Measurement uncertainty in environmental analysis

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Measurement uncertainty in environmental analysis

  1. 1. Measurement Uncertainty in Environmental Analysis Todd R. Crawford, BA Center for Toxicology and Environmental Health, LLC
  2. 2. Standard Procedures <ul><li>ISO 1993 – ‘Guide to the Expression of Uncertainty in Measurement’ </li></ul><ul><li>NIST 1994 – ‘Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results’ </li></ul>
  3. 3. Uncertainty Statements <ul><li>Inform the reader of how sure the writer is that the answer lies within that range. </li></ul>
  4. 4. Uncertainty Statement? <ul><li>“…if by any chance our value is shown to be in error by more than 1 part in 1,000, we are prepared to eat the apparatus and drink the ammonia.” </li></ul><ul><li>Attrib. Dr. C.H. Meyers </li></ul>
  5. 5. Error <ul><li>Error is a single value – an idealized concept, error cannot be known exactly. </li></ul><ul><li>Error is the difference between the true value and the result of the measurement. </li></ul>
  6. 6. Uncertainty <ul><li>Uncertainty is the state of knowledge about the unknown error. </li></ul><ul><li>Uncertainty is given as the range in which the error is to be expected. </li></ul>
  7. 7. Accuracy <ul><li>Accuracy is the closeness of a measurement to the true value. </li></ul><ul><li>Accuracy is expressed as the percent recovery. </li></ul>
  8. 8. Precision <ul><li>Precision is the closeness of agreement between repeated measurements. </li></ul><ul><li>Precision is expressed as the relative standard deviation (RSD). </li></ul>
  9. 9. Measurement Uncertainty in Environmental Analysis <ul><li>The true value cannot be known. </li></ul><ul><li>The reported value is a ‘consensus’ value. </li></ul>
  10. 10. Reporting Uncertainty (ISO) <ul><li>Reporting is required when: </li></ul><ul><ul><li>The information is relevant to the application of the results, </li></ul></ul><ul><ul><li>When the client requires it, </li></ul></ul><ul><ul><li>When the Uncertainty affects compliance with a specification limit. </li></ul></ul>
  11. 11. Evaluating Uncertainty (ISO) <ul><li>Evaluating is required for calibrations. </li></ul><ul><li>Procedures for estimating uncertainty are needed and need to be applied. </li></ul>
  12. 12. Data Quality <ul><li>The Uncertainty of a result is a quantitative indication of its quality. </li></ul><ul><li>Quantitative Uncertainty allows comparison of the results. </li></ul><ul><li>Measurement Uncertainty needs to be taken into account when interpreting the data. </li></ul>
  13. 13. TPH in Soil <ul><li>Bioremediation of a former petroleum terminal to residential clean-up standards. </li></ul><ul><li>Approximately 500,000 cy of soil treated. </li></ul><ul><li>Approximately 15,000 samples analyzed on-site over five years. </li></ul><ul><li>On-site data demonstrated compliance with the remedial criteria. </li></ul>
  14. 14. Interlaboratory Study <ul><li>Measure the Accuracy and Precision of Five Labs Analyzing TPH in Soil by GC-FID. </li></ul><ul><li>Homogenous soil containing biodegraded diesel fuel with 10% moisture. </li></ul><ul><li>Soil submitted in blind triplicates to each lab. </li></ul><ul><li>Each lab is certified for this analysis. </li></ul>
  15. 15. Summary Results (n=15) <ul><li>Control Lab </li></ul><ul><ul><li>TPH = 921 ppm </li></ul></ul><ul><ul><li>RSD = 2.4% </li></ul></ul><ul><li>Independent Labs </li></ul><ul><ul><li>TPH = 755 ppm </li></ul></ul><ul><ul><li>RSD = 35% </li></ul></ul>
  16. 16. Assessing Measurement Uncertainty <ul><li>Any set of numbers (n>3) has a mean and standard deviation. </li></ul><ul><li>Complex statistical tests are beyond the expertise of most data users. </li></ul><ul><li>Most data sets are too small to assess with any confidence. </li></ul>
  17. 17. Charting to Evaluate Uncertainty <ul><li>Rank the data in order of concentration. </li></ul><ul><li>Normalize the data against the maximum value. </li></ul><ul><li>Chart the value against its rank, where rank equals 1/n. </li></ul><ul><li>Determine the slope and correlation coefficient. </li></ul>
  18. 21. Conclusion <ul><li>Forensic environmental investigations require evaluation of measurement uncertainty. </li></ul><ul><li>Quantitative results are most useful when reported in the context of their measurement uncertainty. </li></ul>
  19. 22. Conclusion <ul><li>Measurement uncertainty is significantly improved with the familiarity of the laboratory to the analysis. </li></ul><ul><ul><li>On-site is better than Off-site! </li></ul></ul><ul><li>Measurement uncertainty can be evaluated (more effectively) by charting. </li></ul>
  20. 23. Acknowledgements <ul><li>John E. Ross, de maximis, Inc. (Charter International Oil Company) </li></ul><ul><li>James S. Smith, Les Eng, Trillium, Inc. </li></ul><ul><li>Willem Schre ü der, Principia Mathematica, Inc. </li></ul><ul><li>Glenn Millner, Center for Toxicology and Environmental Health, LLC </li></ul>

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