Update on Mercury Calibration Gas Standards and Traceability

Scott Hedges

US EPA, Clean Air Markets Division

2009 EPRI CEM User Group Conference

St. Louis, MO

May 14, 2009

Background

A significant collaborative effort between EPRI, industry, and EPA has been ongoing and has produced a technically feasible and implementable mercury monitoring program

EPA is fully committed to continuing this collaborative effort

Providing NIST-traceable mercury (Hg) calibration gas standards and protocols is one of the remaining technical issues being addressed

Update on NIST-Traceable Hg Calibration Gas Standards

EPRI, NIST, OAQPS, ORD, CAMD and industry continue their collaborative work to provide NIST traceability for elemental and oxidized mercury calibration standards

To date, NIST has certified and recertified their NIST Prime elemental Hg generator, has certified the first round of the Vendor Prime generators. Vendor Prime generator recertification is expected to begin in the near future.

Manufacturers have also shipped elemental mercury gas cylinders to NIST to establish NIST traceability. This work is in progress.

EPA has also worked with Western Research Institute, ORD, NIST and vendors to conduct variability, stability, and uncertainty testing of mercury gas generators and cylinders

Phased Approach to NIST Traceability

We are implementing a phased approach to NIST traceability

– The first phase (last year and the first period of 2009) was devoted to conducting a field and laboratory Hg calibration gas demonstration program and writing interim elemental and oxidized Hg traceability protocols

– The second phase (beginning shortly) is devoted to certifying Hg calibration gas generators to establish NIST traceability

Mercury Calibration Gas Field and Laboratory Demonstration Program

EPRI sponsored and provided funding to conduct the Hg calibration gas field and laboratory demonstration program that was completed earlier this year

The program was a collaborative effort between EPRI, calibration gas and monitoring equipment vendors, NIST, RMB, Emission Monitoring Inc., WRI, participating utilities, and EPA

Key elements of the program included:

– An assessment of the field performance of Hg gas generators (the program focused on the elemental Hg generators)

– An evaluation of the stability of elemental Hg compressed gas cylinders

Demonstration Program

The demonstration program informed the development of the Hg gas traceability protocols by:

– Providing data on the quality (stability, repeatability, variability) and uncertainty of elemental Hg gas generators and cylinders

– Developing and testing draft bracketing procedures to compare elemental Hg gas generators and cylinders in the laboratory and in the field against “certified” (NIST-traceable) reference generators

The demonstration program determined certification acceptance criteria, QA objectives, frequency of recertification, and qualification criteria for elemental Hg calibration gas standards

Demonstration Program

Developed ongoing QA procedures and finalized procedures/spreadsheets to calculate uncertainty

Provided input to the procedures written into the interim traceability protocols

  Developed and set-up the necessary infrastructure for

affected sources and equipment vendors to implement the field program and ultimately the traceability protocols

How Traceability Works

Mercury Gas Standard Traceability

A NIST-traceable Hg gas standard (produced by a generator or cylinder) is one that has been compared and certified directly (without intermediate standards) to another Hg gas standard that has been certified by NIST at its laboratories

Traceability can be simply defined as an “unbroken chain of comparisons” to a primary NIST standard

Gas standards must be traceable to a measured, not theoretical, concentration

Mercury Calibration Gas Standards

What We Need:

Mercury calibration gas standards of known quality and uncertainty

  How well Hg generators and cylinders perform

under the “best” conditions, and in the field

Mercury Calibration Gas Standards

Definition and use of uncertainty:

A statistical predictor of closeness to a true measurement or reference (minus bias or systematic error)

Uncertainty is often confused with measurement error which includes systematic error or bias

 

Mercury Calibration Gas Standards

Definition and use of uncertainty : Combined uncertainty is the sum of each individual

uncertainty propagated down the “unbroken chain”(e.g, uncertainty of NIST Prime +Vendor Prime + any transfer standard such as a certified Field Reference generator + field or User generator)

Expanded uncertainty includes a coverage factor or multiplier to the combined uncertainty that is essentially comparable to a confidence interval (for this program a coverage factor of 2 was used)

 

Interim Mercury Gas Traceability Protocols

We are finalizing interim traceability protocols for elemental and oxidized Hg gas generators and expect to make them available to states and other groups for use within the next month

The procedures of these interim protocols will be replaced when final EPA traceability protocols for elemental and oxidized Hg generators are completed

Interim Elemental Mercury Gas Traceability Protocol

NIST Traceability for Elemental Hg Generators

NIST calibrates individual set points (calibration concentrations) that are used to certify Vendor Prime generators, vendors certify Field Reference generators which in turn are used to certify elemental Hg generators already in the field

NIST certifies Vendor Prime generators against their NIST prime generator that has been certified using ID-ICP/MS

NIST Traceability for Elemental Hg Generators

It is the responsibility of the vendor to qualify and certify their field generators using the procedures in the interim traceability protocols

It is the responsibility of the end user to perform the quality assurance checks once their generators have been certified and placed into field operation

Interim Elemental Hg Gas Protocol

Establishes the quantitative output (i.e., “calibration”) of elemental Hg generators

Determines the expanded uncertainty of the gas standards produced by elemental Hg generators

Specifies the maximum allowable uncertainty for elemental mercury gas generators

Establishes generator output traceability to NIST by means of an unbroken chain of comparisons between each generator and a NIST-certified generator

Interim Elemental Hg Protocol – Qualification Tests

Qualification tests are required for each elemental Hg generator model

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.

Interim Elemental Hg Protocol – Certification Tests

The interim elemental Hg gas protocol specifies the procedures for certifying elemental Hg generators (i.e., procedures to establish the “NIST traceable” quantitative output)

The output from the candidate 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*.

Alternatively, a candidate 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.

* A bracketing procedure is a means of directly comparing the outputs of two elemental Hg gas generators. The purpose of the procedure is to certify the output of a “candidate” generator with the output from a certified generator.

Interim Elemental Hg Protocol - QA/QC and Recertification

Following generator certification, periodic QA/QC procedures are required to verify that the generator continues to provide accurate and reliable calibration standards.

– These checks and procedures may be performed using a certified Field Reference generator or other device (e.g., compressed gas cylinder, sorbent trap, etc.)

Recertification of a generator is typically required every 8 calendar quarters to maintain NIST traceability. Recertification may also become necessary when certain malfunctions, operating conditions, or other factors identified by the user or manufacturer occur.

Interim Oxidized Mercury Gas Traceability Protocol

Oxidized Hg Gas Traceability

The interim oxidized Hg gas traceability protocol reflects the current procedures for generating and quantifying mercuric chloride (HgCl2) calibration standards. HgCl2 is a form of oxidized mercury.

Traceability of HgCl2 Generators

An established approach for direct measurement of HgCl2 gas standards is not currently available

The interim oxidized mercury gas protocol relies on establishing NIST traceability of the individual components of the HgCl2 generating systems followed by calculation of a combined and expanded uncertainty.

Work is underway by ORD, NIST and others to evaluate one or more direct HgCl2 measurement approaches

Traceability of HgCl2 Generators

Once completed, this direct measurement approach will be used in the final oxidized Hg gas protocol

The interim traceability protocol provides procedures to:

– Establish the quantitative performance (i.e., “calibration”) of components of HgCl2 generators

– Determine the uncertainty contributed by each of these components in order to calculate the reference value and combined expanded uncertainty of the standards produced by the HgCl2 generators

Traceability of HgCl2 Generators

The reference value for an evaporative HgCl2 generator output concentration at specific operating conditions is based upon the feed rate and concentration of the HgCl2 solution and the associated volumetric diluents, including the contribution from evaporated water

The combined uncertainty for each reference value is based on the combined individual standard uncertainties of the HgCl2 solution, the HgCl2 solution feed rate, additional water feed rate (if any) and the carrier/diluent volumetric flow. The expanded uncertainty is calculated using a coverage factor of 2.

Traceability of HgCl2 Generators

Two basic types of HgCl2 generators are currently in use:

– An evaporative HgCl2 generator, which produces gas standards of known concentration by vaporization of aqueous solutions of HgCl2 and quantitative mixing the resultant vapor with a diluent carrier gas;

– A reactor or oxidizer that converts the output from an elemental Hg generator to HgCl2 by means of a chemical reaction with chlorine gas.

Certification of Evaporative HgCl2 Generators Generator certification is based on calculating the uncertainty of the

following components: – Working solution concentration – Liquid feed rates– Carrier gas flow rate

The HgCl2 concentration of the working liquid feed solution is established through the use of commercially available NIST-traceable HgCl2 liquid standards

Liquid feed rates are established by gravimetrically calibrating the feed rate meter relative to a NIST- traceable mass using a balance

Certification of evaporative HgCl2 generators requires:– Traceability to NIST for each of the inputs– Determining combined and expanded uncertainty of the generated

concentrations

Certification of Generators That Convert Elemental Mercury to HgCl2

Certification is based on the reference and uncertainty values from the certified elemental Hg generator

Chlorine dilution and humidification must be taken

into account to determine the reference value of the HgCl2 concentration

The combined uncertainty of the HgCl2 calibration

standards is calculated based on the uncertainty of the elemental Hg generator, the water vapor dilution (if applicable), the chlorine dilution, and possibly the reactor (oxidation) efficiency

Quality Assurance for HgCl2 Generators

Performance assessments:

– For CEM systems with independent elemental Hg and evaporative HgCl2 generators, the daily calibration error test and weekly system integrity check constitute direct comparisons of the two generators with the system integrity check results used to determine if the evaporative HgCl2 generator is performing within acceptable tolerances.

– For HgCl2 generators that convert elemental Hg, acceptable performance is based on periodic checks of the oxidation efficiency

Recertification of HgCl2 Generators

At a minimum, HgCl2 generators shall be recertified at least once every 8 calendar quarters

Conclusions

A significant collaborative effort between EPA, EPRI, NIST, industry and others is near completion to provide traceable mercury calibration gas standards and protocols

Interim traceability protocols are expected to be available within a month

Questions?

Comments?

Thank You