Hach DR/850 Procedures Manual page 44

CHEMICAL ANALYSIS INFORMATION
Method Detection Limit (MDL)
Hach provides a value called the Estimated Detection Limit (EDL) for all
programs. It is the calculated lowest average concentration in a deionized
water matrix that is different from zero with a 99% level of confidence.
Specifically, it is the upper 99% confidence limit for zero concentration
based on the calibration data used to prepare the pre-programmed
calibration curve. Do not use the EDL as the 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 (see most current edition).
The USEPA defines the method detection limit (MDL) as the minimum
concentration that can be determined with 99% confidence that the true
concentration is greater than zero. Since the MDL will vary from analyst
to analyst, it is important that analysts determine the MDL based on their
unique 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 estimated detection
limit (EDL) value stated in the Method Performance section of the
analysis procedure.
2. Prepare a laboratory standard of the analyte in deionized water which
is free of the analyte that is 1 to 5 times the estimated detection limit.
3. Analyze at least seven portions of the laboratory standard and record
each result.
4. Calculate the average and standard deviation (s) of the results.
, continued
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