MEMORANDUM

TO: Docket EPA

- HQ

- OAR

- 2002

- 0058

FROM: Peter Westlin, EPA/OAQPS/SPPD

DATE: April 10, 2012

SUBJECT: Measurement Detection C apabilities 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 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 c apabilities of the EPA ’s instrumental test

methods in 40 CFR part 60, appendices A1

- 8 , including EPA Method 10 for CO

.

B. Discussi on

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 p rotocol 2

**•** ± 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

– 97/121For low concentration instrume nt calibration span values, Method 7E allows absolute

calibration error

, interference

, and drift values of ± 0.5 0 ppmv. The t raceability p rotocol 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 met hod allows the tester to correct for half the drift determined

for each test. This correction effectively decreas es the zero or low level drift to 1.5 percent rather
than 3.0 percent for higher level calibration span values. This measured concentration adj ustment

would be 0.2 5 ppmv for low level pollutant concentration measurements rather than the default

0.5 0 ppmv for pollutant concentration measurements and 0.25 percent for O 2 or CO 2
concentration measurements rather than 0.5 0 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 qual ity criteria.
Many of the instrumental test methods, including Method 10, refer to Method 7E for the
performance criteria. The equation s 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. 0 2 0 *S) 2 + (0. 0 25 *S) 2 + (0. 0 15 *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 calibration span allowances
S = Test specific calibration span value

Method 25A has me thod 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 differ s 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

T able 1 shows example MDL values for each of the EPA instrumental test methods for a
range of c alibration 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

- spe cific 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

concentratio n 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 (R M DL) and floor adjustment

In earl ier guidance to SPPD, w e 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 i s to define a n 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 R M DL. Determining the R M DL 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 NO x from a set of ICR data

, Method 7E would typically call for an instrumental calibration high lev el 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 calibra tion error, measurement drift,

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

this NO x example would be 0.76 ppmv. In another exa mple, for a CO floor limit of 70 ppmv, Method 10 would call for a calibration spa n 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*R M DL) 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 t he calculated floor or emissions l imit would need no adjustment. T he expe cted

measurem ent im precision 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 p p mv ( < 70 ppm v )
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 sub stituting 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 (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 c alibration span value t hat 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.