DETECTION LIMITS, PQLs AND NJQLs

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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 CANT SET PERMITLEVELS _at_ 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
  levels _at_ 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