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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 docket 1 in January 2011 in response to comments about the proposed boiler MACT ru le 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 capabil ities 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 st ructured 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, interfer ence, calibration gas, and drift requir ements 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 cal ibration error,

 ± 2.5 percent for inte rference error,

 ± 2.0 percent for calibration gas following the traceability protocol 2 , and

 ± 3.0 percent for drift.

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 Standa rds” September 1997 as amended, EPA–600/R–97/1214/10/2012 v3

For low concentration instru ment calibration span values, Method 7E allows absolute calibration error, interfer ence, 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 lim it (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 ha lf 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 concen tration measurements rather than the default 0.50 ppmv for pollutant concentration measurements and 0.25 percent for O 2 or CO 2 concentration measurements rather than 0.50 percent. We can estimate the equivalent of the MDL fo r a measurement with an instrumental test method (e.g., EPA Methods 3A, 6C, 7E, 10, 20, a nd 25A) including the measurement of CO emissions using a square root formula and the me thod prescribed allowable data quality criteria. Many of the instrumental test methods, incl uding Method 10, refer to Method 7E for the performance criteria. The equations for calculating MDL for these methods are:
MDL d = ((0.50) 2 + (0.50) 2 + (0.25) 2 + (0.10) 2 ) 1/2

MDL s = ((0.020*S) 2 + (0.025*S) 2 + (0.015*S) 2 + (0.020*S) 2 ) 1/2
Where:
MDL d = Calculated method detection limit using default allowances
MDL s = Calculated method detection limit using percent of calibrati on span allowances S = Test specific calibration span value

Method 25A has method performance criteria th at differ slightly from those in Method 7E. Because of these differences, there is no de fault MDL value and the equation for calculating
MDL values relative to calibrati on span values for Method 25A m easurements differs from the equation used for Method 7E related methods:
MDL s = ((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.4/10/2012 v3

C. Applying these results to the fl oor 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 emissi ons 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 ru le writer should replace any reported measured concentration value with the test-specific MDL va lue where the test-specific MDL is greater than the reported measured concentration value. Othe rwise, the rule writer should use the measured concentration values as reported in calcul ating 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 le vel (RMDL) and floor adjustment In earlier guidance to SPPD, we recommended pr ocedures 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 th at is representative of the data to be used in establishing the floor or emissions limit. This va lue is termed the representative method detection limit or RMDL. Determining the RMDL for instrume ntal 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 cal culate a RMDL that is direclty related to the floor value or the applicable emissions limit. For example, if we have estab lished a floor value of 5 ppmv NO x from a set of ICR data, Method 7E would typically call for an instrume ntal calibration high leve l 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 instru ment calibration error, measurement drift, interference allowance, and calibra tion gas error. As shown in the table below, the RMDL for
this NO x example would be 0.76 ppmv. In another ex ample, for a CO floor limit of 70 ppmv, Method 10 would call for a calibration span va lue of 140 ppmv. The RMDL for this example would be 5.7 ppmv.4/10/2012 v3

The second step in the process is to calcu late three times the representative method detection level (3*RMDL) and comp are that value to the calculated floor or emissions limit. If
3*RMDL were less than the calc ulated floor or emissions limit calculated from the upper prediction limit (UPL), we would conclude th at 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*RMD L were greater than the floor value or emission limit, we would conclude that the ca lculated floor or emi ssion 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 emissi ons limit (i.e., increasing the floor to be equal to 3*RMDL). This adjusted value would ensure measuremen t uncertainty is adequately
addressed in the floor or the emissions limit.4/10/2012 v3

Table 1 - Example MDL values for instrume ntal test methods measurements

Method MDL (default) MDL (mid-range) MDL (high range) Method 3A (O 2 or CO 2) 0.76 % (@ < 19 %
O 2 or CO 2 span) 3 1.0 % (@ 25 % O 2 or CO 2 span) Method 6C (SO 2 ) 0.76 ppmv (@ < 19 ppmv span) 4.1 ppmv (@ 100 ppmv span) 20 ppmv (@ 500 ppmv span) Method 7E (NO x ) 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 (NO x and
SO 2 ) 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.