Ask The Expert Webinar Series

EPA's Revision to the 40 CFR Part 136 Method

Detection Limit (MDL) Procedure

Richard Burrows, Ph.D. – Corporate Technical Director

A Revision to the Method

Detection Limit

EPA finalized a revision to the 40

CFR Part 136 MDL procedure in the

Federal Register

This is a final rule published on August 28th

Effective September 27th 2017

Presentation Summary

• Part 1 – History of the MDL

• Part 2 – What the MDL is

• Part 3 – Problems with the MDL

• Part 4 – Why we need a MDL

• Part 5 – Details of the modifications

• Part 6 – How the modifications improve the procedure

• Part 7 – What does this mean to labs and data users?

History of the MDL

Further developments

1984 USEPA the MDL procedure 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. The ML is also promulgated at this time.

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 and modified ML definition, which provided an opportunity for a legal challenge of the MDL and ML.

2000 USEPA entered into a settlement agreement with the Alliance of Automobile Manufactures, Chemical Manufacturer’s Association, Utility Water Act Group and AFPA.

Further Developments

2002 USEPA issues a Technical Support

Document of Detection and Quantitation

Regulations under the Clean Water Act

(TSD).

2003 Draft revised MDL published

2003 Consensus letter submitted to Assistant

Administrator of Office of Water signed by 31

parties urging EPA to consider a scientifically

sound approach to the detection and

quantification issue.

Yet More

2005 Federal Advisory Committee on Detection

and Quantification (FACDQ) formed by

USEPA Office of Water as a result of the

2000 Settlement Agreement

2007 FACDQ completes their work issuing a final

report with recommendations, with Office of

Water to complete a post FACDQ pilot study

based on FACDQ recommendations.

And even more….

2010 TNI forms Environmental Methods Measurement Expert Committee based on a USEPA grant to address Calibration, Detection, Quantification and other measurement issues.

2011 Final report on Post FACDQ pilot study issued, recommending further evaluation with additional methods and analytes.

2013 TNI EMEC (renamed Chemistry committee) completes work on a MDL revision and submits to EPA

2014 EPA completes internal review of the revised MDL and makes minor modifications

2015 EPA publishes revised MDL as part of a Methods Update Rule

2017 Signed by EPA Administrator Scott Pruitt

What is the MDL?

Lloyd Currie’s original

concept

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

Not routinely used in environmental testing

(included in the DOD QAPP)

Equals the MDL equals the TNI LOD

Conceptually, equals TNI LOQ, EPA ML and EPA LLOQ

MDL

MDL The method detection limit (MDL) is defined as the

minimum measured concentration of a substance that can be

reported with 99% confidence that the measured concentration

is distinguishable from method blank results

MDL

0 MDL 40 CFR Part 136

0 LC Currie’s Critical Level

3.14 x Standard Deviation

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

MDL and Currie’s LD

Currie’s LD is the minimum true concentration that is

reliably detected (i.e., gives a result above the MDL)

LD (LOD)

0 MDL -- 40 CFR Part 136

0 LC LD Currie’s Detection Level

0 DL (LODDOD) DOD

MDL

1% chance of false negative

What does this mean

regarding verification?

• MDL can be verified by examining blank results

• MDL cannot be verified with spiked samples

• (Curries LD could be verified with spiked samples)

Problems with the Old MDL

Blank bias

Old 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

0 MDL 40 CFR Part 136

0 LC Currie’s Critical Level

3.14 x Standard Deviation of 7 spikes

Lead in Particulate Matter

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7

Ultrasonic extraction Quartz filter blanks

Blank result

MDL(S)

X+ts

Variance and Verification

• Old MDL assumes that short term and long term

variance are the same

• Variability of instrument response in one batch is

the same as variability of instrument response

over the course of a year???

• Old MDL has no verification that results obtained

are reasonable

Why Do we need a MDL?

Reason #1 that we need MDLs

We need to make the Quantitation limit meaningful

• Applies to MRL, LLOQ, or any quantitation limit

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7 8

Re

po

rte

d C

on

cen

tra

tio

n

Replicate Number

9

Mean recovery

90% recovery, 9% RSD

Spike #1 #2 #3 #4 #5 #6 #7 Mean MDL

10 9 8.3 9.8 9.3 8.1 8.6 10.0 9.0 2.29

Spike True Value = LLOQ

Calculated MDL MDL = 2.29

Reported results

(without MDL)

ND ND ND ND ND ND 10

MDL ZERO LOQ

If you run 100 spikes at LLOQ…

What if you have 70% average recovery?

Assume 10% RSD

Now 99% False

Negative Rate

Reason #2 that we need MDLs

MDLs are needed in risk assessment

• Handling non-detects

~ Substitute a value such as ½ detection limit or

detection limit

~ More sophisticated methods such as Maximum

Likelihood estimation and Regression on Order

statistics

− These still benefit from a detection limit as low as

possible

If we do not have a detection limit, the Quantitation

limit will become the new Detection limit

Details of the Modifications

First, what stays the same?

• Fundamental concept 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?

• Fundamental approach is unchanged

• Describe the distribution as Student’s t times the

standard deviation of results

Revised MDL Procedure

Flow Chart

Run 7 low level

spikes

Run 7 method

blanks

Do

results

meet

qual ID?

Calculate MDLs Calculate MDLb

Set higher of

MDLs and MDLb

as MDL

Fix problem or

raise spike level

and repeat

No

Yes

INITIAL DETERMINATION OF MDL

Spikes and Blanks

Run 7 low level

spikes

Run 7 method

blanks

Do

results

meet

qual ID?

Calculate MDLs Calculate MDLb

Set higher of

MDLs and MDLb

as MDL

Fix problem or

raise spike level

and repeat

No

Yes

INITIAL DETERMINATION OF MDL

Qualitative identification

Run 7 low level

spikes

Run 7 method

blanks

Do

results

meet

qual ID?

Calculate MDLs Calculate MDLb

Set higher of

MDLs and MDLb

as MDL

Fix problem or

raise spike level

and repeat

No

Yes

INITIAL DETERMINATION OF MDL

Qualitative identification

MDLS = Students t x Standard Deviation of the spikes

MDLB = Mean of blanks + Students t x Standard Deviation of the Blanks

Run 7 low level

spikes

Run 7 method

blanks

Do

results

meet

qual ID?

Calculate MDLs Calculate MDLb

Set higher of

MDLs and MDLb

as MDL

Fix problem or

raise spike level

and repeat

No

Yes

INITIAL DETERMINATION OF MDL

Compare MDLS and MDLB

Run 7 low level

spikes

Run 7 method

blanks

Do

results

meet

qual ID?

Calculate MDLs Calculate MDLb

Set higher of

MDLs and MDLb

as MDL

Fix problem or

raise spike level

and repeat

No

Yes

INITIAL DETERMINATION OF MDL

Details, details

• Spiking level

• 2-10 times estimated MDL

• Run spiked replicates in at least 3 separate

preparation and analysis batches

• Multiple instruments

• At least 2 spike replicates on each instrument

• If blanks give ND, MDLB does not apply

• Addendum for MDL determined on a specific

matrix

• No 10X rule

• Use all method blanks unless batch was rejected

Does the 10X rule protect

against MDLs that are too low?

True 1 2 3 4 5 6 7

100 100 103 99 102 97 98 102

Results within =/- 3%

Mean RSD Std Dev MDL

100 2.3% 2.3 7.1

True 1 2 3 4 5 6 7

1.0 1.0 1.4 0.8 1.3 0.7 0.8 1.3

Results within =/- 40%

Mean RSD Std Dev MDL

1.0 29% 0.29 0.90

Ongoing verification

Quarterly date collection /

verification

36

Analyze at least one spike

on each instrument (2 if only

one instrument)

Do results meet

qualitative ID?

No actions needed

Correct problem

and repeat or

repeat initial at

higher

concentration

No

Yes

Annual recalculation

37

Collect Spike Data

and Recalculate

MDLS

Collect Blank Data

and Recalculate

MDLB

Is the newly

calculated MDL

within 2X of the

existing MDL?

Change MDL to

the newly

calculated MDL

Option: Leave the

MDL unchanged

or change to the

newly calculated

MDL

NO YES

Method blank Example -

Ammonia

38

0.016 (99th)

0.014 (last 50)

0.013 (all data)

4 results >

MDLs; MDLs

not valid

Why didn’t we do…..

Calibration design

LOD and MDL

Signal to noise

LOQ

Remove spiking requirement when blanks work

Calibration design

Hubaux-Vos

ASTM D6091

• H-V assumes constant variance – does not work

• Complex, D6091 depends on calibration fit, assumes minimal

calibration error

• Labor intensive – only really works if detection limit is very

stable

• Problems with positive blank bias

LOD and MDL

Pros

• More comprehensive description of detection

• Technically sound

Cons

• Difficult to properly estimate LD

• Pretty close to typical LQ

Signal to noise

• No consideration of sample prep

• No consideration of blank bias

Limit of Quantitation

LOQ

TNI LOQ

1.5.2.2.2 Ongoing verification of the LOQ

The laboratory shall prepare and analyze a minimum of one (1)

LOQ verification sample spiked at the same concentration as the

initial LOQ verification on each instrument during each quarter in

which samples are being analyzed for each quality system matrix,

method, and analyte.

a) Results of each LOQ verification sample analysis shall be

evaluated at the time of the testing and shall meet the qualitative

identification criteria in the method and laboratory Standard

Operating Procedure (SOP) and the quantitated result shall be

greater than the DL and meet the laboratory established

accuracy criteria.

Remove spiking

requirement when blanks

work

• Good idea!

• Maybe a next step

How the modifications

improve the procedure

• 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

How the modifications

improve the procedure

• 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 to

the MDL

• Ongoing verification

• Very consistent with EPA office of Water MRL,

EPA ORCR LLOQ and the proposed TNI LOQ

What does this mean to

labs?

• Clear requirements

• Sensible MDLs

• Level playing field

• Low transition costs since existing data can be

used

• Some additional organizational requirements

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

How much will MDLs

change?

• Analytes with minimal or no detects in blanks, eg

most GC/MS analytes at normal levels:

Not Much

• Analytes with frequent detects in blanks, eg,

metals, very low level PAH, some 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, potentially quite a bit

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EPA’S Revision To The 40 CFR Part 136 Method

Detection Limit (MDL) Procedure

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