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Method Detection Determining a Method Detection Limit when the blank value is contaminated with the analyte

Pierre writes:

Hi, Knowing that the detection limit or LOD is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value), how can you determine a Method Detection Limit when the blank value is contaminated with the analyte? The level of contamination of Mg on 10 different wipes is 222,5 µg/L (mean) with a SD of 15,8.


Pierre,

The MDL is based on a method's ability to determine an analyte in a sample matrix, regardless of its source of origin. This is based on the variance for the analysis of samples. At least 3 levels are analyzed about 10 times each and the standard deviations of the results are estimated. These standard deviations are plotted to obtain a s0 extrapolated. The MDL at 95% confidence is defined as 3 x S0. If your blanks are as precise as indicated then the MDL will still be pretty low. However, if there is a high degree of variability in the blank then this approach will point this out.

Serving you in chemistry,
 

Paul R. Gaines, Ph.D.
CEO of Inorganic Ventures & Fellow Chemist

DISCLAIMER: Advice offered by the chemists at Inorganic Ventures is intended for the individual posing the question. Feel free to contact us to verify whether these suggestions apply to your unique circumstances.