Hach DR/850, Procedures Manual, Page: 45 / 460

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