Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

1 

### Interim EPA Traceability Protocol for Qualification and Certification of Elemental Mercury Gas Generators

1.0 INTRODUCTION

For regulatory purposes, mercury (Hg) calibra tion standards of know n concentration and known uncertainty are needed to quality assure data recorded by Hg continuous emission monitoring systems (CEMS) and measurements ma de with instrumental Hg reference test methods. This interim EPA traceability protocol reflects the current state of development of procedures for generating and quantifying elemental Hg calibration standards. The procedures of this interim protocol will be replaced when a final EPA traceability protocol for elemental Hg generators is issued.

This interim traceability protocol focuses on tw o basic issues. It pr ovides procedures to: (1) establish the quantitative ou tput (i.e., “calibrati on”) of elemental Hg generators; and (2) determine the expanded uncertainty of the gas standards produced by the elemental Hg generators. This protocol specifies th e maximum allowable uncertainty for the gas standards. Demonstrating that a particular generator meets the requirements contained in this document establishes that the generator out put is traceable to the National Institute of Standards and Technology (NIST) by means of an unbroken chain of comparisons of the generator to a NIST certified re ference generator. For CEMS applications, this protocol applies only to Hg monitoring system span values greater th an or equal to 5.0 micrograms per cubic meter (μg/m 3 ).

Section 2, “Overview,” explains the steps in the process leading to elemental Hg generators certification and identifies activities performe d by NIST and activities and responsibilities of the generato r manufacturer or others. Section 3, “Definitions” presents important definitions related to generator ce rtification and the calcu lation of uncertainty. Section 4, “Qualification Test s,” contains provisions designed to demonstrate the adequacy of a particular gene rator model for the intended fiel d application(s). Section 5, “Certification,” details the sp ecific procedures for the quantitative determination of the candidate generator output. Section 6, “Uncertainty,” summarizes the method used to calculate and report the expa nded uncertainty of the generator output. Section 7, “Quality Assurance,” details approaches for periodic assessments providing assurance of continued acceptable elemental Hg generator performance.

The need for NIST-traceable elemental Hg gene rators (or lack thereof) in a particular regulatory application depends on the nature of the Hg emission standards that must be met and the associated monitoring and testing requirements. These issues are addressed during formal rulemaking processes or by the regulatory agency responsible for implementing the standards, and are not addressed in this or other EPA traceability protocols.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

2 2.0 OVERVIEW

In the simplest of terms, elemental Hg ge nerators produce known c oncentrations of Hg by passing a controlled gas stream through the headspace of a temperature-controlled reservoir containing elemental mercury, a nd then dynamically blending the mercuryladen gas stream with a controlled dilution ga s stream. The temperature of the reservoir, the flow rate of the carrier gas sweeping through the reservoir headspace, the chamber pressure, and the flow rate of the dilution gas stream are carefully monitored and controlled to produce the n ecessary Hg concentrations. The specific design, configuration, and features of an elemental generator are le ft to the judgment of the

manufacturer and are not constrained by explicit design specifications or EPA requirements.

Qualification tests are required for each elemental Hg generator model. For the purposes of this interim protocol, the tests that ar e routinely performed by the manufacturer will suffice, provided that the test results demonstrate that the generator model has adequate stability and repeatability and can operate re liably over a stated range of conditions (e.g., back pressures, ambient temperatures, and ot her applicable factor s). These tests may either be performed on each individual gene rator or on selected generators that are representative of the model. The manufact urer must provide the end user with a statement of disclosure that explains pr oper set-up and use of the generator and documents the range of conditions over wh ich reliable generator operation can be expected.

NIST has developed a fundamental analyti cal method for quantifying the concentration of elemental Hg in gas mixtures to provide traceability to the Inte rnational System of Units (SI). The method is isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) 1 . The output of a NIST Prime elemental Hg generator 2 , which is

maintained at NIST Chemical Science a nd Technology Laboratory in Gaithersburg, Maryland, has been certified by this method.

NIST uses the NIST Prime generator to cert ify “Vendor Primes” (which are elemental Hg generators provided by the ma nufacturers), by direct comp arisons of the generator outputs at a series of Hg concentrations. 3,4,5 When a Vendor Prime certification is complete, NIST issues a Report of Analysis to the party submitting the generator. The report describes the experimental procedures used and identifies the specific Vendor Prime set points with the corresponding cer tified Hg concentrations and expanded

uncertainties.

This Interim EPA Traceability Protocol specifies the procedures for establishing the “NIST traceable” quantitative output of elemen tal Hg generators that are employed in regulatory applications for emission monitori ng or testing. These generators may be certified either: (a) at the ma nufacturer’s facilit y, prior to shipping, by direct comparison to a certified Vendor Prime; or (b) at the si te where the generator will be used (or at another convenient location), by direct compar ison to an elemental Hg generator that has been previously certified against a Vendor Prime.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

3 The manufacturer or equipment supplier perf orming the certifications must maintain documentation of all generator ce rtifications. The test date, certified Hg concentrations, and uncertainties must be provided in writing to the user. All certification test records must be available for inspecti on by regulatory officials.

Following the initial certifica tion of a generator, it can be used in accordance with the

manufacturer’s written inst ructions and operational sp ecifications for regulatory applications in emission monitoring or tes ting. Periodic quality assurance (QA) checks and/or quality control (QC) pr ocedures are required to verify that the generator continues to provide accurate and reliab le calibration standards. Rece rtification of a generator is required periodically to maintain NIST traceab ility. Recertification may also become necessary when certain malfunctions, operati ng conditions, or other factors identified by

the user or manufacturer occur.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

4 3.0 DEFINITIONS

3.1 Bracketing procedure means an experimental design whereby a measurement system is used to compare the output from a reference standard to the output from a candidate generator.

3.2 Candidate generator means an elemental Hg generator that is being tested for initial certification, rece rtification, or quality assurance, by comparing its output to that of

a reference standard.

3.3 CEMS means continuous emission monitoring system.

3.4 Certified candidate genera tor output concentration means the arithmetic average of the calculated output concentra tions of a candidate generator, when a bracketing procedure is performed at a given concentration level.

3.5 Combined standard uncertainty means an estimate of th e standard uncertainty, calculated by propagation-of-e rror, for measurements that are characterized by multiple sources of uncertainty.

3.6 Control chart means a graphical representation of how the output ratio varies over time, when a series of bracketi ng procedures (e.g., initial certification an d ongoing quarterly performance evaluations) is performed with a particular certified Field Reference generator and a particular User generator.

3.7 Coverage factor means a multiplier used to calculate an expa nded uncertainty from a standard uncertainty. A larg e coverage factor increases th e confidence that the “true”

value is within the bounds of a stated uncertainty. For a stat istical “normal” distribution of measurements, a coverage factor of 2 gives a level of confidence of approximately 95%.

3.8 Detector linearity uncertainty means an uncertainty component related to how closely a detector response approximates a straight line when plotted against known injected Hg concentrations.

3.9 Elemental Hg generator model means a specific confi guration identified by the instrument system design, including:

(a) The use of specific gas flow rate measurement/control devices, Hg evaporation chamber, temperature controllers, temp erature or pressure measurement and compensation devices, and other components,

(b) The physical arrangement of principal components,

(c) The specific electronics configur ation and signal processing, andInterim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

5 (d) The specific software/firmware version and data processing algorithms, as implemented by a particular manufacturer a nd subject to an identifiable quality

assurance system.

Note : The following changes to an elemental Hg generator do not constitute a model change. However, the manufacturer must keep records that document and provide a satisfactory explanation of all such changes:

 Minor changes to software, firmware, or data outputs that do not affect data processing algorithms or status outputs;

 Software installed on external device s, (including ex ternal computer systems), and used for processing of the generator output to generate reference values, operational information, or to activate alarms;

 Substitution or use of new measurement components, provided that the generator manufacturer verifies th at the performance of the new components is as good, or better, than the replaced components.

3.10 Expanded uncertainty means a standard uncertain ty (or combined standard uncertainty) multiplied by a coverage factor to provide a wider confid ence interval within

which the “true” value is believed to lie.

3.11 External adjustment means either (1) a physical ad justment to a component of the elemental Hg generator that affects its ou tput or performance, or (2) an adjustment applied by the data acquisition system (for example, mathematical adjustment to compensate for drift) that is external to the generator. External adjustments are made at the election of the end user but may be subj ect to various regulatory requirements.

3.12 Field Reference generator means an elemental Hg generator that has been certified by comparing its output to that of a Vendor Prime, using procedures that are more rigorous than those required for certification of a User generator.

3.13 Final certified candidate ge nerator output concentration means the arithmetic average of the calculated output concentratio ns produced by a candi date generator at a given concentration level, based on data from a bracketing procedure.

3.14 Intrinsic adjustment means an automatic and essential feature of an elemental Hg generator that provides for the internal control of specific components or adjustment of the generator output in a manner consiste nt with the manufacturer's design of the instrument and its intended operation. Intrin sic adjustments are either non-elective or are configured according to fact ory recommended procedures; they are not subject to change by the user.

3.15 Measurement stability means an uncertainty compon ent related to how much the readings obtained with a measurement devi ce change during a measurement interval.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

6 3.16 NIST means the National Institu te of Standards and Technology in Gaithersburg, Maryland.

3.17 NIST Prime means an elemental Hg generator that has been certified by NIST, using isotope dilution induc tively coupled plasma mass spectrometry (ID-ICP-MS).

3.18 NIST Traceability means, with respect to elemental Hg gas standards, that either: (a) the concentrations of elementa l Hg produced by a generator; or (b) the concentrations of elemental Hg in comp ressed gas cylinders are traceable to a NIST Prime by an unbroken chain of comparisons.

3.19 Output ratio means the ratio of the output fr om a candidate generator to the output from a certified referen ce standard, when a bracketing procedure is performed.

3.20 Reference standard means an elemental Hg gene rator or other device (e.g., compressed gas cylinder) that is used as the standard for the certif ication, rece rtification, or quality assurance of a candidate generator.

3.21 Reference uncertainty means the uncertainty of the traceable reference to which the candidate is being compared. For the purposes of this pr otocol, this uncertainty comes

from the certification of the Vend or-Prime or Field-Reference ge nerator that is used as the

reference standard in a bracketing procedure(s).

3.22 Repeatability means an uncertainty component re lated to successi ve measurements of the same quantity under th e same conditio ns, based on data co llected in a single experiment.

3.23 Reproducibility means an uncertainty component related to successive experiments involving the same me asured quantity, but not necessarily under id entical conditions.

3.24 Standard uncertainty means the quantitative value of an uncertainty component expressed as the standard deviation of a distribution of measurements.

3.25 Type A uncertainty component means an uncertainty component that is “local” to the measurement process, and is evaluate d based on statistical da ta collected alongside

the measurement data.

3.26 Type B uncertainty component means an uncertainty component that is “external” to the measurement process. The stat istical data for this type of component is

often limited.

3.27 Uncertainty means a statistical predictor of how close measurements are to the true value of the quantity being measure d, minus any bias or systematic error.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

7 3.28 User generator means an elemental Hg generator that is employed in regulatory applications for emission monitoring or testing.

3.29 Vendor Prime means an elemental Hg genera tor that has been certified by comparing its output to that of a NIST prime.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

8 4.0 QUALIFICATION TESTS

Manufacturers are required to conduct qualification tests to de monstrate the adequacy of each elemental Hg generator model (as define d in Section 3.8). Documentation of the qualification test procedures and results must be available upon request for inspection by EPA and other regulatory officials. Certai n other information must be provided to the end user by means of a “Manufacturer’s Disc losure” (see Section 4.1) to show that the generator model can operate reli ably over the range of conditions that are expected to be

encountered in the intend ed field application(s).

4.1 Manufacturer’s Disclosure

As part of each User generato r’s certification, a “Ma nufacturer’s Disclosure” is required.

This disclosure must include documentati on and other relevant information for the elemental Hg generator model, in accord ance with Sections 4.1.1 through 4.1.4, below. This information shall be provided to each end- user, in the generator manual and/or other

documents.

4.1.1 Design Description

The generator manufacturer shall provide a description of the generator design and operation in sufficient detail to afford users with a factual basis to assess the parameters that can reasonably be expected to affect ge nerator performance. The description must identify the critical components, features fo r the control of (or co mpensation for) changes in external influences, alarms or fault i ndicators, and necessary external devices or

connections.

4.1.2 Range(s)

The manufacturer shall specify the nomina l concentration range(s) (minimum and maximum Hg concentrations in units of μg/m 3 ) over which the elemental Hg generator

model has been demonstrated to meet all of the manufacturer’s specifications. The description of the range should explain the pa rameter values that must be changed or selected by the user to establish the effec tive operating range and/or Hg concentration values for a specific generator in a particular measurement application.

4.1.3 Operational and Environmental Conditions

The manufacturer shall explicitly state the operational range or lim itations within which the generator model can be expected to operate reliably and meet all of the manufacturer’s specificati ons, for such concerns as:Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

9 4.1.3.1 Carrier Gas Supply

The minimum and maximum compressed air or nitrogen supply pressure, the allowable variation in supply pressure, th e required carrier gas flow ra te, and the quality of carrier gas required.

4.1.3.2 Back Pressure

The range of back pressures over which the generator model has been tested and can deliver reliable Hg gas concentrations.

4.1.3.3 Enclosure Temperature Operating Range

The range of ambient temperatures over which the thermal stability of the generator

model has been tested and can deliver relia ble Hg gas concentrations, and the time required for the generator to equilibrate to a change in ambient temperature.

4.1.3.4 Line Voltage

The acceptable line voltage limits over which th e generator can provide reliable Hg gas

concentrations.

4.1.4 Operation, Maintenance, and QA

The manufacturer shall provide written procedures for inst allation, start-up, operation and maintenance, and quality assurance (QA) of the elemental Hg generator. The manufacturer shall identify those activities and/or QA check/maintenance intervals, or other factors that may need to be adjusted based on site-specifi c conditions or other application factors. The manufacturer shal l also identify the specific conditions and factors that require recertif ication of the generator, in cluding, as applicable, known malfunctions or failures, exposure to unacceptable operating conditions, component replacements, component recalibrations, or other conditions and/or factors.

4.2 Interim Model-Specific Tests

For the purposes of this interim protocol, th e manufacturer shall pr epare, keep on file, and make available upon request, a report desc ribing the qualification tests performed to ensure that the elemental generator model (a s defined in Section 3.8) will operate reliably and produce stable, repeatable gas concentratio ns over a stated range of conditions. The report shall indicate whether th e qualification tests were pe rformed on each individual generator or on selected generators that are representative of the model, and shall

document the results of the tests.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

10 4.3 Standardized Tests

In the final version of this protocol, EPA plans to require all manufacturers to perform and pass a series of standardized qualific ation tests for each el emental Hg generator model, to evaluate such things as generator stability with respect to time and temperature, set point reproducibility, and the effects of chan ges in back pressure, supply pressure, and line voltage on generator performance. S ections 4.3.1 through 4.3.5, below, are reserved for these test procedures. Until the final traceability protocol is issued, the interim model-specific qualification te sts described in Section 4.2 w ill suffice for the purposes of

initial certification. However, successful completion of the standardi zed qualification tests is a prerequisite for initial generator certifications that are performed after the standardized qualification tests are inco rporated into the final protocol.

4.3.1 Set Point Repeatability Test [Reserved]

4.3.2 Thermal Stability Test [Reserved]

4.3.3 Back Pressure Test [Reserved]

4.3.4 Supply Pressure Test [Reserved]

4.3.5 Line Voltage Variation Test [Reserved]Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

11 5.0 CERTIFICATION

The certification protocols descri bed in this document are identical in concept to those used by NIST and are similar in implementa tion. In order to certify elemental Hg generators, manufacturers must first take steps to have one or more Vendor Primes certified by NIST at specific set points. To date, fifteen (15) certified Hg concentrations within the nominal range from 0.5 to 38 μg/m 3 have been provided by NIST for this

purpose.

There are two options for certifying a Us er generator and establishing its NIST traceability. The output from the User generato r can be compared dire ctly to the output from a certified Vendor Prime at the particul ar concentrations required by the user, by means of the “bracketing procedure” described in Sections 5.1 through 5.3, below. This approach is used when the manufacturer or equipment supplier ca n certify the generator prior to shipping it to the user. Alternatively, a User generator can be certified at the field location where it is used, or at another c onvenient location, by direct comparison with a Field Reference generator that has been prev iously certified by direct comparison to a Vendor Prime. The Field Reference genera tor certification procedures, which are presented in Section 5.4, below, are more rigorous than those required for a User generator, in order to minimize the eff ects of the additional transfer standard.

Within this document, zero-, low-, mid-, and hi gh-level gases are designated as Z, L, M, and H, respectively. The detector (analyzer) responses to the output from each of the generators during the bracketing proc edure are identified as follows:

RS* = Reference standard generator C = Candidate generator U = User generator

V* = Vendor Prime FR = Field Reference generator (prior to certification) FR* = Field Reference generator (after certification)

( Note : Generators with * designations have certified outputs)

5.1 Bracketing Procedure

A bracketing procedure is a means of directly comparing the outputs of two elemental Hg gas generators. The purpose of the procedur e is to use the outp ut from a certified “reference standard” generator to certify th e output of a “candidate” generator. For the purposes of this interim protoc ol, the certified reference stan dard is either a Vendor Prime or a certified Field Reference generator. The outputs from the gene rators are introduced alternately to an analyzer so that each res ponse from the candidate generator is bracketed by a pair of responses from the reference sta ndard generator. At each gas level, the bracketing procedure requires a minimum of four responses from the certified reference standard generator and three responses from the candidate generator. An example of aInterim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

12 bracketing procedure injection sequence for a given gas level is provided below in

Table 1.

Table 1. Example of Bracketing Procedure Injection Sequence

Generator ID Response ID

Reference Standard RS* 1 Candidate C 1 Reference Standard RS* 2 Candidate C 2 Reference Standard RS* 3 Candidate C 3 Reference Standard RS* 4

5.1.1 Bracketing Procedure for User Generator Certifications

(a) Either a Vendor Prime or a certified Field Reference generator may be used as the reference standard to certify a candidate User generator. At a minimum, the bracketing procedure requires a three by thr ee matrix (3x3) to compare U to V* or FR* (i.e., three comparisons at each of three different concentration levels—see Section 5.3). More than one User genera tor at a time can be certified using a single Vendor Prime or Field Reference gene rator. If this approach is followed, sequential responses from each User ge nerator are recorded in-between the Vendor Prime (or Field Reference gene rator) responses, a nd linear (time) interpolation of the Vendor Prime (or Fi eld Reference generator) responses is

used to correlate with the exact time of each intervening User generator’s

response.

(b) Table 2 illustrates the bracket ing procedure at three typi cal gas concentrations for a User generator certification, designated as “high”, “mid”, and “low”. In this example, a certified Field Reference genera tor is used as the reference standard. At the option of the manu facturer, the order of in troduction of the three gas concentrations shown in Table 2 (i.e., high, mid, and low) may be reversed.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

13 Table 2. Bracketing Procedure Sequential Steps

for User Generator Certification a

Z 1 FR* H-1

U H-1 FR* H-2

U H-2 FR* H-3

U H-3 FR* H-4 FR* M-1

U M-1 FR* M-2

U M-2 FR* M-3

U M-3 FR* M-4 FR* L-1

U L-1 FR* L-2

U L-2 FR* L-3

U L-3 FR* L-4 Z 2

( Note : In Table 2, and throughout the remainder of this section , replace FR*

with V* if a Vendor Prime generator is used as the reference standard for the bracketing procedure.)

(c) From these data, an ”out put ratio” (R) is calculated for each value of U. The calculation of R is illustrated by Equation 1a, below, for U H-1 , the first response to the output from the User gene rator at the high level.

a The nomenclature for Table 2 is: Z 1 = first zero response; FR* H-1 =Field Reference generator, high level, first response; U H-1 = User generator, high level, first response; etc.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

14    

 

   

 

         

  

2 *

2 *

1 1

H H H
FR FR U R (Equation 1a)

Where:

R = Output Ratio

U H-1 = First response to User ge nerator output at high level (zero corrected, if appropriate) b

FR * H-1 = Response to Field Reference generator output, pre U H-1 (zero corrected, if appropriate)

FR * H-2 = Response to Field Reference generator output, post

U H-1 (zero corrected, if appropriate)

(d) The average of the FR * H-1 and FR * H-2 responses in the denominator of Equation 1a provides an estimate of the value of FR * when U H-1 was actually recorded. Note, however, that this equation only applies to equally-spaced measurements when one candidate generator is being certified. If th e spacing is not equal or more than one candidate generator is be ing certified with a single reference standard, a more rigorous, time-interpolat ed formula in Equation 1b below is used

to estimate FR *

.

*

2 *

1 1

1 2 1 1

1 2 1 2



 





  

  

   

    H

FR FR FR U

H

FR FR U FR H

FR FR U R



## t t t t

t t t t

*
H *
H *
H H

*
H *
H H *
H (Equation 1b)

The 

# t values in Equation 1b are time stamps associated with the U and

* FR readings. Equations 1a and 1b are math ematically identical when the three 

# t
values are equally spaced.

(e) At each calibration gas level, the R values from Equation 1a or 1b (whichever applies) are averaged arithmetically. The n, the value of “Y”, i.e., the calculated User generator output, is determined. Equation 2, below, illustrates the calculation of Y for the high level gas. Manufacturers’ spread sheets are generally used to perform this calculation 6,7,8,9 :

b See Section 5.2, below, for a discussion of zero correctionInterim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

15 

#   

#  

H FR H R X Y

H *

 (Equation 2)

Where:

Y H = Calculated User ge nerator output for the high level gas

*

H FR X = Certified high-level ou tput concentration of the Field Reference generator

H R = Average output ratio for the high level gas

5.1.2 Stepwise Procedures

(a) At each concentration level, perf orm a minimum of three (3) comparisons as follows:

(1) Turn on all components of the measur ement system including the certified reference standard generator, c the User generator, the measurement detector (analyzer) and any associated necessary equipment and allow them to reach thermal equilibrium and stable operation with air (or nitrogen) supply connected and carrie r gas flowing for both generators. The following procedure is recomme nded to assess stable operation. Conduct a 15-minute stability check of both generators at the outset. Generator output may be assumed to be stable if no more than 0.10 μg/m 3

drift is observed over the 15-minute interval.

(2) Ensure that the generator intern al data logging func tions are properly configured and operational, that th e detector internal data logging functions are properly configured and operational, and that any external computers or data logging equipm ent are properly configured and operational so that complete documentation of the certification test procedures is available for future inspection.

(3) Ensure that the two generators, the analyzer, and any support equipment are operating within the prescribed limits for all key parameters as specified by the manufacturer.

( Note : If, at the outset or any other time during the process, test results demonstrate that equilibra tion is not achieved, or th at any key parameter is not within acceptable limits, stop the experiment, investigate and correct the problem. Valid data collected up to that point in the test may be

retained. Discard all data collected at non-equilibrium conditions or with any key parameter outside of prescribed limits; such data are not valid and may not be used for certificati on of the User generator).

c The certified reference standard generator may e ither be a Vendor Prime or a certified Field Reference

generator.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

16 (4) Set each generator to the first set point (i.e., the high-level or low-level point, as deemed appropriate) and allow all components of the measurement system to attain equilibrium.

(5) If the manufacturer’s system utilizes a continuous detector, it shall be calibrated as follows. After equilibri um has been established, zero the detector by injecting zero gas and ma king any necessary zero adjustments. Once the zero has been set, calibrate the detector by injecting the highlevel calibration gas from the referen ce standard generator and making any necessary span adjustments. Repeat these steps, if necessary, to optimize the analyzer calibration relative to th e reference standard generator.

( Note : Adjusting the analyzer to provide the correct responses to the reference standard generator is cons idered to be a valuable diagnostic technique to detect changes in the measurement system that can adversely affect the experimental procedure.)

(6) Alternatively, the detector may be set up using the manufacturer’s standard operating procedure.

(7) Direct the output of the reference standard generator to the detector and obtain a response (V * or FR * , as applicable). It is recommended that the recorded response at each test poin t be based on at least 5 minutes of stable readings for continuous systems, or one reliable batch measurement for gold trap collection systems.

(8) Direct the output of the User ge nerator to the det ector and obtain a response (U).

(9) Switch back to direct the output of the reference standard generator to the detector and obtain a response (V * or FR * , as applicable).

(10) Repeat steps 7 through 9 two more times to achieve triplicate responses for the User generator at that concentration level.

(11) Repeat steps 7 through 10 for the second and third concentration levels. Be sure to allow system to reach equilibrium before recording any responses. The third concentrati on level completes the 3x3 matrix.

(12) Check the zero response of the de tector, without making any adjustments.

(b) The manner in which the responses are recorded depends on the analyzer (detector). If it is a continuous analyzer , record readings at appropriate set intervals and compute the av erage over the desired time period. If the analyzer concentrates the sample and an integrat ed signal is obtained for the generator output, record the result of one analysis cycle or average multiple analysis cyclesInterim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

17 using all stable measurement system responses. For systems that have two traps (“A” and “B”) to concentrate the sample, use the same designated trap (A or B) throughout the sampling procedure.

5.2 Data Processing for User Generator Certifications

To determine the actual output of the User ge nerator, first correct each response for any zero offset of the detector, if appropriate. Zero correction is required only for an instrument that does not perform an automa tic baseline correction for every reading. If zero correction is required, calculate the inte rpolated zero offset based on the time when a specific response was recorded as follows:



##            

 

  

2 1 2 1

1 1 t t Z Z t t Z Z i

i (Equation 3)

Where: Z i = Calculated zero offset at t i Z 1 = Initial zero response

Z 2 = Final zero response

t i = Time of response to be zero corrected

t 1 = Time initial zero response was recorded

t 2 = Time final zero response was recorded

(a) Then, correct the response at t i for the zero offset using the following equation:

i i c Z U U

  (Equation 4)

Where: U

c = Response at t i corrected for zero offset

U i = Actual response at t i Z i = Zero offset at t i

(b) The data set in Table 3 provides an exam ple of how each response is corrected for zero offset over the course of a test period.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

18 Table 3. Zero Corrections for an Example Data Set

Time (min) Zero Response Interpolated

Zero FR i * Response d FR
c *

Zero-Cor. U i Response d U
c Zero-Cor.

0 0.00

10 0.04 10.00 9.96

20 0.08 9.85 9.77

30 0.12 10.20 10.08

40 0.16 9.96 9.80

50 0.20 10.25 10.05

60 0.24 10.00 9.76

70 0.28 10.40 10.12

80 0.32

(c) Next, calculate R, the output ratio for each value of U

c (see Section 5.1.1, above). Table 4, below, illustrates these calculations , for the example data set presented in

Table 3.

Table 4. Example Calculation of User Generator Output (With Zero Correction)

Time (min) FR i * Response FR

c *

Zero-Cor. U i Response U

c Zero-Cor. Output Ratio (R)

10 10.00 9.96

20 9.85 9.77 0.975

30 10.20 10.08

40 9.95 9.80 0.974

50 10.25 10.05

60 10.00 9.76 0.968

70 10.40 10.12

(d) Next, average the three cal culated output ratios arithmeti cally. For this data set,

the value of H R is 0.972.

(e) Finally, use Equation 2 in Section 5.1.1, above, to determine Y H , the calculated candidate (User) generator ou tput concentration at high level. For this example
data set, *

H FR X , the certified high level output concentration for the Field

d Note that the User and Field Reference generator responses can be in various units of measure (e.g., μg/m 3 , volts, counts, etc.), depending on the exact set up of the measurement system.Interim Elemental Mercury Gas Tr aceability Protocol -- July 01, 2009

19 Reference generator, is assumed to be 10.0 μg/m 3 . Therefore, the value of Y H from Equation 2 is 9.72 μg/m 3 , based on the zero-corrected responses.

(f) Apply the same calculation procedures to each of the other concentration levels. A spreadsheet can be used to perform the calculations.

(g) If zero-correction is not necessary fo r the detector, use the uncorrected values (FR i * and U i ) in the calculations, as shown belo w in Table 5. For this example data set, the value of Y H is 9.73 μg/m 3 , based on the uncorrected responses.

Table 5. Calculation of User Generator Output (Without Zero Correction)

Time (min) FR i * Response U i Response Output Ratio

10 10.00

20 9.85 0.975

30 10.20

40 9.96 0.974

50 10.25

60 10.00 0.969

70 10.40

(h) For each concentration level ≥ 4.0 μg/m 3

, calculate the relative standard deviation (RSD) of the output ratios, using Equation 5.



#  

100 1 1 2

x

x n x x

RSD n

i i





 

# 

 (Equation 5)

Where:

RSD = Relative st andard deviation, percent

x = Arithmetic average of the output ratios

x i = Individual, “i th ” output ratio n = Number of output ratios

(i) The RSD must not exceed 1.0%. If the RS D value is exceeded, the test is invalid and must be repeated.

(j) The calculated RSD values for the out put ratios in Tables 4 and 5 are 0.4% and 0.3%, respectively, and are therefore acceptable.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

20 5.3 Selection of Set Points for User Generator Certifications

Field Reference and Vendor Prime generato rs are certified at specific set points to produce known concentrations of elemental Hg. These generators must be operated at NIST certified concentrations when they are us ed as reference standard generators. The

certified concentrations of a Field Re ference generator ar e established by the manufacturer and are based upon the certified concentrations of a Vendor Prime. The

certified concentrations of a Vendor Prime generator are established by an agreement between NIST and the party submitting the generator to NIST. 10 The exact certified concentrations may differ somewhat from the nominal concentrations requested.

The concentration levels that are needed fo r a User generator may be specified by an applicable regulation. For example, the regu lations for a CEMS application may require the user to perform linearity tests at three le vels using elemental Hg concentrations of 20 to 30% of span, 50 to 60% of span, and 80 to 100% of span. For practical reasons, it is better to certify the User generator at concen trations near the middle of the specified ranges to provide some working margin to a void a particular point being either on the cusp of or outside the desired range. Other factors, such as adjustment for calibration gas humidification may also need to be considere d. The particular con centrations necessary for certification of a User generator are es tablished by agreement between the user and the equipment supplier.

Any type or model of elemental Hg generato r certified as a Vendor Prime or as a Field Reference generator may be used to certify a User generator. The Vendor Prime (or Field Reference generator) and User generators ne ed not be the same type and model. However in practice, equipment suppliers ar e likely to choose to compare the same or similar models. Furthermore, some generato r manufacturers may restrict the parameters that are varied to produce the necessary con centrations for a given span. For example, the Hg reservoir temperature and diluent gas flow rate may vary only between units with different spans, but for a give n span, both of these parameters are always held constant and only the flow rate of the carrier ga s sweeping through the reservoir headspace is varied. In such cases, the equipment supp lier may choose to have the Vendor Prime (or Field Reference) generator certified for va rious groups of parametric values or combinations of parameter settings. 11 In this situation, the ge nerator manufacturer shall specify the appropriate Vendor Prime (or Field Reference generator) certified concentrations or Vendor Prime (or Field Re ference generator) parameter combinations

that are selected for certification of User generators in accordance with a written procedure. The written procedure must be av ailable for inspection by regulatory agencies

upon request.

After selecting the most appropriate Ve ndor Prime (or Field Reference generator) certified concentrations for comparison with th e User generator, the actual set-points for the User generator can be verified or adju sted to produce the low, mid, and high-level concentrations in the desired ranges. The se t points or parameter settings for the User generator are not required to be identical to the set points or parameter settings of the Vendor Prime (or Field Reference generator). However, the certified concentrations ofInterim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

21 the User generator must either be within ± 30% or ± 0.5 μg/m 3 of the certified Vendor Prime (or Field Reference generator) concentrat ions (whichever is le ss restrictive).

5.4 Bracketing Procedure for Field Reference Generator Certifications

When performing a dual-bracketing procedure to certify a Field Reference generator, a

Vendor Prime must be used as the reference st andard and the Field Reference generator is the candidate generator. Once a Field Referen ce generator has been certified, it may then be used to certify, recertify, or perfor m on-going data quality assessments of User

generators.

(a) At a minimum, the Field Reference ge nerator certification consists of a three by three bracketing matrix performed in dup licate ((3x3) x 2), comparing the output of the Field Reference generator to th e output of the Vendor Prime. Table 6, below, provides a summary of this certification procedure.

(b) Follow all of the steps in Section 5.1.2, above for each 3x3 bracketing matrix. All preparations, precautions, and procedures specified for the cer tification of User generators also apply to Field Reference generators. If the second 3x3 matrix is commenced immediately after the first one , Z-2, the final zero response from the first matrix, may serve as Z-3, the initia l zero response for the second matrix (see Table 6).Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

22 Table 6: Field Reference Generator Certification Dual-Bracketing Procedure e



### Field Reference Generato r Bracketing Procedure

Set 1 Set 2

Z-1 Z-3

V*-H-1-1 V*-H-2-1

FR-H-1-1 FR-H-2-1

V*-H-1-2 V*-H-2-2

FR-H-1-2 FR-H-2-2

V*-H-1-3 V*-H-2-3

FR-H-1-3 FR-H-2-3

V*-H-1-4 V*-H-2-4

V*-M-1-1 V*-M-2-1

FR-M-1-1 FR-M-2-1

V*-M-1-2 V*-M-2-2

FR-M-1-2 FR-M-2-2

V*-M-1-3 V*-M-2-3

FR-M-1-3 FR-M-2-3

V*-M-1-4 V*-M-2-4

V*-L-1-1 V*-L-2-1

FR-L-1-1 FR-L-2-1

V*-L-1-2 V*-L-2-2

FR-L-1-2 FR-L-2-2

V*-L-1-3 V*-L-2-3

FR-L-1-3 FR-L-2-3

V*-L-1-4 V*-L-2-4

Z-2 Z-4

(c) Follow the data processing steps in S ection 5.2, above, for both the first and second repetitions of the 3x3 bracketing matrix. For each bracketing set where

the concentration level is ≥ 4.0 μg/m 3

, calculate the relative standard deviation (RSD) of the output ratios, using Equati on 5 in Section 5.2. For any bracketing set where the RSD exceeds 1.0%, that set is invalid and must be repeated.

(d) For detectors that require zero correc tion, Table 7, below, provides an example of how each response is corrected for zero o ffset over the course of a test period. Table 7 illustrates the high-le vel gas injections for the first and second data sets.

e The nomenclature for Table 6 is: Z-1= zero response 1; V*-H-1-1=Vendor Prime, high level, Set 1,

response 1; FR-M-2-3 =Field Reference generator, mid level, Set 2, response 3; etc.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

23 Table 7: Zero Corrections for an Example Data Set

Time (min) Zero Response Interpolated

Zero V i * Response V

c *

Zero-Cor. FR i Response FR

c Zero-Cor.

0 0.00

10 0.02 10.00 9.98

20 0.04 9.85 9.81

30 0.06 10.20 10.13

40 0.08 9.96 9.88

50 0.10 10.25 10.13

50 0.12 10.00 9.88

70 0.14 10.40 10.23

240 0.48

250 0.49 10.45 9.96

260 0.50 10.15 9.65

270 0.51 10.50 9.99

280 0.52 10.20 9.68

290 0.53 10.48 9.95

300 0.54 10.22 9.68

310 0.55 10.50 9.95

360 0.60

(e) For each matrix, de termine the output ratios (R H ) and the calculated Field Reference generator output value (Y H ) using the basic approach shown in Section 5.1.1 (the bracketing procedure for User generator certifications)

, replacing U with FR (since the Field Reference genera tor is now the candidate), and replacing

FR * with V * (since the Vendor Prime is now the reference standard). Table 8, below, illustrates these calculation pro cedures for the zero-corrected responses from the example data set presented in Ta ble 7. For this example data set, the
certified concentration of the Vendor Prime ( *

H V X ) is assumed to be 10.0  g/m 3

.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

24 Table 8: Example Calculation of Field Reference Generator Output

(With Zero Correction)

Time (min) V i * Response V

c *

Zero-Cor. FR i Response FR

c Zero-Cor. Output Ratio

10 10.00 9.98

20 9.85 9.81 0.975

30 10.20 10.14

40 9.96 9.88 0.974

50 10.25 10.15

60 10.00 9.88 0.968

70 10.40 10.26

250 10.45 9.96

260 10.15 9.65 0.967

270 10.50 9.99

280 10.20 9.68 0.971

290 10.48 9.95

300 10.22 9.68 0.973

310 10.50 9.95

(f) The final certified candidate (Field Refe rence) generator output concentration is the arithmetic average of the two calculated Y H values obtained with Equation 2. For this example data set at the hi gh gas level, the final value of Y H is 9.71 μg/m 3

, based on the zero-corrected responses.

(g) Apply the same calculation procedures to each of the other c oncentration levels. A spreadsheet may be used to perform the calculations.

(h) If zero-correction is not necessary fo r the detector, use the uncorrected values (V i * and FR i ) in the calculations, as shown in Table 9, below. For this example data set at the high gas le vel, the final value of Y H is 9.72 μg/m 3 , based on the uncorrected responses.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

25 Table 9: Example Calculation of Field Reference Generator Output

(Without Zero Correction)

Time (min) V i * Response FR i Response Output Ratio

10 10.00

20 9.85 0.975

30 10.20

40 9.96 0.974

50 10.25

60 10.00 0.969

70 10.40

250 10.45

260 10.15 0.969

270 10.50

280 10.20 0.972

290 10.48

300 10.22 0.974

310 10.50Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

26 6.0 UNCERTAINTY

This section describes the cal culation method that is used to determine the expanded uncertainty of elemental Hg concentrat ions produced by candidate generators.

6.1 Uncertainty Calculation---General Approach

The first step in the process of determining the uncertainty of a measured quantity is to establish the “standard uncertainty” of each component. For a Type A component where

statistical data are avai lable, the standard uncertainty is the standard deviation of a series of repeated measurements rela ted to the component. For a Type B component, standard uncertainty is established based on either (1 ) statistical data pr ovided by the external source, or (2) a “worst case” estimate conve rted to a comparable standard deviation.

Once all standard uncertainties are establishe d, they are combined through propagation of error to arrive at a single combined uncertain ty value representing the overall uncertainty of the measurement. Finally, the expanded uncertainty is established by multiplying the combined uncertainty by a coverage fact or, the magnitude of which depends on the required confidence level. To determine the expanded uncertainty for Hg concentrations produced by elemental mercury generators, a coverage factor of 2 has been selected, approximating a 95% confidence interval for the “true” value.

The uncertainty assigned to the output concen tration of an elemental mercury generator is accumulated at each step in a chain of compar isons. The process starts with the primary measurement standard, NIST’s Isotope Dilution Induced Coupled Plasma Mass Spectroscopy (ID-ICP-MS) method, which is us ed to certify the NIST Prime generator and has its own uncertainty. Then, when the NIST Prime generator is compared to a Vendor Prime generator, additional uncertain ty components are added, based on NIST’s evaluation of its measurement systems and procedures.

The Vendor Prime generator is the starting point for the uncertainty calculations in this protocol. The combined uncertainty of the Vendor Prime generator is one component of the uncertainty for a candidate generator’s cer tification (where a candi date can be either a User generator or a Field Reference genera tor). This protocol describes how the

combined uncertainties of the Vendor Prim e and the other components are accumulated to determine the combined uncertainty of the candidate generator’s Hg concentrations. When a Field Reference generator is used as a transfer standard between a Vendor Prime and a User generator, the accumulated uncerta inties of both comparisons must be taken into account when calculating the User generator concentr ation uncertainty.

6.2 Uncertainty Components

In addition to the uncertainty of the refere nce standard, the follo wing four sources of uncertainty have been identified for assigning concentration values to candidate generator setpoints: (1) detector linear ity; (2) measurement stabilit y; (3) repeatability; and (4) reproducibility. The first three component s apply specifically to a comparisonInterim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

27 (bracketing) procedure. The reproducibility component applies to the overall process of transferring traceability from the refe rence gas standard to the candidate.

For the mathematical details of how uncertain ty is calculated, refer to the Companion Document to this interim protocol 12 . In practice, these calculations are incorporated into the same spreadsheet files that calculate the certified concen trations. EPA has developed several example spreadsheets, based on the eq uipment commercially available at the time of this writing. Both the Companion Docume nt and the spreadsheets are available in support of this interim protocol.

6.2.1 Detector Linearity Uncertainty

[Reserved]

6.2.2 Measurement Stability

The measurement stability uncertainty calcu lation applies a statistical test to the assumptions that are built into the br acketing technique (e .g., no non-linear drift, wavering responses, or excessive ly “noisy” measurements). This uncertainty component

is calculated based on the st andard error of the reference and candidate measurement points, assumed to lie on two straight lines when plotted versus time.

6.2.3 Repeatability

Repeatability attempts to quantify, independently of measurement issues such as linearity and stability, the precis ion of the output ratio determina tions within a single bracketing procedure. It is calculated as the differ ence between (1) the standard deviation of the individual bracket ratios, and (2) the statisti cal contribution of measurement instability to

the standard deviation of the ratios.

6.2.4 Reproducibility

Reproducibility is a measure of how precise ly a bracketing procedure’s result can be duplicated. Because of the differences between the certification pro cedures (as described in Section 5), reproducibility may be estimat ed differently depending on what type of generator is being certified (i.e., Field Refe rence generator, or User generator) and how many times the bracketing procedure is repeated.

Field Reference generator certifications must include a calculated reproducibility component in the uncertainty calculation. Ei ther the “statistical” or the “bound on bias” approach may be used, at the analysts discretion, based on guidance provided in Appendix A. For User generator certifications , in addition to these two options, there is a third, i.e., use a Type-B default value equal to 0.5 % of the cert ified value. The results of field testing have shown that this Type-B reproducibility is routinely achievable.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

28 6.3 Combined and Expanded Uncertainty

Once calculated, all of the individual compone nts (sources) of uncertainty (described above) are combined using propagation of error. First, the combined uncertainty of each bracketing procedure is calculated (assuming zero uncertainty for the reserved linearity component). Then, these uncertainties are co mbined into a single comparison uncertainty (dimensionless, because it is th e uncertainty of the comparis on ratio determination). This comparison uncertainty is multiplied by the re ference concentration and combined with the reproducibility and refere nce concentration uncertainti es to calculate the overall combined uncertainty in concentration units. The expanded uncertainty is this combined uncertainty multiplied by a coverage factor of 2.

6.4 Uncertainty Acceptance Criteria

At each concentration level, the results of the bracketing certification procedure are acceptable if the expanded uncertainty of the elemental mercury generator concentration,

calculated in accordance with Section 6.3 a bove, does not exceed 5.0 percent of the certified value, or is not more than 2.0 per cent above the Vendor Prime uncertainty at the closest set point f , whichever is less restrictive.

f If the candidate certification setpoint does not ex actly match the reference standard setpoint, base the alternative specification on the cl osest reference standard setpoint.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

29 7 .0 QUALITY ASSURANCE

This section specifies: (a) th e periodic data quality a ssessments that are required following initial certification of an elementa l Hg generator, to provide assurance of continued acceptable performance; and (b ) the required frequency of generator

recertification.

7.1 Periodic Data Quality Assessmen ts of Certified User Generators

(a) Periodic data quality evaluations of the gene rator shall be conducted as follows:

(1) At least once each calendar quarter, except where otherwise indicated in Section 7.1.2, below; and

(2) Following any malfunction, repair, or co rrective action that may reasonably be expected to affect the generato r’s output, but does not necessitate

recertification of the generator (see Section 7.3, below).

(b) Any of the data quality a ssessment procedures (Options A through D) described in Sections 7.1.1 through 7.1.4, below, may be used for the performance

evaluations.

(c) Except as otherwise provided in Secti on 7.1.2.1(e), below, if the generator does not meet the acceptable performance criteria , subsequent data from the generator

are considered invalid until either:

(1) Another performance evaluation is successfully completed; or

(2) The generator is recertified in acc ordance with the bracketing and data processing procedures in Section 5, above

7.1.1 Option A---Field Reference Generator Comparison

The required performance evaluations of a User generator may be done by direct comparison to a certified Field Reference generator. If this option is selected:

(a) Use the bracketing and data processing pr ocedures described in Sections 5.1 and 5.2, above, except that the performance evaluation is conducted at only one

concentration level rather than three. Perform three replicates to provide three output ratios, average the ratios arithmeti cally, and then calculate Y (see Section 5.1.1, above).

(b) For continuous emission monitoring syst em (CEMS) applications, perform the comparison with the User generator output set at the certified Hg concentration that is used for daily calibration erro r checks. The certified set point Hg concentration from the Field Reference ge nerator must be within ± 30% or ± 0.5Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

30 μg/m 3 (whichever is less restrictive) of the concentration used for daily

calibrations of the CEMS.

(c) The performance of the User generator is acceptable if the Y value obtained from the bracketing procedure is within ± 5.0% or ± 0.5 μg/m 3 (whichever is less restrictive) of the User generator’ s certified set point concentration.

7.1.2 Option B---Permeation Tube Comparison

Directly compare the User generator output to a permeation device that generates elemental Hg. The manufacturer must dem onstrate that any model-specific methodology used for this comparison is consistent with the generic approach described in Section

7.1.2.1.

7.1.2.1 Generic Approach

(a) Use a permeation tube whose output is c onsistent and predictable over time and under the conditions of its actual operation and use.

(b) Compare the permeation device and the User generator using an Hg analyzer. After taking any system corrective actions or making modifications that would affect the analyzer response to the pe rmeation device or elemental generator, determine an initial (“base”) ratio of an alyzer responses to the permeation device and to the elemental generato r, as described in paragra ph (c) of this section, to serve as a reference point for subsequent periodic performance evaluations. It is

recommended that the base ratio be estab lished as soon as pract icable after initial certification, and re-establis hed as soon as practicab le after any subsequent corrective actions or system modifi cations that may affect the ratio.

(c) To determine the base ratio, make alternating measurements of the output from the permeation tube and the User generator until at least one pair of readings has

been obtained. g For CEMS applications, perfor m the comparison procedure at a single concentration level, preferably (but not nece ssarily) corresponding to the level of the daily calibration error check. Before recording each measurement,

allow sufficient time to ensure that a stab le analyzer reading is obtained. For each pair of readings, calculate (A perm /A gen ), the ratio of the analyzer responses to the output from the permeation device and the el emental generator. If more than one pair of readings is obtained, the base ra tio shall be the arithmetic average of the

calculated ratios.

(d) For the periodic data quality assessme nts, repeat the comparison procedure described in paragraph (c). Only one pa ir of readings is required. Because permeation device will not have EPA traceability and will not have been

g Although one pair of stable readings is the mini mum requirement, it is recommended that three pairs be

obtained and averaged, to provide a more robust base ratio.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

31 calibrated against a reference method, perform this periodic performance evaluations at least monthly (prefera bly weekly), for CEMS applications.

(e) The performance of the User generator is acceptable if the calculated value of (A perm /A gen ) output ratio of the generator (or th e average ratio, if more than one pair of readings is obtained) is within ± 5.0% of the base ratio determined in paragraph (b) of this section. If this performance criterion is not met:

(1) One of the other data quality assessment procedures in Section 7.1 of this protocol may, if available, be used to demonstrate acceptable generator performance h . However, if this approach proves to be unsuccessful and it appears that the generato r is actually malfuncti oning, follow the provisions of Section 7.1(c), above, re-establis hing the base ratio, if necessary; or

(2) If none of the other da ta quality assessment procedures in Section 7.1 of this protocol is available, the permeati on tube comparison procedure described in paragraph (c) of this Section may be repeated up to three times over a three-day (72-hour) period, beginning immediately after completing the initial test. If the test is not passe d within that 72-hour period, follow the provisions of Section 7.1(c), above, re-establishing the base ratio, if

necessary.

7.1.2.2 Model-Specific Methodologies

EPA has reviewed and found the following model-specific methodology for performing periodic QA assessments of User generators with permeation tubes to be sufficient to satisfy the QA requirements of this interim prot ocol. The procedures are detailed in an April 3, 2008 document prepared by Tekran Instruments 13 . A spreadsheet for the efficient processing of results has been developed and is available 14

.

The Agency is persuaded that the intern al permeation tube technique developed by Tekran provides meaningful insight into the actual performance of the elemental generator, and is consistent with the generi c approach described in Section 7.1.2.1 of this protocol. The viability of this approach has been demons trated through long term field studies at multiple industrial facilities. All performance requirements in Section 7.1.2.1(e) of this protocol apply to the Tekran-specific procedures.

h Failure to meet the 5.0% performance criterion do es not necessarily indicate a generator malfunction.

Fouling of the Hg analyzer may be the cause, in which case, performing corrective maintenance on the analyzer is the approp riate follow-up action.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

32 7.1.3 Option C---Sorbent Tube Comparison

Use adsorbent (“sorbent”) tubes to sample the User generator effluent. Perform an independent analysis of the samples to confir m the quantity of Hg that is generated.

(a) Conduct the comparison with the User ge nerator producing a certified set point Hg concentration. For CEMS applica tions, the set point shall be the Hg

concentration that is used fo r daily calibration error checks.

(b) Perform all sorbent tube sampling accordi ng to the procedures (including all of the QA procedures) of document No. 14 cite d in the “References ” section of this interim protocol. 15 Perform a minimum of three re plicates of paired samples. Average the three results.

(c) Calculate the difference between the averag e Hg concentration measured with the sorbent tubes and the generator’s certified set point Hg concentration. The performance of the User generator is accepta ble if this difference is within ± 5.0% of the certified set point con centration, or if the absolute value of the difference is no greater than 0.5 μg/m 3 (whichever is less restrictive).

7.1.4 Option D---Cylinder Gas Comparison

Evaluate the performance of a User gene rator by direct comparison to a compressed calibration gas cylinder having an EPA traceable elemental Hg concentration (see Sections 7.1.4.2 and 7.1.4.3, below). If the analy zer used for the comparisons is sensitive to variation in the oxygen concentration in th e sample gas, perform the comparisons only if the oxygen concentrations in the compresse d gas cylinder and in the User generator effluent are the same, or are known accura tely and precisely so that appropriate corrections can be applied.

7.1.4.1 Comparison Procedure and Criteria

(a) Perform the direct comparison using the pr ocedures for introd uction of calibration gas to an elemental Hg analyzer as well as the nesting procedure and data processing described in document No. 15 c ited in the “References ” section of this interim protocol. 16

( Note : A specially coated regulator (suc h as a SILTEK coated regulator body, CGA Nipple, and Isolation Valve) shoul d be used with the compressed gas cylinder)

(b) Before proceeding, the following stabilit y check of the output from the cylinder is recommended. Inject cylinder gas and observe the output. The output may be assumed to be stable if no more than 0.10 μg/m 3 of drift is observed over a period

of at least 2-3 minutes.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

33 (c) Perform a bracketing procedure, consis tent with Section 5.1.2 and Table 1, above, using the compressed gas cylinder as th e reference standard and the User generator as the candidate generator. Perform four replicate cylinder gas injections and three generator gas injections, recording the stable responses of the detector to all injections. Determine three output ratios, average the ratios arithmetically, and calculate the value of Y, in a manner consistent with Section 5.1.1, above.

(d) For CEMS applications, perform th e comparison with the User generator producing the certified se t point Hg concentration that is used for daily calibration

error checks. The certified concentrati on of the compressed gas cylinder must be within ± 30% of that set point concentration.

(e) Use the average output ratio and the cer tified cylinder concentration to calculate the average User generator output concen tration, in a manner consistent with Section 5.2, above. The performance of th e User generator is acceptable if its calculated average output concentration is within ± 5.0% or ± 0.5 μg/m 3
(whichever is less restrictive) of th e certified set point Hg concentration.

7.1.4.2 Protocol Gas Mixtures

After NIST-certified compressed gas mixt ures become available to commercial calibration gas suppliers, EPA will revise its traceability protocol for gaseous calibration

standards 17 , to include procedures for preparing elemental Hg Protocol Gas mixtures.

Until such time as the referenced EPA traceab ility protocol document is revised, the procedures that are used for preparing SO 2 Protocol Gas Mixtures may be followed by the commercial gas suppliers.

7.1.4.3 Hg Compressed Gas Mixtures Certified Relative to a Vendor Prime

Commercial gas suppliers may elect to certify elemental Hg compressed gas mixtures relative to the output of a Vendor Prime elem ental Hg generator. If this option is selected, the gas supplier shall develop a nd maintain a detailed written procedure describing the exact procedures used to certify cylinders relativ e to a Vendor Prime including all quality assurance measures to en sure the validity of the process, and shall make these procedures available to EPA or other air pollution control agencies upon

request.

7.2 Quality Control Procedures for Field Reference Generators

The following quality control (QC) proce dures are required for Field Reference generators that are used as reference standa rds to certify, recertif y, or perform periodic QA evaluations of User generators. Each Fiel d Reference generator that is used for these purposes must have a current (non-expired ) certification or rece rtification status.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

34 7.2.1 Preliminaries

Before each use of a Field Reference generator for a certification, rece rtification, or QA application, ensure that the generator is operating properly. The manufacturer’s written procedures and specifications for proper opera tion and maintenance of the generator must be strictly followed.

7.2.2 Output Ratio Check

The following output ratio check is required on each day that a Field Reference generator is used to certify, recer tify, or quality-assure data from a Us er generator (or generators):

(a) Perform the preliminary procedures de scribed in Section 7.2.1, above. Then, initiate the stepwise bracketing proce dure described in Section 5.1.2, above.

(b) Make no additional adjustments to the analyzer after zeroing it and calibrating it with a high-level gas produced by the Fiel d Reference generator (Step (5) in Section 5.1.2).

(c) Prior to, during, or immediately after the bracketing procedure, adjust the output from the Field Reference generator to produce a mid-level gas, and record the analyzer response to the gas.

(d) Calculate the ratio of the high-level and th e mid-level gas outputs, from paragraphs (b) and (c), above. Also calculate this out put ratio using the certified high-level and mid-level set point concentrations from the initial certification of the Field Reference generator (or, if the generator ha s been recertified, from its most recent recertification).

(e) If the two ratios calculated in paragraph (d), above, agree to within ± 5.0%, the performance of the Field Reference generato r is acceptable. If this criterion is not

met:

(1) Corrective action (which may invol ve recertification—see Section 4.1.4, above) is required before the Field Re ference generator may be used to certify, recertify, or perfor m quarterly evaluations of User generator(s); and

(2) If the mid-level gas injection in pa ragraph (c), above, o ccurred either during or immediately after a brack eting procedure, the results of that te st shall be

invalidated.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

35 7.2.3 Use of Control Charts

(a) When a particular Field Reference ge nerator is used to perform a series of bracketing tests on a particul ar User generator (e.g., th e initial certification and subsequent quarterly data quality assessmen ts), a control chart shall be prepared and shall be updated after each test. The control chart is used to track the output

ratio of the User and Field Reference generators from one bracketing test to the next. This ratio provides a means of a ssessing generator stability over the longterm. Significant changes in this ratio i indicate possible generator malfunctions.

(b) If, based on an evaluation of a control chart, generator malfunction is suspected:

(1) Investigate the apparent problem. If the control chart evaluation is found to be in error (i.e., a “false positive”) and generator malfunction cannot be confirmed, no further action is required.

(2) However, if an actual genera tor malfunction is discovered:

(i) Corrective action (which may i nvolve recertification of the generator—see Section 4.1.4, above) is required; and

(ii) The bracketing procedure for whic h the control chart indicated a possible malfunction is invalid and must be repeated, following the

corrective action.

(iii) When a malfunction of the Fiel d Reference generator (but not the User generator) is found, data fr om the User generator are not impacted. However, if a User generator malfunction is discovered, data from that generator are c onsidered invalid from the hour of completion of the invalidated brack eting procedure unt il the bracketing procedure has been successfully completed.

(c) Control charts are especially useful when a single Field Reference generator is used to assess the data quality of multiple User generators over an extended period of time. For example:

(1) If the output ratios fo r all of the User generators have changed very little since the previous test, it is likely that the Field Reference generator and the User generators are all operating properly; or

(2) It is likely that a particular User generator is malfunctioning if its output ratio has changed significantly i since the last test, but there has been no corresponding change in the other Us er generator output ratio(s); or

i As a “rule of thumb” if the output ratio changes by more than 5 percent from one test to the next, this is

considered si gnificant.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

36 (3) If the output ratios for the User generators have shifted significantly f in the same direction since the previous test , the Field Reference generator may be malfunctioning.

7.3 Recertification

Following initial certifi cation, all User generators and Fiel d Reference generators must be periodically recertified to maintain NIST traceability.

(a) User generators shall be recertified:

(1) Whenever any event, condition, or factor that triggers recertification (as identified

in the Manufacturer’s Disclosure ) occurs--see Section 4.1.4, above; and

(2) Once every 8 calendar quarters, if not required for any other reason.

(b) Field Reference generators shall be recertified against a Vendor Prime annually (i.e., once every four calendar quarters) , except as otherwise indicated in paragraph (c) of this section.

(c) The interval between re quired recertifications of a Fi eld Reference generator may be extended, up to a maximum of 8 calenda r quarters from the calendar quarter of the last test, as follows:

(1) The recertification test deadline ma y be extended by one quarter if the 3x3 bracketing procedure described in Sections 5.1 and 5.2, above, is successfully completed in the calendar quarter in which the recertification test is due. In this bracketing proced ure, the Field Reference generator is the candidate generator and any avai lable generator that has a current (non-expired) certification may be used as the reference standard. The results of the test are acceptable if, at each gas concentration level (L,M,H), the calculated average ou tput concentration of the Field Reference generator is within ± 5.0 % or ± 0.5 g/m (whichever is less restrictive) of the set point concentrati on from its original certification or

from its most recent recer tification (as applicable);

(2) Additional one-quarter extensions of the recertification test deadline (up to three) may be obtained, one quarter at a time, by successfully repeating the 3x3 bracketing procedure desc ribed in paragraph (c)(1).

(3) As an alternative to ex tending the recertification te st deadline one quarter at a time, as described in paragraphs (c)(1) and (c)(2), the test deadline may be extended by four calendar quarte rs if the following more rigorous, one-time test is successfully comple ted in the calendar quarter in which

the recertification test is due. Perform the 3x3x2 bracketing procedure described in Section 5.4, above with th e Field Reference generator as theInterim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

37 candidate generator. The reference standard shall be a generator that has a current (non-expired) certification and is independent of the Field Reference generator (i.e., its certific ation was not derived from the Field Reference generator). The results of the test are acceptable if, at each gas concentration level (L,M,H ), the calculated average output concentration of the Field Reference generator is within ± 5.0% or ± 0.5 g/m (whichever is less restrictive) of the set point concentration from its original

certification or from its most recent recertification (as applicable).

(d) Each elemental generator that is used as the standard for recertification testing must have a current, active (i.e., non-e xpired) certificati on or recertification

status.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

38 References

1 Long, S., “Method Protocol for Calibrati on of Elemental Mercury Gas Generation

Devices Using Isotope Dilution Cold-VaporGeneration Inductively Coupled PlasmaMass Spectrometry,” Version 2.01, National Institute of Standards and Technology (NIST), Gaithersburg, MD

2 “NIST Prime Certification Points”, Spreadsheet provided to US EPA by NIST.

3 Mitchell, G., Dorko, W., Long, S., “Repor t of Analysis – Traceability of Mercury

Measurements,” EPA Reference DW-13-92214701.

4 Mitchell, G. “Report of Analyses – Quan tification of Selected Set-Points for One

Thermo and One Tekran Mercury Generator – Numbers 20555 and 3015,” August

2008.

5 Leigh, S., “Memorandum – Uncertainty Calc ulations for the Cer tification of Mercury

Generator Outputs”, 1-14-08.

6 McRanie, R. and Roberson, W., “Field Co mparison Check of Installed Elemental Hg

Calibrator Versus a Reference Calibrator --Thermo Procedure”, RMB Consulting & Research, Inc., April 7, 2009.

7 Thermo Data Reduction Template-- RM B Consulting & Research, Inc., 08/27/08

8 McRanie, R. and Roberson, W., “Field Co mparison Check of Installed Elemental Hg

Calibrator Versus a Reference Calibrator --Tekran Procedure”, RMB Consulting & Research, Inc., May 1, 2009.

9 Tekran Data Reduction Template --RM B Consulting & Research, Inc., 08/27/08

10 Tentative agreement on these points was reached by all members of the workgroup at

the time of the EPRI CEMS Users Conference in Nashville, May 13-14, 2008.

11 See for example, McRanie, R and Schaedlich, F., “Technical Note Concerning Hg Calibrator Points and Transfer Protocol”, May 6, 2008.

12 Companion Document, “Treatment of Elemental Mercury Gas Generator

Certification Data and Calculation of Un certainty in Support of the Interim EPA Traceability Protocol for Qualification and Certification of Elemental Mercury Gas Generators”; U.S. EPA; July 2009.Interim Elemental Mercury Gas Tra ceability Protocol -- July 01, 2009

39 13 McRanie, R. and Roberson, W., “Using Internal Permeation Source as a Potential QA/QC Check of Installed Elemental Hg Calibrator”, RMB Consulting & Research,

Inc.

, April 2, 2008.

14 “Tekran Perm Source Ratio Template R01”

15 Kinner, L.L., “Adsorbent (“Sorben t”) Tube Sampling of Gaseous Mercury Generator/Calibrators and Compressed Ga seous Mercury Cylinders”, Revision 5, Emission Monitoring, Inc., May 26, 2008.

16 Peeler, J., “Field Evaluation of Hg Compressed Gas Standards”, Emission Monitoring, Inc., June 18, 2008.

17 “EPA Traceability Protocol for Assay and Ce rtification of Gaseous Calibration Standards”, EPA-600/R-97/121, September 1997.