detection Limit" or MDL. The
MDL is the lowest concentration that is different from zero with a 99% level of
confidence. A measurement below this MDL is highly suspect.
The MDL is not fixed; it varies for each reagent lot, instrument, analyst, sample
type, etc. Therefore, a published MDL may be a useful guide, but is only accurate
for a specific set of circumstances. Each analyst should determine a more
accurate MDL for each specific sample matrix using the same equipment,
reagents, and standards that will routinely be used for measurements.
Hach provides a sensitivity value (concentration change equivalent to an
absorbance change of 0.010 abs) as an estimate of the lower detection limit of
each test. The sensitivity value may be treated as an EDL for the purposes of
MDL determination. It can be considered a good starting concentration when
determining a MDL. Do not use the EDL for MDL. The conditions for MDL
determination must be exactly the same as the conditions used for analysis. The
EDL may be useful to the analyst as a starting point in determining a MDL, or as
a way to compare methods. Measurements below the EDL may also be valuable
because they can show a trend, indicate the presence of analyte and/or provide
statistical data. However, these values have a large uncertainty.
This method is in accordance with the USEPA definition in 40 CFR, Part 136,
Appendix B in the 7-1-94 edition. The USEPA defines the method detection limit
(MDL) as the minimum concentration that can be determined with a 99% level of
confidence that the true concentration is greater than zero. Since the MDL will
vary from analyst to analyst, it is important that the MDL be determined under
actual operating conditions.
The procedure for determining MDL is based on replicate analyses at a
concentration 1 to 5 times the estimated detection limit. The MDL value is
calculated from the standard deviation of the replicate study results multiplied
by the appropriate Student's t value for a 99% confidence interval. For this
definition, the MDL does not account for variation in sample composition and
can only be achieved under ideal conditions.
1. Estimate the detection limit. Use the Hach sensitivity value stated in the
Method Performance section of the analysis procedure.
2. Prepare a laboratory standard of the analyte, 1 to 5 times the estimated
detection limit, in deionized water that is free of the analyte.
5940022ChemAnal.fm
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