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DETECTION LIMITS, PQLs AND NJQLs
Presented by Stu Nagourney NJDEP, OQA
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MDL DEFINITION Defined at 40 CFR 136 Appendix D
The minimum concentration of a substance that can be measured and reported with 99-percent confidence that the analyte concentration is greater than zero and is determined from the analysis of a sample in a given matrix containing the sample
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OTHER DEFINITIONS IDL RL ML
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MDL CALCULATION Seven (7) replicates at ~ [5 times] the background noise: should be at or near the low calibration standard Multiply by (z) distribution factor 3.14 time the S.D. = MDL The MDL is more dependent upon precision than accuracy Labs. must demonstrate that they can see a signal at the MDL
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MDL ISSUES TO CONSIDER Many regulatory standards near detection limit
Lab credibility often associated with detection limits Maze of tests and matrices make it difficult to find a universally applicable technique Calculating detection limits can be expensive
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WHY YOU CAN’T SET PERMIT LEVELS @ THE MDL
MDLs vary by day, lab., analyst & method Precision at low levels are usually poor Good for false-positives; false negatives may be reported 50% of the time Inherent uncertainties make enforcing permit MDL values impossible
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MDLs to PQLs PQL: Practical Quantitation Limit
PQL = Average (Median) MDL x 5 Sources of MDL data Interlaboratory low-level studies Applying statistics to low-level cal. data Method data where available Most data is now 5-12 years old!!!
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PQLs to NJQLs The Department recognizes that the current PQL data is based upon out-dated technology OQA already requires certified labs. to collect MDL data by method and media What we need is current, representative data and a way to manage it
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NJQLs & NJDEP OQA is contracting for the development of a database to collect MDL data from NJ certified labs. and then generate NJQLs (avg. MDL x 5) NJQLs to be generated by analyte, matrix, method DW WP SHW DEP programs will have a choice in selecting an NJQL for their individual needs
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