“How Low Can We Go – A Statistical Approach to Determine Method Detection
Limits
(MDLs) for Various Metals”
Alex Shoemaker, John Gauf, Nathan Wall
NC State University
Clients: Kristen Benedict and Steffan Johnson, USEPA, OAQPS
The USEPA’s Clean Air Act requires that industrial, commercial, and institutional
sources meet hazardous air pollutant (HAP) emission standards for various metals.
Method 29 is a promulgated Federal Reference Method that ensures the analyses
of metal emissions are consistent across facilities. The purpose of this project is
to analyze paired laboratory data submitted during the rulemaking process to
determine method detection limits (MDLs). An MDL is defined as the minimum
concentration of an analyte that can be measured with confidence that is greater
than zero. This applies to ten of the Method 29 metals (antimony, arsenic, beryllium,
cadmium, chromium, cobalt, lead, manganese, nickel, and selenium). For each metal, the
precision of the paired data was plotted against its average concentration. The upper
99% confidence interval estimate for the y-intercept represents the MDL. This
statistical approach appears consistent with the MDLs that were already specified in
Method 29 and suggests that laboratory type significantly influences the data. Further,
after we determine that the method is only able to measure a certain detection limit,
we will discuss how EPA might assess variability in non-detectable values.