The Fundamentals of Understanding and Evaluating Laboratory Environmental Data

Transcription

1 Ask The Expert Webinar Series The Fundamentals of Understanding and Evaluating Laboratory Environmental Data Dr. William Anderson Senior Chemical and Analytical Project Consultant

2 Typical Quantification of Environmental Analytes 2

3 Calibrations and Data Flags The Use of Surrogates The Use of Internal Standards 3

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6 Detection Limit versus Quantification Limit 6

7 What is the Reporting Limit or RL? Defines where quantification is commenced without data flags Defines hard quantification Data is bracketed by calibration points 7

8 What is a Method Detection Limit or MDL? Is statistically determined Is a level that says with 99% confidence that a hit is greater than zero Since the MDL is statistically derived, it has a confidence interval around it 8

9 The Implications are Important! Example: If MDL = 2 ppb At a 95% confidence interval, what is the range from the upper and lower confidence levels? 9

10 At 2 ppb: Lower Statistical Bound Upper Statistical Bound 1.28 ppb 2 ppb 4.4 ppb At a concentration of the analyte at 2 ppb, we are 99% confident that the analyte will be detected. Our ability to find 2 ppb with 95% confidence limits is represented by a range of results from 1.28 to 4.4 ppb. Statistically they are the same number! 10

11 11 If the MDL = 2 ppb, then an incorrect reporting convention is:

12 The MDL was Promulgated in CFR Part 136, Appendix B See: Development of Compliance Levels from Analytical Detection and Quantitation Levels EPA Document Number PB

13 Precision and Accuracy Precise, But Not Accurate 13

14 Precision and Accuracy Accurate, But Not Precise 14

15 Precision and Accuracy Accurate and Precise 15

16 The Use of Surrogates Definition and Description of Use Examples of Chemistry at Work 16

17 The Use of Surrogates A surrogate is a chemically similar compound to the ones being investigated but not expected to occur in the environmental samples. The final data is not adjusted for surrogate recoveries. 17

18 Concentration 100 ppb Surrogate Standard 100 ppb Surrogate Standard Analyte Surrogate Recovery = 100% Analyte = 100 ppb Concentration 100 ppb Surrogate Recovery = 25% Analyte = 100 ppb Concentration 100 ppb Surrogate Recovery = 150% Analyte = 100 ppb 18

19 Surrogates for Volatiles by Method 8260B CD 3 D D Cl Cl D C C D D D Toluene-d 8 D D D 1,2-dichloroethane-d 4 F Br F C Br H Dibromofluoromethane Br 4-Bromofluorobenzene 19

20 The Use of Internal Standards (IS) Definition and Description of Use Examples of Chemistry at Work 20

21 The Use of Internal Standards (IS) An internal standard is a compound that is chemically similar to analytes being determined and spiked directly onto a test sample at the analytical instrument used to perform the analytical test at a known concentration. The response of the instrument to the known level of internal standard is used to quantitate the analyte in the sample. 21

22 22 Internal Standard 100 ppb

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