1. An Update to the 40 CFR Part 136 Method Detection Limit
The New MDL Procedure
An Update to the 40 CFR Part 136
Method Detection Limit
Procedure
Richard Burrows
TestAmerica Inc.
TNI Chemistry Expert Committee
Jerry Parr
The NELAC Institute

2. Presentation Summary Part 1 – History of the MDL
Part 2 – What the MDL is
Part 3 – Problems with the MDL
Part 4 – Details of the new procedure
Part 5 – How the modifications improve the procedure

3. History of the MDL

5. Developments Since 1983
1984 MDL is promulgated in 40 CFR Part 136, Appendix B for use in the wastewater program and defined as 3.14 times the standard deviation of seven low level spiked blanks.
1985 The MDL is widely adopted by other programs within EPA and written into many state and federal regulations.
1999 USEPA published Method 1631B for analysis of mercury using the old MDL approach, which provided an opportunity for a legal challenge.
2000 USEPA entered into a settlement agreement with the Alliance of Automobile Manufactures, Chemical Manufacturer’s Association, Utility Water Act Group and AFPA.
water quality based effluent limitations

6. And even more….
2005 Federal Advisory Committee on Detection and Quantification (FACDQ) as a result of the Settlement Agreement
2007 FACDQ completes their work issuing a final report with recommendations
2010 TNI forms Chemistry Expert Committee to address Calibration, Detection, Quantification and other measurement issues.
2013 Chemistry committee completes work on a MDL revision and submits to EPA
2015 EPA publishes revised MDL as part of a Methods Update Rule

7. What is the MDL?

8. Lloyd Currie’s Original Concepts
LC The lowest result that can be reliably distinguished from a blank
LD The lowest amount present in a sample that will reliably give a result that is above LC
LQ The lowest amount that gives quantitative results
EPA: The minimum measured concentration of a substance that can be reported with 99% confidence that the measured concentration is distinguishable from method blank results
Analytical Chemistry: 1968

9. MDL and Currie’s LD MDL LD
Currie’s LD is the minimum true concentration that is reliably detected (i.e., gives a result above the MDL)
MDL LD
1% chance of false negative
MDL
LC LD

10. What the MDL is (and is not):
MDL = Lowest result that can be distinguished from blanks
Or, lowest result that means there is actually something in the sample
MDL ≠ Lowest amount in a sample that can be reliably detected

11. What does this mean regarding verification?
MDL can be verified by examining blank results
MDL cannot be verified with spiked samples

12. Problems with the Current MDL

13. Is the Current MDL Procedure a Viable LC?
Assumptions in the MDL
1. There is no blank contamination
2. Short term standard deviation is the same as long
term standard deviation
3. A response at the MDL indicates analyte detection

14. Blank Bias
Current MDL assumes blank results are centered around zero
If blanks are not centered around zero, then the MDL will be too low and many false positives will result.
MDL
3.14 x Standard Deviation of 7 spikes
MDL 40 CFR Part 136
LC Currie’s Critical Level

15. MDLs Calculated Below Blank Levels
Source: EPA Memo January 3, 2013

16. Procedure Does Not Reflect Long-Term Variance

17. Calculated MDLs Below Level of Detection
Data from EPA MDL Study of Method 524.2

18. THE NEW PROCEDURE

19. Fundamentals Stay the Same
Definition is unchanged
What is the lowest result that is qualitatively reliable, i.e., the lowest result that reliably indicates the analyte is in the sample?
Calculation is unchanged
Describe the distribution as Student’s t times the standard deviation of results

20. Required Elements of the Procedure
Reflect current operating conditions
Incorporate entire analytical process
New: Include data from low-level spikes and method blanks analyzed over multiple days
New: Include criteria for evaluating false positives in blanks
New: Include criteria for evaluating qualitative identification
Calculate the MDL as Student t X SD
New: Include SD of blanks

21. The Procedure Estimate an initial MDL Determine the initial MDL
Evaluate the results
Compute the MDL
Collect additional data periodically
Verify annually
Initial
On-going
The 10X rule is gone!
MDL may be determined in specific matrix

22. 1. Estimate an Initial MDL
The mean plus three times the standard deviation of a set of method blanks, OR
The concentration value that corresponds to an instrument signal/noise in the range of 3 to 5, OR
Three times the standard deviation of spiked blanks, OR
That region of the standard curve where there is a significant change in sensitivity, OR
Instrumental limitations, OR
Previously determined MDL.

23. 2. Determine the Initial MDL
Select a spiking level, typically 2 – 10 times the estimated MDL. *
Analyze a minimum of 7 spikes and 7 blanks.
Include at least three batches on three separate days.
Existing data may be used if generated within the last 2 years.
Samples must be distributed across all of the instruments.
A minimum of two spikes and two blanks on different days for each instrument.
* Spiking levels in excess of 10 times the estimated detection limit may be required for analytes with very poor recovery

24. 3. Evaluate the Results
If any result from the spiked samples does not meet the qualitative identification criteria* or does not provide a result greater than zero then repeat the spikes at a higher concentration.
* A set of rules or guidelines for establishing the identification or presence of an analyte.  Qualitative identification does not ensure that quantitative results can be obtained.

25. 4. Calculate the MDL MDLb = X + Sb * t of blank results
MDLs = Ss * t of spike results
Reported MDL = Greater of MDLb or MDLs
If none of the method blanks give numerical results for an individual analyte, the MDLb does not apply.
If some (but not all) of the method blanks for an individual analyte give numerical results, set the MDLb equal to the highest method blank result.
If more than 100 method blanks are available, set MDLb to the level that is no less than the 99th percentile of the blank results.

26. 5. On-going Data Collection
At least one spike and one blank on each instrument per quarter
At least 7 spikes per year
At least 7 blanks per year
If more than 5% of the spikes do not return positive numerical results that meet all identification criteria, then the spiking level must be increased and the initial MDL re-determined

27. 6. Annual Verification
Recalculate MDLb and MDLs from blank and spike results.
Include all data over a two-year period.
If the verified MDL is within a factor of 3 of the existing MDL, and fewer than 3% of the method blank results have numerical results above the existing MDL, then the existing MDL may be left unchanged. Otherwise, adjust the MDL to the new verification MDL.

28. An Improved Procedure Accounts for Problems in Current Procedure
Blank contamination
Long-term variance
Actual detectability
Ensures Scientific Reliability
Benefits Labs and Data Users
Valid MDLs should not change much

29. Accounts for Blank Bias
Sensible MDLs when there is blank bias
1980 Lead in tuna results overstated by 1000X due to blank contamination
2004 EPA Episode 6000 data Chromium by ICPMS, 1400% recovery at the MDL and 600% recovery at the ML due to blank bias
2013 Multi-lab blank detection rates
8270 SIM 6.4%
8921B 16%
ICPMS 8%
2014 Lead in particulate matter
All blanks in the validation study exceeded the MDL
This problem is getting worse because of the need for low level data and increasing sensitivity of instrumentation

30. Accounts for Variance Long term vs. short term bias
The difference varies from method to method and lab to lab, but can be large
Long term bias is what matters when it comes performance

31. Ensures Scientific Reliability
Low level spikes must meet qualitative criteria for identification
Low level spikes must be greater than the mean of blank results

32. What does this mean to labs?
Clear requirements
Scientifically reliable MDLs
Level playing field
Low transition costs since existing data can be used
Some additional organizational requirements

33. What does this mean to data users?
MDLs that make sense
Much lower rate of false positives, especially for ICP, ICPMS and some general chemistry tests
Easier to compare labs
In general, more reliable data = better decision making

34. How much will MDLs change?
Analytes with minimal or no detects in blanks, e.g., most GC/MS analytes at normal levels
Not Much
Analytes with frequent detects in blanks, e.g., metals, low level PAH, general chemistry tests
Depends
If the lab is currently adjusting MDLs to avoid excessive false positives
Not much
If the lab has been pushing MDLs below levels justified by the blanks
Quite a bit

35. Status of the Procedure
Expected to be final later this year
Only minimal changes are expected
New TNI Standard encourages use of this procedure
Labs could begin implementing now, except where not allowed

36. TNI Chemistry Committee 2010-2015
Thank You
TNI Chemistry Committee
Richard Burrows, Chair TestAmerica Laboratories
Dan Dickinson
New York DOH
Francoise Chauvin New York City DEP
Brooke Connor USGS
Anand Mudambi US EPA OSA
Tim Fitzpatrick
Florida DEP
Nancy Grams
Advanced Earth Technologists
Lee Wolf
Columbia Analytical Services
Scott Siders Illinois EPA
John Phillips Ford Motor Company
Gary Ward Oregon DOH
Valerie Slaven Teklab Inc.
Colin Wright Florida DEP
Gale Warren New York DOH
91 meetings