DRINKING-WATER- QUALITY DATA:

Transcription

1 A New Approch to DRINKING-WATER- QUALITY DATA: Lowest- Concentrtion Minimum Reporting Level Downloded vi on July 10, 018 t 00:43:18 (UTC). See for options on how to legitimtely shre published rticles. jupiterimges JOHN J. MARTIN THE CADMUS GROUP, INC. STEPHEN D. WINSLOW SHAW ENVIRONMENTAL, INC. DAV ID J. MUNCH U.S. EPA OFFICE OF GROUND WATER AND DRINKING WATER One of the key fctors in ensuring the integrity nd relibility of drinkingwter-qulity dt is the level of confidence ssocited with the nlyticl results. Precision (reproducibility of dt) nd ccurcy (closeness of mesured vlue to true vlue) re criticl dt-qulity objectives (DQOs) for drinking-wter regultions. Trgeting these objectives, the U.S. EPA hs developed process for determining the single-lbortory lowestconcentrtion minimum reporting level (LCMRL). The LCMRL is the lowest true concentrtion for which future nlyte recovery is predicted (with t lest 99% confidence) to fll between 50 nd 150% A new procedure focuses on precision nd ccurcy in nlyticl mesurements. (inclusive). This recovery intervl hs been used in recent gthering of occurrence dt, including the Informtion Collection Rule nd the Unregulted Contminnt Monitoring Regultion. Although the LCMRL is lbortory- nd nlyte-specific vlue, LCMRLs from multiple lbortories cn be used to determine minimum reporting level (MRL) tht cn be considered for ntionl ppliction for ny environmentl regultory progrm. The LCMRL, nd by ssocition the MRL, llows for simultneous incorportion of precision nd ccurcy in nlyticl mesurements. As result, specified confidence is inherent in ny nlyticl result t or bove the MRL. To mke policy decisions regrding the protection of humn helth, including decisions bout whether to regulte prticulr nlyte, EPA needs dt of known precision nd ccurcy. Recently, Federl Advisory Committee on Detection nd Quntittion Approches ws estblished to review nd consider forml doption of detection nd quntittion procedures. The LCMRL procedure is one of mny being considered. Debte nd reserch bout procedures for determining detection limits (i.e., presence) nd quntittion levels (i.e., relible quntittion) hve been ongoing for decdes (1 14). Mny of those procedures consider only the precision of the mesurements (1 11), wheres others tke both precision nd ccurcy into ccount (1 14). However, of those tht consider both precision nd ccurcy, ll but the LCMRL procedure pply precision nd ccurcy independently of ech other. The LCMRL procedure pplies them simultneously. Thus, the dvntge of the LCMRL procedure over others is tht the determintion of the LCMRL is simultneously influenced by the vrince of the mesurements nd the prescribed DQOs of ±50% of the true vlue. The other procedures re discussed in detil by Winslow et l. (15). 007 Americn Chemicl Society Februry 1, 007 / Environmentl Science & Technology n 677

2 One of the key issues ssocited with replicte nlyses over rnge of concentrtions nonconstnt vrince over the rnge hs been investigted nd ddressed by mny reserchers over the pst severl yers (8, 9). LCMRL procedures lso ddress the problem of nonconstnt vrince (see Supporting Informtion). These LCMRL procedures hve been documented by EPA in further detil (15 17). Interlbortory study: proof of concept An interlbortory study ws conducted to evlute the soundness of the LCMRL concept nd to determine whether LCMRLs could be used to estblish relistic, yet relible, MRLs. The gol ws to obtin three complete sets of dt from ech of seven EPA methods currently in use for the nlysis of regulted nlytes in drinking wter. A primry nlyte ws selected from ech method; other nlytes included in the study re beyond the scope of this rticle. Ech dt set consisted of seven replicte nlyses t ech of four concentrtions. Thus, n LCMRL could be clculted for ech nlyte nd lbortory, nd the LC- MRLs could be used to determine MRLs, s described lter. The nlyticl methods, primry nlytes, nd spiking concentrtions re summrized in Tble 1. One of the three lbortories initilly selected for inclusion in the study ws not set up to perform ll of the nlyticl methods tht were the subject of the study. Thus, to obtin three complete dt sets for ech of the seven nlyticl methods, nd to provide s representtive cross section of lbortories s possible, five lbortories were selected: one commercil lbortory, one stte lbortory, one Public Wter System lbortory, nd two EPA regionl lbortories. As prt of the study plnning process, it ws determined tht lowest spiked (or true) concentrtion of ~ ech lbortory s method detection limit (MDL) ws the minimum level to which lbortories could resonbly be expected to clibrte nd report dt. Therefore, s mtter of prcticlity, the rnge of true concentrtions used to determine LCMRLs ws tilored to fit into the rnge between the MDL nd the prcticl quntittion limit (PQL) of ech primry nlyte. Severl exceptions to the twice-the-mdl guideline cn be seen in Tble 1 if the lowest true concentrtion is compred with the MDL. Some of these were ssocited with clibrtion ner the MDL to crete seril dilutions tht llowed for mximum overlp of spiking concentrtions, nd some exceptions were mde to decide whether concentrtions ner the MDL could be included in n LCMRL determintion. The mount of experience of individul lbortories with these nlytes ws the bsis for these decisions. The primry nlytes were selected on the bsis of the following criteri: the selected nlytes T A B L E 1 Anlyticl methods, primry nlytes, nd true concentrtions EPA method number Primry nlyte Lb number MDL True concentrtion (μg/l) (μg/l) Low Intermedite High 00.7 Chromium (67.7 nm); (06.1 nm) 00.9 Cdmium Nitrte s N Atrzine N/A b Heptchlor N/A b ,4-Dichlorophenoxycetic cid 55. Hexchlorobenzene Method detection limit. b Not pplicble. c Estimted c Spiking rtio (high/low) 678 n Environmentl Science & Technology / Februry 1, 007

3 must hve estblished mximum contminnt levels (MCLs) nd PQLs, nd the selected nlytes must ech consist of single component (unlike toxphene, for exmple). Anlytes were processed through ll steps of ech nlyticl method, including extrction (where pplicble) nd the use of method-required preservtives. Seven replicte smples, t ech of the four predetermined spiking concentrtions, were prepred nd nlyzed by prticipting lbortories with ech pplicble method. LCMRLs nd MRLs were determined from the dt by the procedures described in the following sections. LCMRL determintion process The following steps were used to determine the LC- MRL. All of these steps cn be performed utomticlly with EPA s LCMRL clcultor (17) or mnully by procedures discussed by Winslow et l. (15) nd in EPA s LCMRL sttisticl protocol (16). An exmple grph is presented in Figure 1. First, for ech nlyte, the mesured concentrtion (y xis) ws plotted ginst the spiked or true concentrtion (x xis). The replicte dt were summrized in Microsoft Excel nd imported into STATA sttisticl softwre. Second, the LCMRL dt were regressed with ordinry lest squres (OLS) nd vrince-weighted lest squres (VWLS; see Supporting Informtion), which were performed simultneously by STATA. The regression line ws not forced through the origin. Regression ws performed with stright-line regression eqution, nd 99% prediction intervl ws constructed round the regression line. Third, the STATA output ws downloded to Microsoft Excel, including plots for ech regressed dt set, with lines corresponding to 50% nd 150% recovery of the spiked concentrtion. The LCMRL is determined by dropping perpendiculr lines to the x xis, strting t the intersections of the upper nd lower prediction-intervl bounds with the 50% nd 150% recovery lines. The lrger of the two x vlues is the LCMRL (0.8 µg/l from Figure 1), becuse this concentrtion is the lowest level for which the upper nd lower recovery requirements re met. See the Supporting Informtion for detils bout the clcultion of the LCMRL from the intersections of the prediction-intervl limits with the 50% nd 150% recovery lines. The DQOs of precision nd ccurcy re depicted in Figure 1. Precision is reflected by the vrince, or spred, of the dt t ech true concentrtion. Accurcy is reflected by the degree to which the dt points re clustered bout their true vlue. Note tht the results t the highest true concentrtion (0.5 µg/l) re bised slightly high nd re centered closer to 0.6 µg/l thn 0.5 µg/l. Both precision nd ccurcy dictte the bredth of the prediction intervl nd, hence, the mgnitude of the LCMRL. High precision (i.e., low vrince) nd high ccurcy (i.e., low bis) will tend to tighten the prediction intervl nd will move the intersection points with the recovery lines to the left, thus reducing the LCMRL. Low precision (i.e., high vrince) nd low ccurcy (i.e., high bis) will move F I G U R E 1 Exmple LCMRL determintion LCMRL determintion from multiconcentrtion replicte nlyses. LCMRL = 0.8 µg/l. Mesured concentrtion (µg/l) True concentrtion (µg/l) the intersections to the right, rising the LCMRL. LCMRLs were determined for ech lbortory nd nlyte in the study. The men of the LCMRL vlues from vrious lbortories ws clculted for ech nlyte, nd MRLs were determined s follows. When LCMRLs from three lbortories were vilble, the MRL ws clculted from Eqution 1: MRL = Men + 3s (1) where Men is the verge of the three LCMRL vlues, nd s is the stndrd devition of the three LC- MRL vlues. When LCMRLs from only two lbortories were vilble, the MRL ws clculted from Eqution : MRL = Men + 3 LCMRL1 LCMRL () In this cse, the bsolute vlue of the difference between the LCMRLs serves s surrogte for the stndrd devition, becuse of the uncertinty of estimting stndrd devition with only two dt points. In sttisticl theory (Chebyshev s inequlity), n intervl of 3 stndrd devitions round the men of ny distribution incorportes the mjority (t lest 88.9%) of the dt points (18). The MRL for ech nlyte ws determined by then rounding the vlue obtined from either Eqution 1 or Eqution to one significnt digit. The MRL is designed to be ntionlly ttinble vlue for lbortories tht re to prticipte in prticulr regultory progrm. The LCMRL is lbortory-specific vlue, but the MRL must be more flexible in tht it represents resonble performnce metric for numerous lbortories. The use of 3s provides tht flexibility by ensuring tht high percentge of lbortories cross the ntion will be ble to meet the DQOs, s indicted by Chebyshev s inequlity. Becuse MRLs re derived from LCMRLs, the DQOs tht re inherent to the LCMRL re retined by the MRL. 150% recovery line Upper limit of 99% prediction intervl Regression line Lower limit of 99% prediction intervl 50% recovery line Februry 1, 007 / Environmentl Science & Technology n 679

4 T A B L E Single-lbortory LCMRLs nd resulting MRLs LCMRL, lowest-concentrtion minimum reporting level; MDL, method detection limit; ML, minimum level; MRL, minimum reporting level; N/A, not pplicble; OLS, ordinry lest squres; PQL, prcticl quntittion limit; VWLS, vrince-weighted lest squres. EPA method Primry LCMRL How determined? Lb MDL ML (3.18 MDL) PQL Threelbortory MRL Two-lbortory MRL (μg/l) from (μg/l) from number nlyte number (μg/l) (μg/l) (μg/l) (μg/l) Eqution Eqution Chromium 1 4 OLS N/A 7.7 OLS VWLS (no 6 OLS 4 13 N/A Cdmium OLS N/A OLS OLS (no 0.33 VWLS N/A Nitrte 1 39 VWLS N/A 50 5 OLS VWLS Atrzine OLS N/A VWLS VWLS N/A N/A (no 0.09 VWLS N/A Heptchlor 515.3,4-Di VWLS N/A VWLS VWLS N/A N/A OLS N/A 1 chlorophen- oxycetic 3 cid OLS OLS Hexchlorobenzene VWLS N/A VWLS VWLS 0.0 b Cnnot be determined. b Estimted. Tble summrizes the individul-lbortory LCMRLs nd possible vlues for the MRL. When the LCMRL for one lbortory is greter thn the highest spiked concentrtion, MRLs re determined from dt from the two remining lbortories with determinble LCMRLs. Removl of outliers llows for inclusion of ll three lbortories in the clcultions for three of the nlytes in Tble. The tble lso includes ech lbortory s MDL, 10 sigm 3.18 MDL, or the minimum level (ML) nd the PQL for ech primry nlyte. Ten sigm hs been proposed by the Americn Chemicl Society (5) nd in EPA Method 1631 (4) s relible level of quntittion. The PQL hs long been used by EPA s level of relible quntittion. The reltionships mong the MDL, the ML, nd the PQL re s follows. The MDL is determined by multiplying the stndrd devition of replicte results t single concentrtion (typiclly seven replictes) by the Student s t vlue for n 1 degrees of freedom nd 99% confidence (3). For n = 7, t = The ML is obtined by multiplying the MDL by 3.18 (4). This is 10 the stndrd devition in the cse of 7 replictes. PQLs hve historiclly been determined through liner regression of proficiency testing (PT) dt (ntionl lbortory pssing rtes), or by multiplying the MDL by 5 or 10 (19). In the regression procedure, the concentrtion t which 75% of prticipting lbortories re predicted to meet n nlyte s cceptnce criteri is set s the PQL. Thus, in generl, MDL < ML < PQL. The min difference between the MRL nd these three trget quntities is tht the MRL is derived from the LC- MRL with simultneous incorportion of precision nd ccurcy. The MDL is detection limit, nd it is bsed on precision only; the ML, multiple of the MDL, is reporting limit, nd it is bsed on precision only. The PQL is reporting limit, nd it cn be estblished s multiple of the MDL. In tht cse, it is bsed only on precision. Alterntively, if the PQL is derived from liner regression of PT dt, it con- 680 n Environmentl Science & Technology / Februry 1, 007

5 siders ccurcy, but it does not explicitly consider precision. Tble demonstrtes how the MRLs tht were clculted for these nlytes relte to the ML nd the PQL. Outliers re discussed in Supporting Informtion nd by Winslow et l. (15). The informtion summrized in Tble indictes tht the MRL typiclly flls in between the vlues for the ML nd the PQL. Chromium nd - 4-dichlorophenoxycetic cid re two exceptions; their MRLs re similr in mgnitude to both their MLs nd PQLs. The simultneous incorportion of both precision nd ccurcy in the determintion of the LCMRL provides confidence in the qulity of the derived MRLs. Further, becuse the MRLs re comprble with (but typiclly less thn) PQLs, the MRL represents reporting level tht meets EPA s need for dt of known qulity. The MRL lso tkes into ccount the bility of lbortories to incorporte precision nd ccurcy into low-level mesurement. The results of the interlbortory study indicte tht use of the LCMRL in determining MRLs is vlid new pproch to ensuring qulity nd consistency in low-level nlyticl mesurements. Simultneous incorportion of precision nd ccurcy results in MRLs tht re generlly lower thn existing PQLs but indictive of low-concentrtion lbortory performnce. The LCMRL procedure is flexible, nd DQOs cn be tilored to fit the needs of ny nlyticl regultory progrm. Acceptnce limits could be mde more or less stringent thn ±50%, nd the confidence levels could be set t ny desired level. John J. Mrtin is n ssocite with the Cdmus Group, Inc., in Wtertown, Mss. Stephen D. Winslow is chemist for Shw Environmentl, Inc., nd n on-site contrctor t the Technicl Support Center of the EPA Office of Ground Wter nd Drinking Wter in Cincinnti, Ohio. Dvid J. Munch is the chemistry lbortory mnger for the Technicl Support Center of the EPA Office of Ground Wter nd Drinking Wter in Cincinnti, Ohio. Address correspondence bout this rticle to Mrtin t jmrtin@cdmusgroup.com. Acknowledgments This work hs been funded in prt by the U.S. EPA, Office of Wter, Office of Ground Wter nd Drinking Wter, Technicl Support Center under contrct 68-C-0-06 to the Cdmus Group, Inc., nd contrct 68-C to Shw Environmentl, Inc. This pper hs been subject to EPA review nd hs been pproved for publiction s n EPA document. Mention of trde nmes or commercil products does not constitute endorsement or recommendtion for use. The uthors pprecite the input of the following individuls who contributed to peer review of the design of the interlbortory study nd LCMRL/MRL determintions nd/or performed lbortory nlyses: Richrd Albert, U.S. Food nd Drug Administrtion (retired); Willim Horwitz, AOAC Interntionl; Willim Foremn, U.S. Geologicl Survey; Andrew Eton, MWH Lbortories; Michel D. Wichmn, University Hygienic Lbortory; Phillip Godorov, Phildelphi Wter Deprtment Lbortory; John V. Morris, EPA Region 5 Lbortory; Lis Wool, EPA Region 6 Lbortory; Willim Tellird nd other personnel, EPA Engineering nd Anlysis Division; nd Brd Venner, EPA Ntionl Enforcement Investigtions Center. Supporting informtion Supporting informtion is vilble for this pper. It contins discussions of the evlution of curve fitting for instrument clibrtion, nonconstnt vrince over the rnge of true concentrtions, nd outlier evlution s well s grphicl exmples of how nonconstnt vrince nd outliers ffect the determintion of n LCMRL. References (1) Currie, L. A. Detection nd Quntifiction Limits: Origins nd Historicl Overview. Anl. Chim. Act. 1999, 391, () Currie, L. A. Limits for Qulittive Detection nd Quntittive Determintion. Anl. Chem. 1968, 40, (3) Glser, J. A.; et l. Trce Anlyses for Wstewters. Environ. Sci. Technol. 1981, 15, (4) EPA Method 1631, Revision E: Mercury in Wter by Oxidtion, Purge nd Trp, nd Cold Vpor Atomic Fluorescence Spectrometry; EPA-81-R-0-019; 00; gov/wterscience/methods/1631e.pdf. (5) Keith, L. H.; et l. Principles of Environmentl Anlysis. Anl. Chem. 1983, 55, (6) Eton, A. D.; Clesceri, L. S.; Greenberg, A. E., Eds. Stndrd Methods for the Exmintion of Wter nd Wstewter, 19th ed.; Americn Public Helth Assocition: Wshington, DC, 1995; Method Detection Level, Prt 1030 E, pp (7) Childress, C. J. O.; et l. New Reporting Procedures Bsed on Long-Term Method Detection Levels nd Some Considertions for Interprettions of Wter-Qulity Dt Provided by the United Sttes Geologicl Survey Ntionl Wter Qulity Lbortory. USGS Open-File Report ; 1999; wter.usgs.gov/owq/ofr_99-193/ofr99_193.pdf. (8) Gibbons, R. D.; Colemn, D. E.; Mddlone, R. F. An Alterntive Minimum Level Definition for Anlyticl Quntifiction. Environ. Sci. Technol. 1997, 31, (9) Americn Society for Testing nd Mterils. 00 Annul Book of ASTM Stndrds, Stndrd Prctice for Interlbortory Quntittion Estimte; ASTM Interntionl: West Conshohocken, PA, 00; ASTM D , Section 11 Wter, 11.01, pp (10) Hubux, A.; Vos, G. Decision nd Detection Limits for Liner Clibrtion Curves. Anl. Chem. 1970, 4, (11) Snders, P. F.; Lippincott, R. L.; Eton, A. A Prgmtic Approch for Determining Quntittion Levels for Regultory Purposes. In Proceedings of the Wter Qulity Technology Conference, Boston, MA, 1996; Americn Wter Works Assocition: Wshington, DC, 1996; pp (1) Oxenford, J. L.; McGeorge, L. J.; Jennis, S. W. Determintion of Prcticl Quntittion Levels for Orgnic Compounds in Drinking Wter. J. AWWA 1989, April, (13) Hertz, C. D.; et l. Minimum Reporting Levels Bsed on Precision nd Accurcy for Inorgnic Prmeters in Wter. In Proceedings of the Wter Qulity Technology Conference, Toronto, Nov 15 19, 199; Americn Wter Works Assocition: Wshington, DC, 199; pp (14) Kimbrough, D. E.; Wkkuw, J. Qulity Control Level: An Alterntive to Detection Levels. Environ. Sci. Technol. 1994, 8, (15) Winslow, S. D.; et l. Sttisticl Procedures for the Determintion nd Verifiction of Minimum Reporting Levels for Drinking Wter Methods. Environ. Sci. Technol. 006, 40, (16) EPA. Sttisticl Protocol for the Determintion of the Single-Lbortory Lowest Concentrtion Minimum Reporting Level (LCMRL) nd Vlidtion of Lbortory Performnce t or below the Minimum Reporting Level (MRL); EPA-815-R ; Nov 004; sfewter/methods/sourclt.html#mlcmrl. (17) LCMRL Clcultor. EPA Office of Ground Wter nd Drinking Wter, 005; (18) Montgomery, D. C.; Runger, G. C. Applied Sttistics nd Probbility for Engineers; John Wiley: New York, (19) Anlyticl Fesibility Support Document for the Six- Yer Review of Existing Ntionl Primry Drinking Wter Regultions (Ressessment of Fesibility for Chemicl Contminnts). EPA-815-R ; EPA Office of Ground Wter nd Drinking Wter: Wshington, DC; 003; www. ep.gov/sfewter/stndrd/review/pdfs/support_6yr_ nlyticl_finl.pdf. Februry 1, 007 / Environmentl Science & Technology n 681