NIST-Traceable Hg Calibration Standards

1. Mercury Calibration Gas Standards and NIST Traceability Scott Hedges, USEPA, CAMDEPRI CEM Users Group MeetingPhoenix, AZMay 9 - 11, 2007

2. NIST-Traceable Hg Calibration Standards • EPA and NIST continue their collaborative work to provide NIST traceability for elemental and oxidized mercury calibration standards • We are expecting to have NIST-traceability protocol documents this year for use in certifying CEMS and sorbent trap monitoring systems • A working draft traceability protocol has been prepared for elemental mercury gas generators • Will incorporate user-friendly uncertainty calculation spreadsheets into the draft

3. Hg Gas Standard Traceability • CAMR requires “NIST Traceable” elemental and oxidized Hg gas standards • A "NIST-traceable" Hg gas generator is one that has been compared and certified directly (without intermediate standards) to another Hg gas generator that has been certified by NIST at its laboratories • Gas standards must be traceable to a measured, not theoretical, concentration

4. How Traceability Protocol Works … • NIST Standard and NIST-Traceable Reference Materials (SRMs and NTRMs) are gas standards used by vendors to: - generate the gas standards used in the field by industry - calibrate instrumentation to establish the concentrations of the field gases sold • EPA Traceability Protocol provides guidance on how to establish the uncertainty of the gases used, traceable to the NIST reference materials

5. How Protocol Process Works for Elemental Hg Generators • NIST certifies vendor-prime generators using ID-ICP/MS • NIST calibrates individual set points that are used to certify vendor-prime generators • EPA traceability protocol uses qualification and certification tests to establish traceability of field generators (user-primes) to NIST-certified generators (vendor-primes) • Protocol includes quarterly QC check procedures to evaluate continuous performance of generators in the field • It is the responsibility of the vendor to qualify and certify their field generators using the guidance in the protocol • It is the responsibility of the end user to perform the quarterly QC checks once their generators have been placed into field operation

6. Traceability Chain for Elemental Hg Generators

7. Qualification Tests for Establishing Traceability • Qualification tests have been designed to examine representative generators for potentially significant operating variables (e.g., back pressure, temperature and flow control, operating environment temperature and voltage, drift, etc) and their effect on candidate generator output performance - To qualify for certification, the output of a candidate generator must be repeatable - Qualification tests designed to demonstrate that the candidate generator output is repeatable over an appropriate range for each variable likely to change between the time and place of certification and its subsequent use - Focus is given to variables that may impact performance in the field

8. Qualification Tests for Establishing Traceability • The objectives of the qualification tests are to demonstrate that: -the candidate generator's concentration output does not change by more than ±1% relative by a given variable over a range likely to be encountered during routine use - the candidate generator's concentration output is repeatable within ±1% relative as the variable is changed over the operational range that is likely to be encountered during use • The relationship between the output and the variable needs to be quantified during the qualification tests • Repeatability must be within ± 1 percent relative for each variable that may change between the point of certification and the point of use • Repeatability tests are based on a series of single point comparisons made between the candidate generator and a stable gaseous mercury calibration standard at six or more different concentrations on each of six or more different days

9. Certification Tests for Establishing Traceability • Certification tests have been designed to establish a quantitative relationship between each field generator and the local vendor-prime generator • Tests are comparable to the NIST certification procedures/method for vendor-prime generators, the difference is that additional statistical / mathematic expressions are being developed that predict mercury concentrations over the entire calibration range of the generator within a known level of uncertainty • These quantitative relationships will be available in easy-to-use spreadsheet software that vendors can use to enter generator set point vs measured concentration • The spreadsheet will then establish the predictive relationship of generator set point to output concentration with a known uncertainty as a relative function of concentration • It is anticipated that the equation will be simply added to the software of each generator so that set point and true concentration agree

10. Certification Tests for Establishing Traceability • Certification requires the measurement of at least 6 comparisons between the candidate generator and a vendor-prime generator at each of no less than 6 NIST-certified concentrations • These comparisons must cover the full range of Hg concentrations (0.2 to 40 µg/m3) and must be spaced equally across at least two days (e.g., 3 comparisons on Day 1 and 3 comparisons on Day 2) • Certification tests are used to establish the 6 comparisons over two days (to determine inter and intra-day repeatability) • Statistical analyses are used to determine output uncertainty • Combined (extended) uncertainty must be within ± 3 percent • We plan to allow a combined uncertainty of ± 5 percent until January 9, 2009 for intra- and inter-day repeatability testing of generators to be completed and interpreted

11. Certification Tests for Establishing Traceability EPA is working on an alternate procedure to allow field certification of candidate generators at three specific concentrations by direct nose-to-nose comparison with the identical concentrations from a vendor-prime generator. The same combined (extended) uncertainty acceptance criterion that is applicable for the six-concentration approach will also apply to the three-concentration approach.

12. Flow Diagram: Candidate Generator Certification

13. Traceability of Elemental Mercury Gas Cylinders • Combined overall uncertainty target is currently ± 3 percent • Currently elemental mercury gas cylinders are prepared/certified according to the Green Book (the existing EPA traceability protocol for gas standards) • Shelf life and stability will be assessed by NIST and EPA with input from vendors and WRI • Cylinders will play a valuable quality assurance role including their use: - in evaluating the field performance of gas generators, and - as stand alone gas standards

14. NIST-Traceable Hg Calibration Standards To date EPA: • Has prepared working draft elemental mercury gas generator traceability protocol • Working draft has been sent to vendors • Is preparing oxidized mercury gas traceability protocol

15. NIST-Traceable Hg Calibration Standards To date NIST: • Has developed draft certification procedures for elemental and oxidized mercury gas generators (vendor-prime generators) • Invited generator vendors to come in and begin certifying elemental mercury gas generators

16. Questions? Comments? THANK YOU!