Measurement uncertainty in environmental analysis


Published on

Published in: Education
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

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>