Determining. quantitation levels for regulatory purposes. A quantitation level is proposed that should be quantifiable by a majority of laboratories.

Transcription

1 REGULATORY GRIDLOCK Determining quantitation levels for regulatory purposes A quantitation level is proposed that should e quantifiale y a majority of laoratories. Paul F. Sanders, R. Lee Lippinott, and Andrew Eaton R The authors desrie an approah for alulating quantitation levels (QLs) that does not require hanges in urrent laoratory praties. The reliale detetion level (RDL), when defined as twie the onentration of the method detetion limit (MDL), provides adequate protetion against oth false-positive and false-negative detetion deisions. When analyte onentrations are at the RDL, the orresponding preision in the instrument response is adequate for analytial purposes. Therefore, the RDL (as determined y a single laoratory) is a reasonale lower limit of quantitation for that laoratory. To determine an interlaoratory QL suitale for regulatory purposes, median interlaoratory MDLs were multiplied y a variale fator (usually 7, determined from atual laoratory performane data). More than perent of the laoratories surveyed had RDLs less than or equal to the alulated QL, indiating that adequate quantitation was attainale at this level and that the QL should e suitale for regulatory purposes. eently, water regulatory agenies and providers have shown onsiderale interest in determining the lowest quantifiale level that is suitale for regulatory purposes. 1 Allowale onentration levels for a ontaminant in a partiular environmental medium (e.g., soil, air, or water) are often ased on health-related risk assessments and are sometimes lower than levels that an e quantitated in a laoratory. For this reason, the lowest quantifiale level frequently eomes the de fato regulatory limit for monitoring and ompliane purposes. The lowest quantifiale level has een given many different names. Current terms inlude the pratial quantitation level 1 JOURNAL AWWA

2 FIGURE 1 Instrument Response area units Sample aliration urve for 1,1-dihloroethene using method 5. r = Conentration μg/l 95 perent onfidene limits are shown as dashed lines, 95 perent predition limits are shown as dotted lines (PQL) defined y the US Environmental Protetion Ageny 9 (USEPA); the limit of quantitation defined y the Amerian Chemial Soiety 1 (ACS); the reliale quantitation level (RQL) suggested y the USEPA Offie of Ground Water and Drinking Water; the minimum level (ML) used y USEPA Offie of Wastewater Enforement and Compliane; 5 the ompliane monitoring quantitation level (Eletri Power Researh Institute); 11 and the method quality ontrol level. 7 Behind this array of terms and the onfliting rationales supporting them lies a ommon need: to estalish the lowest quantifiale level that an e used to evaluate regulatory ompliane. To e useful in this role, the regulatory level should e adequately quantifiale y the majority of laoratories ertified for the partiular ompound and method of interest. At the same time, the regulatory level should e elow the level ahievale y poorly performing laoratories in order to provide inentive for these laoratories to improve their detetion and quantitation levels. The proedure to alulate the regulatory level therefore needs to take into aount atual performane data from the laoratory ommunity. In the past, three major approahes have een used for determining the lowest quantifiale level. First, interlaoratory studies may e onduted, in whih a group of laoratories are hallenged with low-level samples. 9,1,13 This approah may e the most defensile eause it diretly measures the quantitative ailities of an atual laoratory ommunity. The lower limit of quantitation an then e set aording to seleted riteria of preision and auray. However, these studies are time-onsuming and expensive; one of the speifi goals of this study was to investigate more rapid and ost-effetive alternatives to interlaoratory studies. A seond approah to setting the lowest quantifiale level employs statistial tehniques ased on unertainties in the aliration urve. This method is perhaps the most theoretially satisfying eause a quantitation level an e derived from the same aliration urve used to analyze environmental samples. Related approahes have een desried for determining a limit of detetion from aliration data. 1,15 However, suh approahes would require some hanges in urrent aliration praties. The final approah employs a multipliation fator, typially in the range of 3 1, applied to the detetion limit.,5,9,1 Beause it is widely used and does not require hanges in urrent laoratory praties, the multiplier approah was the method investigated in this study. The starting point for determining a lower limit of quantitation using the multiplier approah is having a value for the detetion limit. For environmental ompliane monitoring in the United States under USEPA regulations, a method detetion limit (MDL) is urrently determined aording to speifi guidelines. 1 The MDL is not suitale as a regulatory level for the following reasons: (1) it varies from laoratory to laoratory; 17 () preision of measurements at this The lowest quantifiale level frequently eomes the de fato regulatory limit for monitoring and ompliane purposes. level are generally poor; 1 and (3) although the MDL provides adequate protetion against false-positive results, protetion against false negatives is inadequate eause samples ontaining ontaminants at a onentration near the MDL will not e reported as deteted 5 perent of the time. 15,17,19 A proposed new parameter, the reliale detetion level (RDL), is twie the value of the MDL and is suffiiently aove the MDL value to ensure that samples MARCH

3 FIGURE Instrument Response area units 1 Instrument Response area units 1 Instrument Response area units f Conentration μg/l Close-up of Figure 1, showing unertainties in predited onentrations for a sample ontaining 1,1-dihloroethene at the RDL of 1 μg/l d d d A B C e f Conentration μg/l e e g h i j f Conentration μg/l a aliration urve, 95 perent onfidene limit, 95 perent predition limit, d mean value of instrument reading, e point on aliration urve orresponding to mean instrument reading, f true onentration of theoretial sample; for part A: points g upper and lower limits (95 perent onfidene level) for instrument response variaility, points H upper and lower limits of estimated onentration ased on instrument variaility; for part B: trae e i onfidene interval for instrument reading of d, j upper limit of estalished onentration ased on onfidene interval; for part C: trae e k predition interval for instrument reading of d, l upper limit of estimated onentration ased on predition interval k a a a l at this onentration will e relialy reported as deteted; thus, the RDL provides adequate protetion against oth false positives and false negatives. 17 This parameter, originally proposed as a detetion limit y Currie, 19 has reently een onsidered in disussions among USEPA, ACS, and the Amerian Soiety for Testing and Materials. 1 The RDL is suitale as a regulatory level provided adequate quantitation at this level is ahievale. However, the RDL is derived from the MDL and will still vary among laoratories. Susequently, eah laoratory would report a different quantitation level (QL) for a partiular method and ompound. To determine a single regulatory level that most laoratories will e ale to ahieve, performane variations within the laoratory ommunity must e taken into aount. In this artile, the authors desrie an approah for determining a lower limit of quantitation that most laoratories should e ale to ahieve, using data readily availale from the laoratory ommunity. The derivation of the preision of analytial measurements at the RDL is examined using two fundamental analytial relationships and the USEPAdefined MDL. This preision proves adequate to justify inluding RDL on the aliration urve as a quantitative level. For purposes of this artile, the authors define the QL as the level that is greater than or equal to the RDLs for a majority of laoratories in a ertified laoratory ommunity. This definition is similar to the definition of the reliale QL proposed y Keith. The term QL is used to represent the proess desried here. To alulate the QL, the median interlaoratory MDL is multiplied y a variale multiplier determined from atual laoratory performane data gathered for the ompound and method of interest. MDL and RDL theoretial onsiderations An MDL an e derived from two fundamental onepts of instrumental analysis the aliration relationship and the minimum distinguishale signal. These two onepts may also e used to determine the preision of measurements made at the USEPA-defined MDL. Throughout this artile, it is assumed that a single laoratory is eing onsidered. The aeptale quantitation range for an analyte in solution is the onentration range for whih there is a linear rela- 1 JOURNAL AWWA

4 TABLE 1 Method 5. QLs, laoratory ahievaility, and urrent MCLs and PQLs for seleted hemials Median Median Perentage of Median Spike Caliration Las Ale Numer MDL Level Low Point Lowest QL to Quantify MCL 3 PQL,5 Compound of Las μgl μg/l μg/l Ratio μg/l at QL μg/l μg/l Benzene Chloroenzene ,-Dihloroenzene ,3-Dihloroenzene ,-Dihloroenzene ,1-Dihloroethane ,-Dihloroethane ,1-Dihloroethene is-1,-dihloroethene trans-1,-dihloroethene Dihloromethane Ethyl enzene Naphthalene Styrene ,1,,-Tetrahloroethane Tetrahloroethene Toluene , 5 1,,-Trihloroenzene ,1,1-Trihloroethane ,1,-Trihloroethane Vinyl hloride Xylenes , 5 tionship etween the instrument response and the onentration of the analyte within the onstraints of a speified onfidene level (usually 95 perent). This relationship defines the linear aliration urve: S = mc + S l (1) in whih S = the magnitude of the analytial signal, m = the slope of the aliration urve, C = the analyte onentration, and S l = the magnitude of the analytial signal for a lank sample. The minimum distinguishale analytial signal has een desried as S min = S l + ks () in whih S min = the magnitude of the minimum detetale analytial signal, S l = the magnitude of the analytial signal for a lank sample, s = the standard deviation of the lank signal in instrument response units (suh as millivolts), and k = a multiplier applied to the value of the standard deviation. If S in Eq 1 is set to S min, C will orrespondingly e equal to C min, the sample onentration that gives the minimum distinguishale analytial signal. Both Eqs 1 and an then e solved for S min S l, and the equations an e omined to give ks = mc min (3) Eq 3 provides the asis for determining the MDL and the preision of measurements at this onentration. USEPA definition of the MDL. Both sides of Eq 3 may e divided y m: ks C = C min () in whih s C = the standard deviation of the instrument response in onentration units. The USEPA defines the MDL as 3.1 times the standard deviation (in onentration units) of seven repliate measurements of a standard onentration (the spike level). 1 An inherent assumption that is made when using unweighted least-squares aliration urves is that the standard deviation of the instrument response is onstant at all onentrations, inluding lank samples, samples at the MDL, and samples at the MDL spiking level. 1 Although this assumption is not neessarily true for the entire working range of the aliration urve, it is approximately orret at onentrations at the low end of the onentration range. 15,17 Therefore, the magnitude of s C in Eq for the lank signal is also appliale at the MDL spike level. Sustituting the multiplier of 3.1 for k in Eq, C min (the onentration that gives a minimum distinguishale signal) eomes the USEPAdefined value for the MDL: 3.1s C = C min = MDL (5) Eq 5 is idential to the MDL equation appearing in USEPA regulations, and it uses a value for s C that is alulated from seven repliate measurements of the onentration at a given spike level. Preision of instrumental measurements at the MDL. Eq 3 also provides the asis for determining the theoretial preision of instrumental measurements at low onentrations. Aording to the USEPA approah, the slope m in Eq 3 is approximated y a response fator S/C, whih is alulated from an average of seven repliate measurements at the MDL spike level. This alulation ignores the magnitude of the lank signal, S l, shown in Eq 1. S l is the y MARCH

5 TABLE Method 5. QLs, laoratory ahievaility, and urrent MCLs and PQLs for seleted hemials Median Median Perentage of Median Spike Caliration Las Ale Numer MDL Level Low Point Lowest QL to Quantify MCL 3 PQL,5 Compound of Las μgl μg/l μg/l Ratio μg/l at QL μg/l μg/l Benzene Caron tetrahloride Chloroenzene Chloroform ,-Dihloroenzene ,3-Dihloroenzene ,-Dihloroenzene ,1-Dihloroethane ,-Dihloroethane ,1-Dihloroethene is-1,-dihloroethene trans-1,-dihloroethene Dihloromethane ,-Dihloropropane Ethyl enzene Naphthalene Styrene ,1,,-Tetrahloroethane Tetrahloroethene Toluene , 5 1,,--Trihloroenzene ,1,1--Trihloroethane ,1,--Trihloroethane Trihloroethene Vinyl hloride Xylenes , 5 TABLE 3 QLs, laoratory ahievaility, and urrent regulatory levels for seleted metals, pestiides, and wastewater ontaminants Median Median Perentage Median Spike Caliration of Las Ale PQL Numer MDL Level) Low Point Lowest QL to Quantify or ML* Method Compound of Las μg/l μg/l μg/l Ratio μg/l at QL μg/l 13. Cadmium ,5. Copper Lead Atrazine ,5 Simazine ,5 Caron tetrahloride Chloroform Dihloroenzenes Tetrahloroethene ,1,1-Trihloroethane Trihloroethene Benzo(a)anthraene Benzo(a)pyrene ,-Dihlorophenol Di(-ethylhexyl)phthalate Pentahlorophenol ,,-Trihlorophenol *MLs reported are from the related methods 1 and 15. interept, and it annot e determined from analysis at a single onentration point. Although ignoring S l is invalid if its magnitude is signifiant, in pratie this quantity is generally small eause the aliration urve usually passes lose to the origin. The response fator is assumed to e onstant at all onentrations, inluding the onentration that gives a minimum distinguishale signal (S min /C min ). Eq 3 an then e reexpressed as or ks = (S min /C min ) X C min () ks = S min (7) Rearrangement of Eq 7 indiates that the multiplier k is quantitatively equal to the signal-to-noise ratio at the minimum distinguishale signal (S min ): 1 JOURNAL AWWA

6 k = S min /s S min /N () in whih N (noise) has een sustituted for s. The signal-to-noise ratio is also desried as the inverse of the relative standard deviation (RSD) of the instrument response: S/N = 1/RSD (9) If S is taken as S min, Eqs and 9 may e omined: k = 1/RSD min (1) in whih RSD min = the relative standard deviation of the instrument response for the minimum distinguishale signal. The value of k is therefore inversely proportional to the RSD of the minimum distinguishale signal for a repliate analysis. As disussed earlier, S min is defined as the instrument signal at the MDL when k is set at 3.1. This value for k is seleted eause it is Student s t value for seven repliate measurements at the 99 perent onfidene level, whih ensures protetion against false-positive detetion deisions. The inverse of this value.31 or 31. perent is the RSD of the instrument response at the MDL. Preision of instrumental measurements at the RDL. These derivations show that the multiplier k, set to 3.1 y the USEPA, determines oth the value for the MDL and desries the preision of the instrument response at this level. As disussed earlier, the RDL is preferale as a onentration level to make a detetion deision eause it also protets against false-negative detetion deisions. Beause the RDL has een defined as twie the MDL, 17 the multiplier k to otain this value would e twie that used for the MDL, or.. If the asolute magnitude of the instrument noise is assumed to e onstant at low levels, the RSD of the instrument response would e 1/. or.159 (15.9 perent) at the RDL. Preision of reported onentrations at the RDL. To translate the preision of the instrument response to a preision for a reported onentration at the RDL, a aliration urve must e used. Figure 1 shows a sample aliration urve for 1,1-dihloroethene using method 5.. The r value for this urve (.99) is typial from a good laoratory. The onfidene intervals shown are routinely used as ontrol limits. The RSD of the instrument response at the RDL (15.9 perent) represents one standard deviation of the instrument noise and an e multiplied y a fator of.5 (Student s t multiplier for seven repliate measurements, two-tailed test) to give an estimate of ±39 perent as the unertainty of the instrument signal at an equivalent onfidene level (95 perent). The laoratory in this example (Figure 1) reported an RDL of 1 μg/l. It an e shown through example that the preision of the instrument response at the RDL lies within the onfines of the onfidene interval for the aliration urve and that the unertainty in the predited onentration aused y instrument noise will e small relative to the onentration interval ounded y the onfidene limits. As shown in the loseup of the low end of the aliration urve (parts A C, Figure ), the instrument response at the RDL (1 μg/l) is, area units (trae d e f). The 95 perent onfidene interval for the signal unertainty at this level (±39 perent) orresponds to ±9, area units (part A of Figure, points g). When a 1-μg/L sample is analyzed, this translates to a reported onentration of 1. ±.3 μg/l (part A of Figure, point f ± points h) eause of variation in instrument response. The estimated preision of a measurement of, area units, ased on the unertainty of the aliration urve, is desried y the onfidene interval (part B of Figure, interval e i). This would lead to a reported onentration of 1. ± 1. μg/l (part B of Figure, point f ± point j). This onentration range is muh larger than the range resulting from variations in the instrument response. Furthermore, although the onfidene interval is ommonly used in pratie, it is theoretially valid only for the mean of repliate measurements. 1 For onentration preditions from individual measurements, it is theoretially more orret to use the predition interval. 1 The estimated preision of a measurement of, area units ased on the predition interval an e desried y onentration interval e k (part C, Figure ). This would lead to a reported onentration of 1. ±.7 μg/l (part C of Figure, point f ± point l). Behind the array of terms and the onfliting rationales supporting them lies a ommon need: to estalish the lowest quantifiale level that an e used to evaluate regulatory ompliane. Clearly, instrument preision at the RDL is muh greater than that assoiated with the aliration urve in this area. As stated earlier, the aliration urve used in this example is typial and was eing used y this laoratory to report onentrations down to its MDL (.5 μg/l). Oviously, reporting onentrations at the RDL (1. μg/l) would e aeptale to this laoratory. The RDL, when defined as twie the USEPA MDL, also offers adequate protetion against oth false-negative and false-positive detetion deision errors; therefore, this para- MARCH

7 FIGURE 3 Distriution of RDLs for seleted ontaminants and methods 1 A 1 B Numer of Laoratories Median RDL =. Median RDL =. Mean RDL = 3. Calulated QL = 5. Numer of Laoratories Mean RDL, alulated QL = Value of RDL μg/l Value of RDL μg/l Lead method 39., numer of laoratories 39 Cadmium method 13., numer of laoratories Numer of Laoratories 1 C Median RDL =. Mean RDL = 3. Calulated QL = 5. Numer of Laoratories 3 1 D Median RDL =. Calulated QL = 1.5 Mean RDL = Value of RDL μg/l Value of RDL μg/l Dihloroenzene method, numer of laoratories 31 Naphthalene method 5., numer of laoratories 11 meter appears to e a justifiale quantitation level for a single laoratory. For the purposes of this artile, the QL is defined as an interlaoratory parameter, speifially a QL suitale for use y a group of laoratories. The RDL is not in itself adequate as a regulatory level eause RDLs will vary from laoratory to laoratory. For this reason, the authors study inorporated a seond omponent: an investigation of the variaility of RDLs determined y different laoratories. Only y omparing a andidate QL with the distriution of the RDLs reported y a representative laoratory ommunity is it possile to determine whether a proposed QL an e quantitated y most laoratories. Calulation of quantitation levels Utilization of laoratory performane data. The authors hose 3 environmentally important hemials that were analytes in one or more of eight standard USEPA methods. Seletion was ased on either New Jersey regulatory needs or ourrene of these ompounds in New Jersey waters (Tales 1 3). The methods hosen were either frequently used or featured state-of-the-art tehniques. The methods were suited for water, drinking water, and wastewater analysis. The -series methods are used to determine individual metal analytes in water. The remaining methods determine multiple analytes. The three 5- series methods are used for organi hemials in drinking water, and the two -series methods determine organi ompounds in wastewater. Fifty-one laoratories ertified y the states of New Jersey or California were surveyed for information routinely reported for quality assurane purposes. Three types of relevant data were easily otainale from the various laoratories: the determined MDL value, the low point on the aliration urve, and the spike level used to determine the MDL. The spike levels and aliration urve low points were generally set y analysts, taking into onsideration the MDL reported in the method of interest. Data were rejeted for this study if the MDL spike level was greater than 5 the reported MDL for a 11 JOURNAL AWWA

8 partiular laoratory or if the information provided on the aliration urve range was inonsistent with the ited method. USEPA guidelines suggest that the spike level should e 1 5X the reported MDL and allow a ratio of up to 1; 1 for this study, however, a ratio of up to 5 was allowed in order to minimize the numer of rejeted laoratories. Using the more onservative 1 1 range would have resulted in disarding several data points. Although the majority of laoratories followed the ratio guidelines, a few laoratories exeeded a ratio of 1, ut only rarely did they exeed a ratio of. One prolem with allowing a spike ratio greater than 1 is that the standard deviation of the instrument noise may egin to vary etween the MDL and the spike level, therey invalidating the assumptions inherent in Eq 5. The alulated MDL would then e affeted y the spike level. However, investigation of the data gathered for this study showed that the reported MDL was independent of the spike ratio, despite the ourrene of ratios greater than 1. This suggested that the assumptions of Eq 5 were not eing violated. The goal of this study was to determine a QL that was ahievale y most of the ertified laoratories; exlusion of data from several of the laoratories would have resulted in a QL that was not refletive of the atual apailities of the laoratory ommunity as a whole. Therefore, the MDL aeptane riteria for this study was more lenient than that of the USEPA in that higher spike ratios were allowed. Variaility of RDLs. For a given hemial and method, RDL distriutions from the laoratories were prepared in order to asertain the variaility of this parameter from laoratories ertified for that method (parts A D, Figure 3). The MDLs reported y the laoratories were multiplied y a fator of to otain the RDLs. RDL values were not normally distriuted. Most FIGURE were lustered within a well-defined range. However, a few RDLs were reported at signifiantly higher levels. As demonstrated earlier, an individual laoratory ould adequately quantify a onentration at its own RDL. Therefore, for a given ompound and method, a QL that most laoratories ould ahieve would e at or near the upper end of the main RDL distriution. Laoratories with RDLs aove the ulk of the distriution (parts A D, Figure 3) would e onsidered poorly performing laoratories and may e exluded. Frequeny Distriution of multiplier values for alulation of QL All methods Method 5. Method 5. Methods and Value of Multiplier Calulation of QLs from laoratory performane data. Although a QL ould e set y visual examination of RDL distriutions (suh as those shown in parts A D, Figure 3), suh a proedure would e sujetive. A reproduile approah needed to e developed. As noted earlier, the approah seleted for this study applies a multiplier to the interlaoratory MDL. Alternatively, the multiplier ould e divided y a fator of and applied to the interlaoratory RDL; however, laoratories report the MDL, making this the more onvenient parameter. The first step of the proess was to selet a representative interlaoratory MDL for the group of laoratories surveyed. This study used the median of the individual MDL values from eah laoratory for a given ompound and method. The nonparametri median value was hosen rather than the mean in order to To determine a single regulatory level that most laoratories will e ale to ahieve, performane variations within the laoratory ommunity must e taken into aount. minimize the influene of values at the upper end of the MDL distriutions. The values at the high end of the distriutions represent laoratories that performed poorly relative to most of the surveyed laoratories. This method of alulating the entral tendeny of the MDL dataset was tested y several data-reje- MARCH

9 tion riteria. In eah method used, the outlying value was signifiant at the 1 perent, 5 perent, and 1 perent levels. The outlying value was rejeted, and the mean value was realulated and ompared with the median value prior to filtering the dataset. In every instane, the mean value onverged to the previously alulated median value. Visually, the advantages of the median over the mean are illustrated in terms of RDL distriutions (parts A D, Figure 3); RDL distriutions appear idential to MDL distriutions ut are at values twie the MDL onentrations. It was visually apparent that when the mean and median RDLs differed signifiantly, the median RDL etter represented the entral tendeny of the ulk of the *Lowest ratio 5, QL 1.1 distriution. To alulate a QL, the authors applied a variale multiplier to the median interlaoratory MDL. The multiplier was determined from the MDL spike level and aliration-urve low-point data supplied y the laoratories for this study. Determining the QL for eah ompound for a given method involved three steps: (1) For eah laoratory, the ratio of the reported MDL spike level to the determined MDL was alulated in order to arrive at the spike ratio. The median ratio was then alulated. TABLE Data sheet for trihloroethene method 5.* The RDL protets against oth falsepositive and false-negative detetion deisions, ut RDLs will vary from laoratory to laoratory. MDL Caliration MDL Spike Caliration MDL Spike Level Low Point Level Low Point La Code μg/l μgl μgl MDL MDL C C C C C C C C Mean Median Maximum Minimum Standard deviation µg/l () For eah laoratory, the ratio of the alirationurve low point to the determined MDL was alulated in order to arrive at the aliration ratio. The median ratio was then alulated. (3) The lower of the two median ratios alulated in steps 1 and was seleted as the multiplier of the MDL to determine the QL. This variale multiplier approah offers two advantages. First, the approah is pratial eause the data needed for its determination are data routinely reported y laoratories (i.e., the MDL, the MDL spike level, and the low point on the aliration urve). Seond, the multiplier is ased on atual laoratory performane data rather than on theoretial onsiderations. Individual aliration and MDL spike ratios for eah laoratory represent multipliers that, if used for that partiular laoratory, would result in a QL set at either the laoratory s MDL spike level or low point on the aliration urve. The laoratory must adequately measure oth of these levels to routinely run the method in a quantitative mode. Therefore, the multiplier for a partiular laoratory is known to give a quantitation level that is ahievale. In step 3, the median MDL is multiplied y the lowest median ratio that is known to give a quantifiale level. This results in a median QL that is known to e ahievale y at least half the laoratories. (Later this artile will illustrate that this QL is, in fat, ahievale y most of the laoratories.) The proedure results in a QL that is method- and hemial-speifi. This method offers two additional useful features. First, the QL value is derived from laoratory performane data, whih means that it serves as a aseline for monitoring the improvement of analytial 11 JOURNAL AWWA

10 sensitivities over time. Seond, this QL is useful to the regulatory ommunity in that it is indiative of the urrent analytial apailities for a partiular ompound and method. Results and disussion Tale is an example of the data worksheet for trihloroethene, method 5., showing data olleted from the laoratories and the alulation of the QL using the floating multiplier approah. In this example, the median MDL (. μg/l) was slightly lower than the mean (. μg/l). The median spike ratio (5) was lower than the median aliration ratio (7). The median MDL was multiplied y 5, resulting in a QL of 1.1 μg/l for this ompound. As shown in Tale, individual laoratory multipliers usually ranged from 3 to 1. There were also a few multipliers etween 1 and. Only oasionally did the ratio exeed. The median multiplier used for QL alulation usually ranged from to 7 (Tales 1 3, Figure ). For methods 5. and 5., the mode of the multiplier distriution was 5. When only wastewater methods were onsidered ( series), the mode of the multiplier was. This results from the somewhat higher spike levels and aliration-urve low-point values typially used in the -series methods (the higher levels eing used in order to allow for matrix effets in these wastewater methods). The final step in assessing the suitaility of the alulated QLs is to asertain what perentage of the laoratories ould adequately quantify at these levels. As disussed earlier, the variale multiplier approah results in a median QL that, y definition, should e quantifiale y at least half of the laoratories eause the multiplier links the MDL to the aliration-urve low point or to the MDL spiking level. However, oth of these analytial levels are typially aove the minimum quantifiale level eause they are usually somewhat greater than the RDL, whih is a fator of only twie the MDL. Beause the RDL represents a suitale minimum quantifiale level, the alulated QL must e ompared with the RDL distriutions of the individual laoratories to determine what perentage of laoratories have RDLs less than or equal to the alulated QLs. As shown in parts A D of Figure 3, the QL typially fell at or near the upper end of the main RDL distriution and eliminated the outlying values reported y poorly performing laoratories. The perentage of laoratories reporting RDLs less than or equal to the alulated QLs for all methods and ompounds are taulated in Tales 1 3. With only a few exeptions, more than perent of the laoratories were ale to adequately quantify at the QL ased on their value for the RDL. Tales 1 3 also list USEPA maximum ontaminant levels (MCLs), PQLs, and draft MLs for the Asuitale QL would e a level that is at or aove the RDLs of most laoratories ertified for the method and ompound of interest. ompounds studied. The MCLs and PQLs are appliale to drinking water and are shown for methods 5., 5., and 57 and for admium. The MLs are appliale to wastewater and are shown with the two wastewater methods ( and 5). All QLs reported in this study were elow the appliale MCLs for the ompounds studied. The QLs were also lower than the PQLs reported y USEPA. The QLs typially were in the range of 1 μg/l for the drinking water methods, whereas the PQLs were reported as 5 μg/l. The QLs are lower than the USEPA PQLs for two reasons: (1) quantitation levels have improved (dereased) sine the USEPA PQLs were pulished and () the proedure for determining QLs desried in this artile differs from that used y the USEPA in alulating PQLs. The USEPA used multiple approahes, inluding interlaoratory studies, a multiple of 5 the MDL, and a multiple of 1 the MDL. Maximum ontaminant level goals (MCLGs) for several of the ontaminants in Tales 1 and are eause the ontaminants have een lassified as arinogens. Benzene, aron tetrahloride, 1,-dihloroethane, dihloromethane, 1,-dihloropropane, tetrahloroethene, and trihloroethene all have MCLs or PQLs of 5 μg/l ut MCLGs of. The study presented in this artile suggests that the laoratory ommunity may e ale to quantitate these ompounds at levels of 1 μg/l if the MCLs were dereased to these levels. Draft MLs reported in Tale 3 were approximately twie as high as the QLs for method ut are roughly omparale to the QLs for method 5 (exept for pentahlorophenol). (The draft MLs were not speifially developed for methods and 5 ut for the related methods 1 and 15.) Conlusions Multipliation of MDLs or RDLs y an appropriate fator to determine a suitale QL has een a ommonly employed tehnique. It is still eing onsidered y agenies suh as USEPA and ACS, as in their reent disussions of the RQL and the ML. 1 5 This study analyzed quality assurane data from 51 ertified laoratories in California and New Jersey in order to develop a variale multipliation fator for alulating QLs from existing data. The authors arrived at a QL that was quantifiale y a majority of the analytial laoratories partii- MARCH

11 pating in the study. This QL, whih is ompoundand method-speifi, most frequently is in the range of X the median interlaoratory MDL. Based on this study, data for a partiular ompound and method from as few as five laoratories may provide meaningful results. The data needed for the QL alulation proedure desried in this artile are availale without any hanges in urrent laoratory analytial praties. The proedure ould e used to alulate ompoundspeifi multipliers to determine QLs as illustrated in this study. Alternatively, the oserved lustering of multiplier values in the range of to 7 ould e used to justify seletion of a onstant multiplier in this range. This proedure should e appliale to any standard method and hemial for whih adequate data are availale. The proedure provides a asis for judging the urrent state-of-the-art quantitative aility of the laoratory ommunity and a sientifi asis for regulations that require an atual reportale onentration value for standards use. Aknowledgment This work was funded through the New Jersey A- Drinking Water Researh Fund. The authors thank Mihael Miller of the Offie of Quality Assurane, New Jersey Department of Environmental Protetion, for supplying New Jersey laoratory data used in this study. This artile does not represent the poliy of the New Jersey Department of Environmental Protetion. Referenes 1. KEITH, L. ACS, EPA, Et. to Redefine MDLs. EnvirofACS, :1:5 (Mar. 199).. KEITH, L. Update on Efforts to Redefine EPA s MDL. EnvirofACS, :: (Aug. 199). 3. KEITH, L. Chaos Revisited EPA s MDL. EnvirofACS, 1:1:9 (Mar. 1993).. KEITH, L. EPA s MDL Issue Where is it Going? EnvirofACS, 1::3 (July 1993). 5. KEITH, L. EPA s Offie of Water Surges Toward One MDL Solution: the ML. EnvirofACS, :1: (May 199).. GIBBONS, R. ET AL. Pratial Quantitation Limits. Chemometris and Intelligent Laoratory Systems, 1:5 (199). 7. KIMBROUGH, D.E. & WAKAKUWA, J. Quality Control Level: An Alternative to Detetion Levels. Envir. Si. & Tehnol., ::33 (199).. EATON, A. Estimation of Interlaoratory MDLs and RQLs. Envir. La. (Apr. May 1993). 9. Fed. Reg., 5:19:9 (Nov. 13, 195). 1. KEITH, L.H. ET AL. Priniples of Environmental Analysis. Anal. Chem., 55:1:1 (193). 11. MADDALONE, R.F. ET AL. Detetion & Quantitation Levels for Compliane Monitoring ACS Natl. Mtg., Chiago, Ill. 1. Fed. Reg., 5:13:599 (July, 197). 13. OXENFORD, J.L.; MCGEORGE, L.J.; & JENNISS, S.W. Determination of Pratial Quantitation Levels for Organi Compounds in Drinking Water. Jour. AWWA, ::19 (Apr. 199). 1. MILLER, J.C. & MILLER, J.N. Statistis for Analytial Chemistry. Ellis Horwood Ltd., West Sussex, U.K. (19). 15. CLAYTON, C.A.; HINES, J.W.; & ELKINS, P.D. Detetion Limits With Speified Assurane Proailities. Anal. Chem., 59::5 (197). 1. USEPA. Definition and Proedure for the Determination of the Method Detetion Limit. CFR 13, App. B:35 (July 1, 199). 17. KEITH, L.H. Environmental Sampling and Analysis A Pratial Guide. Lewis Pul., Chelsea, Mih. (1991). 1. TAYLOR, J.K. Quality Assurane of Chemial Measurements. Lewis Pulishers, Chelsea, Mih. (197). 19. CURRIE, L.A. Limits for Qualitative Deision and Quantitative Determination. Anal. Chem., :5 (19).. SKOOG, D.A. Priniples of Instrumental Analysis. Saunders College Pul., New York (195). 1. JOHNSON, R.R. Elementary Statistis. Wadsworth, Belmont, Calif. (197).. USEPA. Maximum Contaminant Level Goals for Organi Contaminants. CFR 11.5:79 (July 1, 199). 3. USEPA. National Revised Primary Drinking Water Regulations: Maximum Contaminant Levels. CFR 11.:75 (July 1, 199).. Fed. Reg., 57:13:3177 (July 17, 199). 5. Fed. Reg., 5::35 (Jan. 3, 1991).. USEPA Offie of Wastewater Enforement and Compliane. National Guidane for the Permitting, Monitoring, and Enforement of Water Quality-ased Effluent Limitations Set Below Analytial Detetion or Quantitation Levels (Draft). Washington, D.C. (Mar., 199). Aout the authors: Paul F. Sanders is a researh sientist in the Division of Siene and Researh, New Jersey Department of Environmental Protetion (DEP), 1 E. State, Trenton, NJ 5. Sanders is a graduate of the University of California at Davis (PhD degree, agriultural and environmental hemistry) and of San Diego State University (BS degree, hemistry). For the past seven years, Sanders has worked on researh projets at the DEP related to drinking water, surfae water, and groundwater quality. His work has een pulished y Environmental Toxiology & Chemistry, Journal of Soil Contamination, Chemosphere, and the Bulletin of Environmental Contamination & Toxiology. R. Lee Lippinott is a researh sientist in the Division of Siene and Researh, New Jersey DEP, 1 E. State, Trenton, NJ 5. Andrew Eaton is tehnial diretor for Montgomery Laoratories, 555 E. Walnut St., Pasadena, CA JOURNAL AWWA