MEMORANDUM

TO: Docket EPA-HQ-OAR-2002-0058

FROM: Peter Westlin, EPA/OAQPS/SPPD

DATE: April 10, 2012

SUBJECT: Measurement Detection Capabilities for EPA for Instrumental Test Methods

A. Introduction

The EPA prepared a memorandum to the docket1 in January 2011 in response to

comments about the proposed boiler MACT rule and specifically on measurement error 

associated with the analyzers used to quantify CO concentrations. This memorandum presents an 

expanded and updated analysis of the detection capabilities of the EPA’s instrumental test 

methods in 40 CFR part 60, appendices A1-8, including EPA Method 10 for CO.

B. Discussion

The EPA instrumental test methods are structured such that we can assess method 

detection capabilities and other data quality factors based on the test specific calibration span 

value (see http://www.epa.gov/ttn/emc/promgate/method7E.pdf). For example, the allowable 

calibration error, interference, calibration gas, and drift requirements are directly proportional to 

the site-specific instrument calibration span as defined in sections 3.3, 12.6, 13.1, 13.3, 13.4 of 

Method 7E:

• ± 2.0 percent for calibration error, 

• ± 2.5 percent for interference error, 

• ± 2.0 percent for calibration gas following the traceability protocol2

• ± 3.0 percent for drift. 

, and 

 1 Memo from Peter Westlin to Docket EPA-HQ-OAR-2002-0058, January 20, 2011

2 “EPA Traceability Protocol for Assay and Certification of Gaseous Calibration Standards” September 1997 as 

amended, EPA–600/R–97/121
For low concentration instrument calibration span values, Method 7E allows absolute

calibration error, interference, and drift values of ± 0.50 ppmv. The traceability protocol allows 

an absolute zero error of ± 0.1 ppmv in preparing calibration gas standards. An additional factor 

relative to method detection limit (MDL) for methods derived from Method 7E (i.e., Methods 

3A, 6C, 7E, 10 and 20) is that the method allows the tester to correct for half the drift determined 

for each test. This correction effectively decreases the zero or low level drift to 1.5 percent rather 

than 3.0 percent for higher level calibration span values. This measured concentration adjustment 

would be 0.25 ppmv for low level pollutant concentration measurements rather than the default 

0.50 ppmv for pollutant concentration measurements and 0.25 percent for O2 or CO2

concentration measurements rather than 0.50 percent.

We can estimate the equivalent of the MDL for a measurement with an instrumental test 

method (e.g., EPA Methods 3A, 6C, 7E, 10, 20, and 25A) including the measurement of CO 

emissions using a square root formula and the method prescribed allowable data quality criteria. 

Many of the instrumental test methods, including Method 10, refer to Method 7E for the 

performance criteria. The equations for calculating MDL for these methods are:

MDLd = ((0.50)2 + (0.50)2 + (0.25)2 + (0.10)2)

1/2

MDLs = ((0.020*S)2 + (0.025*S)2 + (0.015*S)2 + (0.020*S)2 )

1/2

Where:

MDLd = Calculated method detection limit using default allowances

MDLs = Calculated method detection limit using percent of calibration span allowances

S = Test specific calibration span value

Method 25A has method performance criteria that differ slightly from those in Method 

7E. Because of these differences, there is no default MDL value and the equation for calculating

MDL values relative to calibration span values for Method 25A measurements differs from the

equation used for Method 7E related methods:

MDLs = ((0.030*S)2 + (0.050*S)2 + (0.020*S)2

)

1/2

Table 1 shows example MDL values for each of the EPA instrumental test methods for a 

range of calibration span concentrations.
C. Applying these results to the floor and emissions limits calculations

1. Calculating the floor

Our guidance to ICR respondents for reporting pollutant emissions used to support floor

has been to provide test-specific MDL values in the reports. Also in accordance with our 

guidance, source owners are to identify emissions data which were measured below the MDL

and reported those values as equal to the MDL as determined for that test. In the database used to 

calculate the floor or emissions limit, the rule writer should replace any reported measured 

concentration value with the test-specific MDL value where the test-specific MDL is greater than 

the reported measured concentration value. Otherwise, the rule writer should use the measured 

concentration values as reported in calculating the floor (or BACT emissions limit).

Not all test reports that include instrumental test method data have included test-specific 

MDL values. In cases where the report does not include test-specific MDL data measured with

an instrumental test method, we recommend that the rule writer determine the test-specific MDL

values using the equations outlined above and the reported calibration span values.

2. Representative method detection level (RMDL) and floor adjustment

In earlier guidance to SPPD, we recommended procedures to account for the effect of 

measurement imprecision in calculating a floor using a database that includes reported MDL 

data. The first step is to define an MDL value that is representative of the data to be used in 

establishing the floor or emissions limit. This value is termed the representative method detection 

limit or RMDL. Determining the RMDL for instrumental test methods is an exercise somewhat 

different than for wet chemistry or gravimetric test methods. We can use the approach described 

above to calculate test specific MDL values to calculate a RMDL that is direclty related to the 

floor value or the applicable emissions limit.

For example, if we have established a floor value of 5 ppmv NOx from a set of ICR data, 

Method 7E would typically call for an instrumental calibration high level gas value of about 

twice that value or 10 ppmv. We would calculate the MDL for a calibration span of 10 ppmv 

using the default minimum values for instrument calibration error, measurement drift, 

interference allowance, and calibration gas error. As shown in the table below, the RMDL for 

this NOx example would be 0.76 ppmv. In another example, for a CO floor limit of 70 ppmv, 

Method 10 would call for a calibration span value of 140 ppmv. The RMDL for this example 

would be 5.7 ppmv.
The second step in the process is to calculate three times the representative method 

detection level (3*RMDL) and compare that value to the calculated floor or emissions limit. If 

3*RMDL were less than the calculated floor or emissions limit calculated from the upper 

prediction limit (UPL), we would conclude that measurement variability was adequately 

addressed and the calculated floor or emissions limit would need no adjustment. The expected 

measurement imprecision at that level is from 10 to 15 percent. This is the case for the two 

examples shown in the previous paragraph:

3*RMDL for first example, 3*0.76 ppmv = 2.3 ppmv (< 5ppmv)

3*RMDL for second example, 3*5.7 ppmv = 17 ppmv (< 70 ppmv)

If, on the other hand, the value equal to 3*RMDL were greater than the floor value or 

emission limit, we would conclude that the calculated floor or emission limit does not account 

entirely for measurement uncertainty. If so indicated, we recommend substituting the value 

equal to 3*RMDL for the adjusted floor or emissions limit (i.e., increasing the floor to be equal 

to 3*RMDL). This adjusted value would ensure measurement uncertainty is adequately 

addressed in the floor or the emissions limit.
Table 1- Example MDL values for instrumental test methods measurements

Method MDL (default) MDL (mid-range) MDL (high range)

Method 3A (O2 or

CO2)

0.76 % (@ < 19 %

O2 or CO2 span)3

1.0 % (@ 25 % O2 or 

CO2 span)

Method 6C (SO2) 0.76 ppmv

(@ < 19 ppmv span)

4.1 ppmv 

(@ 100 ppmv span)

20 ppmv

(@ 500 ppmv span)

Method 7E (NOx) 0.76 ppmv

(@ < 19 ppmv span)

1.0 ppmv 

(@ 25 ppmv span)

56 ppmv

(@ 1000 ppmv span)

Method 10 (CO) 0.76 ppmv

(@ < 19 ppmv span)

8.1 ppmv

(@ 200 ppmv span)

0.20 % (2000 ppmv)

(@ 5 % CO span)

Method 20 (NOx and 

SO2)

0.76 ppmv (@ 

< 19 ppmv span)

2.0 ppmv

(@ 50 ppmv span)

12 ppmv

(@ 300 ppmv)

Method 25A (THC) 2.5 ppmv

(@ 40 ppmv span)

12 ppmv

(@ 200 ppmv)

 3 19 ppmv or 19 percent represent the highest calibration span value that would produce an MDL of 0.76 ppmv or 

0.76 percent, respectively, using the Method 7E default calibration error, drift, interference, and calibration gas

allowances.