Chemical Analysis
Chemical Analysis
5940022ChemAnal.fm Page 43
3. Analyze at least seven portions of the laboratory standard and record
each result.
4. Calculate the average and the standard deviation(s) of the results.
5. Compute the MDL using the appropriate Student’s t value (see table below)
and the standard deviation value:
MDL = Student’s t x s
For example:
The EDL for measuring iron using the FerroZine
®
method is 0.003 mg/L. An
analyst accurately prepared 1 liter of a 0.010 mg/L (about 3x the EDL) laboratory
standard by diluting a 10-mg/L iron standard in iron-free deionized water.
Eight portions of the standard were tested according to the FerroZine method
with the following results:
Using a calculator program, the average concentration = 0.010 mg/L and the
standard deviation (s) = 0.0009 mg/L
Based on the USEPA’s definition, calculate the MDL as follows:
MDL for FerroZine method = 2.998 (Student’s t ) x 0.0009 (s)
MDL = 0.003 mg/L (agrees with initial estimate)
Note: Occasionally, the calculated MDL may be very different than Hach’s estimate of the detection
limit. To test how reasonable the calculated MDL is, repeat the procedure using a standard
near the calculated MDL. The average result calculated for the second MDL derivation
should agree with the initial calculated MDL. Refer to 40 CFR, Part 136, Appendix B
(7-1-94), pages 635–637 for detailed procedures to verify the MDL determination.
Run a laboratory blank, containing deionized water without analyte, through
the test procedure to confirm that the blank measurement is less than the
calculated MDL. If the blank measurement is near the calculated MDL, repeat
the MDL procedure using a separate blank for analysis for each portion of
standard solution analyzed. Subtract the average blank measurement from
Number of Test Portions Student’s t Value
7 3.143
8 2.998
9 2.896
10 2.821
Sample # Result (mg/L)
10.009
20.010
30.009
40.010
50.008
60.011
70.010
80.009
MDL Students t s×=
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