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NEW DETECTION LIMIT PROCEDURES: IS THERE A CLEAR WINNER?

NEW DETECTION LIMIT PROCEDURES: IS THERE A CLEAR WINNER? Charles R. Lytle, Jason Dahl, and Elizabeth Farkas charlesl@bes.ci.portland.or.us 503-823-5568 Water Pollution Control Laboratory 6543 N. Burlington Avenue Portland, OR 97203. IDL SET SO THAT CHANCE OF A FALSE POSITIVE IS < 1 %

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NEW DETECTION LIMIT PROCEDURES: IS THERE A CLEAR WINNER?

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  1. NEW DETECTION LIMIT PROCEDURES: IS THERE A CLEAR WINNER? Charles R. Lytle, Jason Dahl, and Elizabeth Farkas charlesl@bes.ci.portland.or.us 503-823-5568 Water Pollution Control Laboratory 6543 N. Burlington Avenue Portland, OR 97203

  2. IDL SET SO THAT CHANCE OF A FALSE POSITIVE IS < 1 % ( = 0.01)

  3. Xave IDL HOW LOW CAN WE GO??

  4. Xave IDL BETTER

  5. IDL Xave AT THIS POINT, WE’VE “MOVED” Xave AS FAR AS WE REALLY CAN GO WITHOUT “LOSING” A LOT OF DATA. WE’VE REACHED THE DETECTION LIMIT! BEST “MOVE” Xave UNTIL CHANCE OF FALSE NEGATIVE < 1% ( = 0.01)

  6. HOW DO YOU GO ABOUT SETTING THE IDL SO THAT  = 0.01 AND THEN HOW DO YOU “MOVE” Xave SO THAT  = 0.01??? YOU ONLY HAVE SEVEN OR SO RESULTS. WILL THIS GIVE A NICE, BELL-SHAPED CURVE? ARE YOU SUPPOSED TO EYE-BALL WHERE THE 1% POINT IS? (no)

  7. YOU USE THE t-STATISTIC WHAT IS THE t-STATISTIC? (OTHER THAN BEING A JUST A BUNCH OF NUMBERS FOUND IN A TABLE AT THE BACK OF SOME BOOK) THE t-STATISTIC IS A FUDGE FACTOR! IT COMPENSATES FOR THE FACT THAT YOU’LL NEVER HAVE ENOUGH RESULTS TO GIVE A “NICE” CURVE.

  8. THE MATHEMATICIANS BOILED IT ALL DOWN TO THIS SIMPLE EQUATION: MDL = ( tn-1,1- = 0.99 ) ● sn NOTE THE USE OF n-1 INSTEAD OF n (n-1 IS CALLED THE “DEGREES OF FREEDOM”) DON’T FREAK OUT BY THE SWITCH FROM β TO 1 – : (THIS IS SO YOU ONLY HAVE TO USE ONE TABLE) MATHEMATICIANS LOVE TO DO THIS SORT OF STUFF JUST TO MAKE EVERYBODY ELSE MISERABLE. (YOU HAVE MY PERMISSION TO DISLIKE MATHEMATICIANS.)

  9. REMEMBER, WE ONLY USE 7 OR SO RESULTS FOR CALCULATING A DETECTION LIMIT. SO HOW “GOOD” IS OUR MDL? (ANSWER: NOT VERY, WHICH IS WHY THE EPA WAS SUCCESFULLY SUED OVER THE USE OF THAT VERY EQUATION!)

  10. IF THE MDL ISN’T SO HOT, HOW ABOUT SETTING THE LIMIT HIGHER SO WE CAN DO AWAY WITH ALL THIS ALPHA-BETA NONSENSE? AND THAT’S JUST WHAT THE EPA (AND LOTS OF OTHER PEOPLE) DECIDED TO DO. AND THEY DECIDED TO CALL THIS A… QUANTITATION LIMIT

  11. IDL Xave OPERATION “GET RID OF BETA” β MDL

  12. QUANTITATION LIMIT = QL=PQL=LOQ=MRL= on and on IDL (=0.01) MDL (β=0.01) MRL (β=0??)

  13. “HOUSTON, WE HAVE A PROBLEM…” THE NORMAL DISTRIBUTION NEVER GOES TO ZERO IN EITHER DIRECTION!!! OR, TO PUT IT ANOTHER WAY….  AND β ARE NEVER ZERO!!! THEY CAN GET EENSY-WEENSY BUT NEVER ZERO

  14. THE MATHEMATICIANS HAVE A WAY OF SAYING THIS: lim β0 OR…IN TERMS OF GETTING TO A QUANTITATION LIMIT (MRL): lim (Xave) = MRL β0 (YOU HAVE MY PERMISSION TO REALLY DISLIKE MATHEMATICIANS!)

  15. WE HAVE NOW GONE “THOUGH THE LOOKING GLASS” AND THERE’S NO HOPE ANYMORE OF EVER GETTING A HAPPY ENDING TO OUR STORY EVER

  16. SO, WHAT DOES ANY SANE PERSON DO? CALL FOR BACK UP!

  17. ENTER THE STATISTICIANS WHO, BY THE WAY, INTENSELY DISLIKE THE CONCEPT OF DETECTION LIMITS (YOU HAVE MY PERMISSION TO LIKE STATISTICIANS)

  18. HOWEVER, BECAUSE OF THAT PESKY LIMIT THE STATISTICIANS CAN ONLY OFFER: ● APPROXIMATIONS ● PROBABILITIES ● COMPLEX & OBTUSE EQUATIONS ● ARCANE MATHEMATICAL SYMBOLS ● SILLINESS ● PAIN ● HUMILIATION ● DESPAIR

  19. “WE BELIEVE DETECTION LIMITS SHOULD GO AWAY!” David Coleman & Lynn Vanatta Statistics In Analytical Chemistry Part 26, Detection Limits

  20. FACED WITH THIS DILEMMA, THE CITY OF PORTLAND WPCL DID THE ONLY THING HUMANLY POSSIBLE: WE APPLIED THE SCIENTIFIC METHOD & GEOMETRIC LOGIC AND CAME UP WITH… TOGA PARTY!

  21. JUST KIDDING, FOLKS. WE WROTE A PAPER!

  22. AN ENTIRE YEAR’S WORTH OF MERCURY DATA: ● HISTORICAL SPIKED REAGENT BLANKS (11) ● HISTORICAL METHOD BLANKS (72) ● SPIKED REAGENT BLANKS (30) ● CALIBRATION STANDARDS (6 SETS OF 5 STDS EACH) FED INTO FOUR METHODS: ● TRADITIONAL MDL METHOD AT 40 CFR 136 ● DL-QL METHOD FROM THE EPA FEDERAL ADVISORY COMMITTEE ON DETECTION AND QUANTITATION ● LC-LD METHOD PROPOSED BY USGS & EAST BAY MUD ● LCMRL METHOD FROM THE EPA OFFICE OF GROUND & DRINKING WATER TECHNICAL SUPPORT CENTER

  23. YOU CAN’T TELL THE PLAYERS WITHOUT A PROGRAM!

  24. TRADITIONAL MDL METHOD ● USED 11 LOW-LEVEL SPIKED BLANKS ● CALCULATED USING AVERAGE OF THE 11 RESULTS AND THE t-STATISTIC FOR 10 DEGREES OF FREEDOM AT THE 99% CONFIDENCE LEVEL (REMEMBER OUR β?)… MDL = ( tn-1,1- = 0.99 ) ● sn ● METHOD DOES NOT CALCULATE A QUANTITATION LIMIT

  25. THE DL-QL METHOD IS SLIGHTLY COMPLICATED!

  26. THE PROPOSED DL-QL METHOD ● USED 72 HISTORICAL METHOD BLANK RESULTS ● DL CALCULATED USING THE k-STATISTIC… DL = MBave + ( kn-1,1- = 0.99) ● sn ● DL TESTED BY CALCULATING THE LOWEST EXPECTED RESULT (LER) USING 30 SPIKED BLANKS… LER = SPIKEave – (tn-1,1- = 0.99) ● sn (SPIKE CAN’T BE ANY LOWER THAN 2X THE DL) ● DL < LER  DL STANDS AS CALCULATED QL BECOMES THE CONC. OF THE 30 SPIKED BLANKS

  27. THE PROPOSED LC – LD METHOD ● USES EITHER A SET OF LOW-LEVEL SPIKED BLANKS OR A SET OF METHOD BLANKS (WE CHOSE THE LATTER)… LC = ( 1-p = 0.99) ● [(n-1)/ 2]½ ● NEXT, ANALYZE A SERIES OF LOW-LEVEL SPIKED BLANKS, EITHER ALONG WITH REGULAR WORK OR AS A SEPARATE SPIKE STUDY (WE CHOSE THE LATTER)… ● IF ALL SPIKES ARE ABOVE THE LC, THE LD BECOMES THE SPIKE CONCENTRATION (NOTE THAT WE USED THE 72 METHOD BLANKS AND 30 SPIKES USED IN THE DL-QL METHOD. HANDY, NO?)

  28. THE PROPOSED LCMRL METHOD ● USES SETS OF CAL STDS: 7 IS BEST, NO LESS THAN 5 (WE USED 5) AND YOU CAN’T USE ANY CAL BLANKS ● RUN AT LEAST 4 DIFFERENT SETS (WE DID 6) ● ENTER ALL RESULTS INTO THE LCMRL SOFTWARE SUPPLIED (FREE) FROM THE EPA TSC IN CINCINNATI ● YOUR DETECTION LIMIT (DL) & LOWEST CONCENTRATION METHOD REPORTING LEVEL (LCMRL) ARE CALCULATED FOR YOU ● NOTE: THE “CONTROL LIMITS” ARE CURRENTLY 50 – 150% (FOR ORGANICS). WE TALKED THE TSC FOLKS INTO CHANGING THEM TO 79 – 121% (LIMITS FOR Hg BY EPA 1631E)

  29. SO…. HOW DID THE HIGH-FALUTIN’ METHODS DO???

  30. THE MORAL OF OUR LITTLE STORY IS… ● USE THE TRADITIONAL MDL METHOD AT 40 CFR 136 UNTIL EPA TELLS YOU NOT TO ● USE YOUR EXPERIENCE AND KSA’s TO SET YOUR MRL (OR LOQ OR QL OR WHATEVER YOU WANT TO CALL IT)… ● BUT THINK ABOUT DOING SOMETHING TO BACK UP YOUR MRL “EDUCATED GUESS” (AT WPCL, WE DECIDED TO GO WITH THE NELAC APPROACH: THE MRL IS THE LOWEST CAL STD THAT RESULTS IN AN ACCEPTABLE CALIBRATION CURVE)

  31. REFERENCES The draft DL-QL method and the various reports and meeting minutes of the Federal Advisory Committee on Detection and Quantitation (FACDQ) may be found online at www.epa.gov.waterscience/methods/det/#faca The background documents and a downloadable executable file of the first version of the LCMRL Calculator may be found online at www.epa.gov/safewater/methods/sourcalt.html The LC-LD method is detailed in Osborn, Kenneth E. & Thomas Georgian (2004). “The Limits of Detection Limits.” Water Environment Laboratory Solutions. 11(2): 6 – 9

  32. ELIZABETH FARKAS KRIS DENNIS JASON DAHL

  33. WHY DO THE MRLs IN MY REPORT BOUNCE ALL OVER THE PLACE? MRL IS NOW 10.0 MRL = 1.0 10.0 5.0 1.0 1.0 NONDETECT NONDETECT ORIGINAL SAMPLE (CAN “SEE” ALL CONC.) SAMPLE DILUTED 1:10 (CAN ONLY “SEE” 10.0)

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