Interim Elemental Mercury Gas Traceability 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) calibration standards of known concentration and 

known uncertainty are needed to quality assure data recorded by Hg continuous emission 

monitoring systems (CEMS) and measurements made 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 two basic issues. It provides procedures to: 

(1) establish the quantitative output (i.e., “calibration”) of elemental Hg generators; and 

(2) determine the expanded uncertainty of the gas standards produced by the elemental 

Hg generators. This protocol specifies the maximum allowable uncertainty for the gas 

standards. Demonstrating that a particular generator meets the requirements contained in 

this document establishes that the generator output 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 reference generator. For CEMS applications, this protocol 

applies only to Hg monitoring system span values greater than or equal to 5.0 

micrograms per cubic meter (μg/m3

). 

Section 2, “Overview,” explains the steps in the process leading to elemental Hg 

generators certification and identifies activities performed by NIST and activities and 

responsibilities of the generator manufacturer or others. Section 3, “Definitions” presents 

important definitions related to generator certification and the calculation of uncertainty. 

Section 4, “Qualification Tests,” contains provisions designed to demonstrate the 

adequacy of a particular generator model for the intended field application(s). Section 5, 

“Certification,” details the specific procedures for the quantitative determination of the 

candidate generator output. Section 6, “Uncertainty,” summarizes the method used to 

calculate and report the expanded 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 generators (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 Traceability Protocol -- July 01, 2009 

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2.0 OVERVIEW 

In the simplest of terms, elemental Hg generators produce known concentrations of Hg 

by passing a controlled gas stream through the headspace of a temperature-controlled 

reservoir containing elemental mercury, and then dynamically blending the mercuryladen gas stream with a controlled dilution gas 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 necessary Hg concentrations. The specific design, 

configuration, and features of an elemental generator are left 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 are 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 reliably over a stated range of conditions (e.g., 

back pressures, ambient temperatures, and other applicable factors). These tests may 

either be performed on each individual generator or on selected generators that are 

representative of the model. The manufacturer must provide the end user with a 

statement of disclosure that explains proper set-up and use of the generator and 

documents the range of conditions over which reliable generator operation can be 

expected. 

NIST has developed a fundamental analytical method for quantifying the concentration 

of elemental Hg in gas mixtures to provide traceability to the International 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 generator2, which is 

maintained at NIST Chemical Science and Technology Laboratory in Gaithersburg, 

Maryland, has been certified by this method. 

NIST uses the NIST Prime generator to certify “Vendor Primes” (which are elemental Hg 

generators provided by the manufacturers), by direct comparisons 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 certified Hg concentrations and expanded 

uncertainties. 

This Interim EPA Traceability Protocol specifies the procedures for establishing the 

“NIST traceable” quantitative output of elemental Hg generators that are employed in 

regulatory applications for emission monitoring or testing. These generators may be 

certified either: (a) at the manufacturer’s facility, prior to shipping, by direct comparison 

to a certified Vendor Prime; or (b) at the site where the generator will be used (or at 

another convenient location), by direct comparison to an elemental Hg generator that has 

been previously certified against a Vendor Prime. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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The manufacturer or equipment supplier performing the certifications must maintain 

documentation of all generator certifications. The test date, certified Hg concentrations, 

and uncertainties must be provided in writing to the user. All certification test records 

must be available for inspection by regulatory officials. 

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

manufacturer’s written instructions and operational specifications for regulatory 

applications in emission monitoring or testing. Periodic quality assurance (QA) checks 

and/or quality control (QC) procedures are required to verify that the generator continues 

to provide accurate and reliable calibration standards. Recertification of a generator is 

required periodically to maintain NIST traceability. Recertification may also become 

necessary when certain malfunctions, operating conditions, or other factors identified by 

the user or manufacturer occur. 
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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, recertification, 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 generator output concentration means the arithmetic 

average of the calculated output concentrations of a candidate generator, when a 

bracketing procedure is performed at a given concentration level. 

3.5 Combined standard uncertainty means an estimate of the standard uncertainty, 

calculated by propagation-of-error, 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 bracketing procedures (e.g., initial certification and 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 expanded uncertainty from 

a standard uncertainty. A large coverage factor increases the confidence that the “true” 

value is within the bounds of a stated uncertainty. For a statistical “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 configuration identified by the 

instrument system design, including: 

(a) The use of specific gas flow rate measurement/control devices, Hg evaporation 

chamber, temperature controllers, temperature or pressure measurement and 

compensation devices, and other components, 

(b) The physical arrangement of principal components, 

(c) The specific electronics configuration and signal processing, and 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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(d) The specific software/firmware version and data processing algorithms, as 

implemented by a particular manufacturer and 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 devices, (including external 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 that the performance of the new 

components is as good, or better, than the replaced components. 

3.10 Expanded uncertainty means a standard uncertainty (or combined standard 

uncertainty) multiplied by a coverage factor to provide a wider confidence interval within 

which the “true” value is believed to lie. 

3.11 External adjustment means either (1) a physical adjustment to a component of 

the elemental Hg generator that affects its output 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 subject 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 generator output concentration means the arithmetic 

average of the calculated output concentrations produced by a candidate 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 consistent with the manufacturer's design of the 

instrument and its intended operation. Intrinsic adjustments are either non-elective or are 

configured according to factory recommended procedures; they are not subject to change 

by the user. 

3.15 Measurement stability means an uncertainty component related to how much the 

readings obtained with a measurement device change during a measurement interval. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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3.16 NIST means the National Institute 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 inductively 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 elemental Hg produced by a generator; or (b) the 

concentrations of elemental Hg in compressed gas cylinders are traceable to a NIST 

Prime by an unbroken chain of comparisons. 

3.19 Output ratio means the ratio of the output from a candidate generator to the 

output from a certified reference standard, when a bracketing procedure is performed. 

3.20 Reference standard means an elemental Hg generator or other device (e.g., 

compressed gas cylinder) that is used as the standard for the certification, recertification, 

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 protocol, this uncertainty comes 

from the certification of the Vendor-Prime or Field-Reference generator that is used as the 

reference standard in a bracketing procedure(s). 

3.22 Repeatability means an uncertainty component related to successive measurements 

of the same quantity under the same conditions, based on data collected in a single 

experiment. 

3.23 Reproducibility means an uncertainty component related to successive experiments 

involving the same measured quantity, but not necessarily under identical 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 evaluated based on statistical data collected alongside 

the measurement data. 

3.26 Type B uncertainty component means an uncertainty component that is 

“external” to the measurement process. The statistical 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 measured, minus any bias or systematic error. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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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 generator that has been certified by 

comparing its output to that of a NIST prime. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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4.0 QUALIFICATION TESTS 

Manufacturers are required to conduct qualification tests to demonstrate the adequacy of 

each elemental Hg generator model (as defined 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. Certain other information must be provided to the 

end user by means of a “Manufacturer’s Disclosure” (see Section 4.1) to show that the 

generator model can operate reliably over the range of conditions that are expected to be 

encountered in the intended field application(s). 

4.1 Manufacturer’s Disclosure 

As part of each User generator’s certification, a “Manufacturer’s Disclosure” is required. 

This disclosure must include documentation and other relevant information for the 

elemental Hg generator model, in accordance 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 generator performance. The description must 

identify the critical components, features for the control of (or compensation for) changes 

in external influences, alarms or fault indicators, and necessary external devices or 

connections. 

4.1.2 Range(s) 

The manufacturer shall specify the nominal concentration range(s) (minimum and 

maximum Hg concentrations in units of μg/m3

) 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 parameter values that must be changed or 

selected by the user to establish the effective 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 limitations within which 

the generator model can be expected to operate reliably and meet all of the 

manufacturer’s specifications, for such concerns as: 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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4.1.3.1 Carrier Gas Supply 

The minimum and maximum compressed air or nitrogen supply pressure, the allowable 

variation in supply pressure, the required carrier gas flow rate, 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 reliable 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 the generator can provide reliable Hg gas 

concentrations. 

4.1.4 Operation, Maintenance, and QA 

The manufacturer shall provide written procedures for installation, 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-specific conditions or other 

application factors. The manufacturer shall also identify the specific conditions and 

factors that require recertification of the generator, including, 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, the manufacturer shall prepare, keep on file, 

and make available upon request, a report describing the qualification tests performed to 

ensure that the elemental generator model (as defined in Section 3.8) will operate reliably 

and produce stable, repeatable gas concentrations over a stated range of conditions. The 

report shall indicate whether the qualification tests were performed on each individual 

generator or on selected generators that are representative of the model, and shall 

document the results of the tests. 
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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 qualification tests for each elemental Hg generator 

model, to evaluate such things as generator stability with respect to time and temperature, 

set point reproducibility, and the effects of changes in back pressure, supply pressure, and 

line voltage on generator performance. Sections 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 tests described in Section 4.2 will suffice for the purposes of 

initial certification. However, successful completion of the standardized qualification 

tests is a prerequisite for initial generator certifications that are performed after the 

standardized qualification tests are incorporated 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]
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5.0 CERTIFICATION 

The certification protocols described in this document are identical in concept to those 

used by NIST and are similar in implementation. 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/m3

 have been provided by NIST for this 

purpose. 

There are two options for certifying a User generator and establishing its NIST 

traceability. The output from the User generator can be compared directly to the output 

from a certified Vendor Prime at the particular 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 can 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 convenient location, by direct comparison with a 

Field Reference generator that has been previously certified by direct comparison to a 

Vendor Prime. The Field Reference generator 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 effects of the additional transfer standard. 

Within this document, zero-, low-, mid-, and high-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 procedure 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 procedure is to use the output from a certified 

“reference standard” generator to certify the output of a “candidate” generator. For the 

purposes of this interim protocol, the certified reference standard is either a Vendor Prime 

or a certified Field Reference generator. The outputs from the generators are introduced 

alternately to an analyzer so that each response from the candidate generator is bracketed 

by a pair of responses from the reference standard 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 a 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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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 C1 

Reference Standard RS*2 

Candidate C2 

Reference Standard RS*3 

Candidate C3 

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 three 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 generator at a time can be certified using a 

single Vendor Prime or Field Reference generator. If this approach is followed, 

sequential responses from each User generator are recorded in-between the 

Vendor Prime (or Field Reference generator) responses, and linear (time) 

interpolation of the Vendor Prime (or Field Reference generator) responses is 

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

response. 

(b) Table 2 illustrates the bracketing procedure at three typical gas concentrations for 

a User generator certification, designated as “high”, “mid”, and “low”. In this 

example, a certified Field Reference generator is used as the reference standard. 

At the option of the manufacturer, the order of introduction of the three gas 

concentrations shown in Table 2 (i.e., high, mid, and low) may be reversed. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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Table 2. Bracketing Procedure Sequential Steps 

for User Generator Certificationa

Z1

FR*H-1

UH-1

FR*H-2

UH-2

FR*H-3

UH-3

FR*H-4

FR*M-1

UM-1

FR*M-2

UM-2

FR*M-3

UM-3

FR*M-4

FR*L-1

UL-1

FR*L-2

UL-2

FR*L-3

UL-3

FR*L-4

Z2

(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 ”output ratio” (R) is calculated for each value of U. The 

calculation of R is illustrated by Equation 1a, below, for UH-1, the first response to 

the output from the User generator at the high level. 

 

a

 The nomenclature for Table 2 is: Z1= first zero response; FR*H-1=Field Reference generator, high level, 

first response; UH-1 = User generator, high level, first response; etc. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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





































  

 



2

*

2

*

1

1

H H

H

FR FR

U R (Equation 1a) 

 

 Where: 

 R = Output Ratio 

 UH-1 = First response to User generator output at high level 

(zero corrected, if appropriate)b

 FR*

H-1 = Response to Field Reference generator output, 

pre UH-1 (zero corrected, if appropriate) 

 FR*

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

UH-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 UH-1 was actually recorded. 

Note, however, that this equation only applies to equally-spaced measurements 

when one candidate generator is being certified. If the spacing is not equal or 

more than one candidate generator is being certified with a single reference 

standard, a more rigorous, time-interpolated formula in Equation 1b below is used 

to estimate FR*. 

*

2

*

1

1

2 1

1 1

2 1

2 1

 



  





 



 

 

 

 

H

FR FR

U FR

H

FR FR

FR U

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 timestamps associated with the U and * FR readings. Equations 1a and 1b are mathematically 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. Then, 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’ spreadsheets are generally 

used to perform this calculation6,7,8,9: 

 

b

 See Section 5.2, below, for a discussion of zero correction 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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   H  YH X FR R H

*  (Equation 2) 

 Where: 

 YH = Calculated User generator output for the high 

level gas 

 *

H XFR = Certified high-level output concentration of the 

Field Reference generator 

 R H = Average output ratio for the high level gas 

5.1.2 Stepwise Procedures 

(a) At each concentration level, perform a minimum of three (3) comparisons as 

follows: 

(1) Turn on all components of the measurement 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 carrier gas flowing for both generators. 

The following procedure is recommended 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/m3

drift is observed over the 15-minute interval. 

(2) Ensure that the generator internal data logging functions are properly 

configured and operational, that the detector internal data logging 

functions are properly configured and operational, and that any external 

computers or data logging equipment 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 equilibration is not achieved, or that 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 certification of the User generator). 

 

c

 The certified reference standard generator may either be a Vendor Prime or a certified Field Reference 

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

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(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 equilibrium has been established, zero the 

detector by injecting zero gas and making any necessary zero adjustments. 

Once the zero has been set, calibrate the detector by injecting the highlevel calibration gas from the reference standard generator and making any 

necessary span adjustments. Repeat these steps, if necessary, to optimize 

the analyzer calibration relative to the reference standard generator. 

(Note: Adjusting the analyzer to provide the correct responses to the 

reference standard generator is considered 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 point 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 generator to the detector 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 concentration level completes the 3x3 matrix. 

(12) Check the zero response of the detector, 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 average over the desired time period. If the analyzer 

concentrates the sample and an integrated signal is obtained for the generator 

output, record the result of one analysis cycle or average multiple analysis cycles 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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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 generator, 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 automatic baseline correction for every reading. If 

zero correction is required, calculate the interpolated zero offset based on the time when a 

specific response was recorded as follows: 

    

  



 







   

1 2

1 2

1 1 t t

Z Z

Z Z t t i i (Equation 3) 

Where: Zi = Calculated zero offset at ti

 Z1 = Initial zero response 

 Z2 = Final zero response 

 ti = Time of response to be zero corrected 

 t1 = Time initial zero response was recorded 

 t2 = Time final zero response was recorded 

 

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

Uc  Ui  Zi (Equation 4) 

Where: Uc = Response at ti corrected for zero offset 

 Ui = Actual response at ti 

 Zi = Zero offset at ti

(b) The data set in Table 3 provides an example of how each response is corrected for 

zero offset over the course of a test period. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

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Table 3. Zero Corrections for an Example Data Set 

Time 

(min) 

Zero 

Response 

Interpolated 

Zero 

FRi* 

Responsed

FRc* 

Zero-Cor.

Ui 

Responsed 

Uc 

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 Uc (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) 

FRi* 

Response 

FRc* 

Zero-Cor. 

Ui 

Response 

Uc 

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 calculated output ratios arithmetically. For this data set, 

the value of R H is 0.972. 

(e) Finally, use Equation 2 in Section 5.1.1, above, to determine YH, the calculated 

candidate (User) generator output concentration at high level. For this example 

data set, *

FRH 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/m3

 , volts, counts, etc.), depending on the exact set up of the measurement system. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

19

Reference generator, is assumed to be 10.0 μg/m3

. Therefore, the value of YH

from Equation 2 is 9.72 μg/m3

, 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 for the detector, use the uncorrected values 

(FRi

*

 and Ui) in the calculations, as shown below in Table 5. For this example 

data set, the value of YH is 9.73 μg/m3

, based on the uncorrected responses. 

 

Table 5. Calculation of User Generator Output 

 (Without Zero Correction) 

Time 

(min)

FRi* 

Response

Ui 

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/m3

, 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 standard deviation, percent 

 x = Arithmetic average of the output ratios 

 xi = Individual, “ith” output ratio 

 n = Number of output ratios 

(i) The RSD must not exceed 1.0%. If the RSD value is exceeded, the test is invalid 

and must be repeated. 

(j) The calculated RSD values for the output ratios in Tables 4 and 5 are 0.4% and 

0.3%, respectively, and are therefore acceptable. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

20

5.3 Selection of Set Points for User Generator Certifications

Field Reference and Vendor Prime generators 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 used as reference standard generators. The 

certified concentrations of a Field Reference generator are 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 for a User generator may be specified by an 

applicable regulation. For example, the regulations for a CEMS application may require 

the user to perform linearity tests at three levels 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 concentrations near the middle of the specified 

ranges to provide some working margin to avoid 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 considered. The particular concentrations necessary 

for certification of a User generator are established by agreement between the user and 

the equipment supplier. 

Any type or model of elemental Hg generator 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 need not be the same type and model. 

However in practice, equipment suppliers are likely to choose to compare the same or 

similar models. Furthermore, some generator manufacturers may restrict the parameters 

that are varied to produce the necessary concentrations 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 given span, both of these parameters are always held constant 

and only the flow rate of the carrier gas sweeping through the reservoir headspace is 

varied. In such cases, the equipment supplier may choose to have the Vendor Prime (or 

Field Reference) generator certified for various groups of parametric values or 

combinations of parameter settings.11 In this situation, the generator manufacturer shall 

specify the appropriate Vendor Prime (or Field Reference generator) certified 

concentrations or Vendor Prime (or Field Reference generator) parameter combinations 

that are selected for certification of User generators in accordance with a written 

procedure. The written procedure must be available for inspection by regulatory agencies 

upon request. 

After selecting the most appropriate Vendor Prime (or Field Reference generator) 

certified concentrations for comparison with the User generator, the actual set-points for 

the User generator can be verified or adjusted to produce the low, mid, and high-level 

concentrations in the desired ranges. The set 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 of 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

21

the User generator must either be within ± 30% or ± 0.5 μg/m3

 of the certified Vendor 

Prime (or Field Reference generator) concentrations (whichever is less 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 standard and the Field Reference generator is 

the candidate generator. Once a Field Reference generator has been certified, it may then 

be used to certify, recertify, or perform on-going data quality assessments of User 

generators. 

(a) At a minimum, the Field Reference generator certification consists of a three by 

three bracketing matrix performed in duplicate ((3x3) x 2), comparing the output 

of the Field Reference generator to the 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 certification 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 initial zero response for the second matrix (see 

Table 6). 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

22

Table 6: Field Reference Generator Certification 

Dual-Bracketing Proceduree 

Field Reference Generator 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 Section 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/m3

, calculate the relative standard deviation 

(RSD) of the output ratios, using Equation 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 correction, Table 7, below, provides an example of 

how each response is corrected for zero offset over the course of a test period. 

Table 7 illustrates the high-level 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 Traceability Protocol -- July 01, 2009 

23

Table 7: Zero Corrections for an Example Data Set 

Time 

(min) 

Zero 

Response 

Interpolated 

Zero 

Vi* 

Response 

Vc* 

Zero-Cor. 

FRi 

Response 

FRc 

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, determine the output ratios (RH) and the calculated Field 

Reference generator output value (YH) 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 generator is now the candidate), and replacing 

FR*

 with V* (since the Vendor Prime is now the reference standard). Table 8, 

below, illustrates these calculation procedures for the zero-corrected responses 

from the example data set presented in Table 7. For this example data set, the 

certified concentration of the Vendor Prime ( *

H XV ) is assumed to be 10.0 g/m3

. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

24

Table 8: Example Calculation of Field Reference Generator Output 

 (With Zero Correction) 

Time 

(min) 

Vi* 

Response 

Vc* 

Zero-Cor. 

FRi 

Response 

FRc 

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 Reference) generator output concentration is 

the arithmetic average of the two calculated YH values obtained with Equation 2. 

For this example data set at the high gas level, the final value of YH is 9.71 μg/m3

, 

based on the zero-corrected responses. 

(g) Apply the same calculation procedures to each of the other concentration levels. 

A spreadsheet may be used to perform the calculations. 

(h) If zero-correction is not necessary for the detector, use the uncorrected values 

(Vi

*

 and FRi) in the calculations, as shown in Table 9, below. For this example 

data set at the high gas level, the final value of YH is 9.72 μg/m3

, based on the 

uncorrected responses. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

25

Table 9: Example Calculation of Field Reference Generator Output 

 (Without Zero Correction) 

Time 

(min)

Vi* 

Response

FRi 

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 10.50 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

26

6.0 UNCERTAINTY 

This section describes the calculation method that is used to determine the expanded 

uncertainty of elemental Hg concentrations 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 available, the standard uncertainty is the standard deviation of a series 

of repeated measurements related to the component. For a Type B component, standard 

uncertainty is established based on either (1) statistical data provided by the external 

source, or (2) a “worst case” estimate converted to a comparable standard deviation. 

Once all standard uncertainties are established, they are combined through propagation of 

error to arrive at a single combined uncertainty value representing the overall uncertainty 

of the measurement. Finally, the expanded uncertainty is established by multiplying the 

combined uncertainty by a coverage factor, 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 concentration of an elemental mercury generator is 

accumulated at each step in a chain of comparisons. The process starts with the primary 

measurement standard, NIST’s Isotope Dilution Induced Coupled Plasma Mass 

Spectroscopy (ID-ICP-MS) method, which is used 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 uncertainty 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 certification (where a candidate can be either a 

User generator or a Field Reference generator). This protocol describes how the 

combined uncertainties of the Vendor Prime 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 uncertainties of both comparisons must be taken 

into account when calculating the User generator concentration uncertainty. 

6.2 Uncertainty Components 

In addition to the uncertainty of the reference standard, the following four sources of 

uncertainty have been identified for assigning concentration values to candidate generator 

setpoints: (1) detector linearity; (2) measurement stability; (3) repeatability; and (4) 

reproducibility. The first three components apply specifically to a comparison 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

27

(bracketing) procedure. The reproducibility component applies to the overall process of 

transferring traceability from the reference gas standard to the candidate. 

For the mathematical details of how uncertainty is calculated, refer to the Companion 

Document to this interim protocol12. In practice, these calculations are incorporated into 

the same spreadsheet files that calculate the certified concentrations. EPA has developed 

several example spreadsheets, based on the equipment commercially available at the time 

of this writing. Both the Companion Document 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 calculation applies a statistical test to the 

assumptions that are built into the bracketing technique (e.g., no non-linear drift, 

wavering responses, or excessively “noisy” measurements). This uncertainty component 

is calculated based on the standard 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 precision of the output ratio determinations within a single bracketing 

procedure. It is calculated as the difference between (1) the standard deviation of the 

individual bracket ratios, and (2) the statistical contribution of measurement instability to 

the standard deviation of the ratios. 

6.2.4 Reproducibility 

Reproducibility is a measure of how precisely a bracketing procedure’s result can be 

duplicated. Because of the differences between the certification procedures (as described 

in Section 5), reproducibility may be estimated differently depending on what type of 

generator is being certified (i.e., Field Reference 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. Either 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 certified value. The results of 

field testing have shown that this Type-B reproducibility is routinely achievable. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

28

6.3 Combined and Expanded Uncertainty 

Once calculated, all of the individual components (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 combined into a single comparison uncertainty 

(dimensionless, because it is the uncertainty of the comparison ratio determination). This 

comparison uncertainty is multiplied by the reference concentration and combined with 

the reproducibility and reference concentration uncertainties 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 above, does not exceed 5.0 percent of the 

certified value, or is not more than 2.0 percent above the Vendor Prime uncertainty at the 

closest set pointf

, whichever is less restrictive. 

 

f

 If the candidate certification setpoint does not exactly match the reference standard setpoint, base the 

alternative specification on the closest reference standard setpoint. 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

29

7.0 QUALITY ASSURANCE 

This section specifies: (a) the periodic data quality assessments that are required 

following initial certification of an elemental Hg generator, to provide assurance of 

continued acceptable performance; and (b) the required frequency of generator 

recertification. 

7.1 Periodic Data Quality Assessments of Certified User Generators 

(a) Periodic data quality evaluations of the generator 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 corrective action that may reasonably 

be expected to affect the generator’s output, but does not necessitate 

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

(b) Any of the data quality assessment 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 Section 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 accordance 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 procedures 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 arithmetically, and then calculate Y (see Section 

5.1.1, above). 

(b) For continuous emission monitoring system (CEMS) applications, perform the 

comparison with the User generator output set at the certified Hg concentration 

that is used for daily calibration error checks. The certified set point Hg 

concentration from the Field Reference generator must be within ± 30% or ± 0.5 
Interim Elemental Mercury Gas Traceability Protocol -- July 01, 2009 

30

μg/m3

 (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/m3

 (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 demonstrate 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 consistent 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 permeation device or elemental generator, 

determine an initial (“base”) ratio of analyzer responses to the permeation device 

and to the elemental generator, as described in paragraph (c) of this section, to 

serve as a reference point for subsequent periodic performance evaluations. It is 

recommended that the base ratio be established as soon as practicable after initial 

certification, and re-established as soon as practicable after any subsequent 

corrective actions or system modifications 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, perform the comparison procedure at a 

single concentration level, preferably (but not necessarily) corresponding to the 

level of the daily calibration error check. Before recording each measurement, 

allow sufficient time to ensure that a stable analyzer reading is obtained. For each 

pair of readings, calculate (Aperm/Agen), the ratio of the analyzer responses to the 

output from the permeation device and the elemental generator. If more than one 

pair of readings is obtained, the base ratio shall be the arithmetic average of the 

calculated ratios. 

(d) For the periodic data quality assessments, repeat the comparison procedure 

described in paragraph (c). Only one pair 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 minimum requirement, it is recommended that three pairs be 

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

31

calibrated against a reference method, perform this periodic performance 

evaluations at least monthly (preferably weekly), for CEMS applications. 

(e) The performance of the User generator is acceptable if the calculated value of 

(Aperm/Agen) output ratio of the generator (or the 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 

performanceh

. However, if this approach proves to be unsuccessful and it 

appears that the generator is actually malfunctioning, follow the provisions 

of Section 7.1(c), above, re-establishing the base ratio, if necessary; or 

(2) If none of the other data quality assessment procedures in Section 7.1 of this 

protocol is available, the permeation 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 passed 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 protocol. The procedures are detailed in an 

April 3, 2008 document prepared by Tekran Instruments13. A spreadsheet for the 

efficient processing of results has been developed and is available14. 

The Agency is persuaded that the internal permeation tube technique developed by 

Tekran provides meaningful insight into the actual performance of the elemental 

generator, and is consistent with the generic approach described in Section 7.1.2.1 of this 

protocol. The viability of this approach has been demonstrated 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 does 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 appropriate follow-up action. 
Interim Elemental Mercury Gas Traceability 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 confirm the quantity of Hg that is generated. 

(a) Conduct the comparison with the User generator producing a certified set point 

Hg concentration. For CEMS applications, the set point shall be the Hg 

concentration that is used for daily calibration error checks. 

(b) Perform all sorbent tube sampling according to the procedures (including all of 

the QA procedures) of document No. 14 cited in the “References” section of this 

interim protocol.15 Perform a minimum of three replicates of paired samples. 

Average the three results. 

(c) Calculate the difference between the average Hg concentration measured with the 

sorbent tubes and the generator’s certified set point Hg concentration. The 

performance of the User generator is acceptable if this difference is within ± 5.0% 

of the certified set point concentration, or if the absolute value of the difference is 

no greater than 0.5 μg/m3

 (whichever is less restrictive). 

7.1.4 Option D---Cylinder Gas Comparison 

Evaluate the performance of a User generator 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 analyzer used for the comparisons is sensitive 

to variation in the oxygen concentration in the sample gas, perform the comparisons only 

if the oxygen concentrations in the compressed gas cylinder and in the User generator 

effluent are the same, or are known accurately and precisely so that appropriate 

corrections can be applied. 

7.1.4.1 Comparison Procedure and Criteria 

(a) Perform the direct comparison using the procedures for introduction of calibration 

gas to an elemental Hg analyzer as well as the nesting procedure and data 

processing described in document No. 15 cited in the “References” section of this 

interim protocol.16

(Note: A specially coated regulator (such as a SILTEK coated regulator body, 

CGA Nipple, and Isolation Valve) should be used with the compressed gas 

cylinder) 

(b) Before proceeding, the following stability 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/m3

 of drift is observed over a period 

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

33

(c) Perform a bracketing procedure, consistent with Section 5.1.2 and Table 1, above, 

using the compressed gas cylinder as the 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 the comparison with the User generator 

producing the certified set point Hg concentration that is used for daily calibration 

error checks. The certified concentration of the compressed gas cylinder must be 

within ± 30% of that set point concentration. 

(e) Use the average output ratio and the certified cylinder concentration to calculate 

the average User generator output concentration, in a manner consistent with 

Section 5.2, above. The performance of the User generator is acceptable if its 

calculated average output concentration is within ± 5.0% or ± 0.5 μg/m3

(whichever is less restrictive) of the certified set point Hg concentration. 

7.1.4.2 Protocol Gas Mixtures 

After NIST-certified compressed gas mixtures become available to commercial 

calibration gas suppliers, EPA will revise its traceability protocol for gaseous calibration 

standards17, to include procedures for preparing elemental Hg Protocol Gas mixtures. 

Until such time as the referenced EPA traceability protocol document is revised, the 

procedures that are used for preparing SO2 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 elemental Hg generator. If this option is 

selected, the gas supplier shall develop and maintain a detailed written procedure 

describing the exact procedures used to certify cylinders relative to a Vendor Prime 

including all quality assurance measures to ensure 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) procedures are required for Field Reference 

generators that are used as reference standards to certify, recertify, or perform periodic 

QA evaluations of User generators. Each Field Reference generator that is used for these 

purposes must have a current (non-expired) certification or recertification status. 
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7.2.1 Preliminaries 

Before each use of a Field Reference generator for a certification, recertification, or QA 

application, ensure that the generator is operating properly. The manufacturer’s written 

procedures and specifications for proper operation 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, recertify, or quality-assure data from a User generator (or generators): 

(a) Perform the preliminary procedures described in Section 7.2.1, above. Then, 

initiate the stepwise bracketing procedure 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 Field 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 the mid-level gas outputs, from paragraphs 

(b) and (c), above. Also calculate this output 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 has 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 generator is acceptable. If this criterion is not 

met: 

(1) Corrective action (which may involve recertification—see Section 4.1.4, 

above) is required before the Field Reference generator may be used to 

certify, recertify, or perform quarterly evaluations of User generator(s); and 

(2) If the mid-level gas injection in paragraph (c), above, occurred either during 

or immediately after a bracketing procedure, the results of that test shall be 

invalidated. 
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7.2.3 Use of Control Charts 

(a) When a particular Field Reference generator is used to perform a series of 

bracketing tests on a particular User generator (e.g., the initial certification and 

subsequent quarterly data quality assessments), 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 assessing generator stability over the longterm. Significant changes in this ratioi

 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 generator malfunction is discovered: 

(i) Corrective action (which may involve recertification of the 

generator—see Section 4.1.4, above) is required; and 

(ii) The bracketing procedure for which the control chart indicated a 

possible malfunction is invalid and must be repeated, following the 

corrective action. 

(iii) When a malfunction of the Field Reference generator (but not the 

User generator) is found, data from the User generator are not 

impacted. However, if a User generator malfunction is discovered, 

data from that generator are considered invalid from the hour of 

completion of the invalidated bracketing procedure until 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 for 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 significantlyi

 since the last test, but there has been no 

corresponding change in the other User 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 significant. 
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36

(3) If the output ratios for the User generators have shifted significantlyf

 in the 

same direction since the previous test, the Field Reference generator may be 

malfunctioning. 

7.3 Recertification 

Following initial certification, all User generators and Field 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 required recertifications of a Field Reference generator may 

be extended, up to a maximum of 8 calendar quarters from the calendar quarter of 

the last test, as follows: 

(1) The recertification test deadline may 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 procedure, the Field Reference generator is 

the candidate generator and any available 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 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); 

(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 described in paragraph (c)(1). 

(3) As an alternative to extending the recertification test deadline one quarter 

at a time, as described in paragraphs (c)(1) and (c)(2), the test deadline 

may be extended by four calendar quarters if the following more rigorous, 

one-time test is successfully completed in the calendar quarter in which 

the recertification test is due. Perform the 3x3x2 bracketing procedure 

described in Section 5.4, above with the Field Reference generator as the 
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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 certification 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-expired) certification or recertification 

status. 

 

 
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References 

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

Devices Using Isotope Dilution Cold-Vapor Generation 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., “Report of Analysis – Traceability of Mercury 

Measurements,” EPA Reference DW-13-92214701. 

4

 Mitchell, G. “Report of Analyses – Quantification of Selected Set-Points for One 

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

2008. 

5

 Leigh, S., “Memorandum – Uncertainty Calculations for the Certification of Mercury 

Generator Outputs”, 1-14-08. 

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

Calibrator Versus a Reference Calibrator--Thermo Procedure”, RMB Consulting & 

Research, Inc., April 7, 2009. 

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

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

Calibrator Versus a Reference Calibrator--Tekran Procedure”, RMB Consulting & 

Research, Inc., May 1, 2009. 

9 Tekran Data Reduction Template --RMB 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 Uncertainty 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 Traceability 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 (“Sorbent”) Tube Sampling of Gaseous Mercury 

Generator/Calibrators and Compressed Gaseous 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 Certification of Gaseous Calibration 

Standards”, EPA-600/R-97/121, September 1997.