
45
CHEMICAL ANALYSIS INFORMATION
, continued
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)
Number of Test Portions
Student’s
t
Value
7
3.143
8
2.998
9
2.896
10
2.821
Sample #
Result (mg/L)
1
0.009
2
0.010
3
0.009
4
0.010
5
0.008
6
0.011
7
0.010
8
0.009
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