Environ. Sci. Technol. 1993, 27, 2692-2699
Method Detection Limits in Solid Waste Analysis David E. Kimbrough’ and Janice Wakakuwa
California Department of Toxic Substances Control, Southern California Laboratory, 1449 West Temple Street, Los Angeles, California 90026-5698
A two-part study is presented to assess the applicability of the US. EPA’s method detection limit (MDL) to the analysis of solid materials. The first part compares MDLs calculated for arsenic, cadmium, molybdenum, selenium, and thallium in soil with the actual method performance on real spiked soils with these analytes at concentrations above and below the calculated MDL. The MDL method is examined to discover its empirical suitability for solid waste analysis and to see if it is the proper theoretical tool for solid matrices. The criteria are the precision and accuracy of results. The results show that the MDL method produces accurate and precise results only in interference-free conditions. This investigation was extended to an interlaboratory study which included the same five soils used above and five other soils spiked with PCBs. A total of 160 accredited environmental laboratories participated in this study. The applicability of the MDL was assessed by measuring the number of qualitative and quantitative errors produced by these laboratories. The results indicate that approximately two-thirds of the reported MDLs produced significant errors. Introduction When determining the concentration of regulated substances in solid matrices (e.g., sediments, sludges, soils, spent catalysts, press cakes, slags, powders, etc.) two types of data are generated: numerical values indicating the amount of an analyte present and “none detected” or “less than” values. A positive numerical value is usually defined within a certain precision and accuracy. A less than value on an analytical report is as much a data point as a numerical value and should be determined with equal precision and accuracy. The most common method for determining this less than value is the U.S.EPA’s method detection limit (MDL) ( I , 2) based on the work of Glazer et al. (3). This method was developed using analyses of organic analytes in water matrices. MDLs have, however, been used extensively for organics and inorganics in solid matrix analyses without an examination of their applicability to the procedures. Less than values are important for a variety reasons. Regulatory agencies often use these values to determine legal action levels, remediation cleanup levels, and other legally binding decisions. Instrument manufacturers and analytical laboratories often use MDLs as marketing tools to demonstrate superior performance. Public debate on environmental issues often revolve around the issue of whether contaminants were “detected” or not in a particular medium. This creates an environmental game of “limbo chemistry” where each participant challenges the others to see “how low can you go?” There has been considerable debate since the promulgation of the MDL procedure on its appropriateness. The debate has focused mainly on statistical theory (e.g., refs 4-6). There has been little study of MDLs from an empirical basis. To examine the applicability of MDLs 2692
Environ. Scl. Technol., Vol. 27, No. 13, 1993
to solids, a two-part study was designed. In the first part, the MDLs are calculated for five analytes in soil: arsenic, cadmium, molybdenum, selenium, and thallium. The calculated MDLs were then compared with the actual method performance on real spiked soils with these analytes at concentrations above and below the calculated MDL. The MDL method is examined to discover its empirical suitability for solid waste analysis and to see if it has the proper theoretical tools for solid matrices. As a continuation of that study, the same materials used for the single laboratory study were analyzed by a large number of laboratories. In addition to the soils used in the first part of the study, five more soils were prepared which were spiked at different concentrations with PCBs. The criteria for assessment is the precision and accuracy of the results.
Method Detection Limit The U.S.EPA’s MDL is the smallest concentration of an analyte in a given matrix and is determined by a given method that has a 99% confidence that this value is not zero. The MDL is hence an either/or procedure; an analyte is present at a concentration that is not zero with 99% confidence or it is not. There is no determination of the accuracy or precision of a value near the MDL. The MDL is a six-step procedure. These procedures are designed to be used on any matrix and for any analyte. The five-step portion of these procedures that are applicable to solid wastes are presented below. (1)Estimate the MDL by one of four procedures: (a) The concentration that corresponds to an instrument signal to noise ratio of 2 . 5 5 . (b) The concentration value that corresponds to 3 times the standard deviation of replicate instrumental measurements for the analyte in reagent water. (c) The concentration value that corresponds to the region where there is significant change in sensitivity at low analyte concentrations. (d) The concentration value that corresponds to the known instrument limitations. (These will be referred to as the estimated MDLs.) (2) Obtain a material corresponding to the matrix type for which the MDL is to be determined. The material must have a concentration of the analyte(s) of interest at 1-5 times (but not to exceed10 times) the estimated MDL. (3) Take a minimum of seven aliquots of the material and process each through the entire analytical procedure. Calculate the results back to solid phase with the appropriate units (such as bg/g). (4) Calculate the standard deviation (S) using eq 1, as follows:
s = (C(x2; - X 2 i ) / n- 11”~ 0013-936X/93/0927-2692$04.00/0
(1)
0 1993 Arnerlcan Chemical Society
( 5 ) Calculate the MDL by eq 2 as follows:
MDL = ts (2) where t is the Student’s value approximate for a 99% confidence level for n - 1. Since n is equal to seven for part 1of this study, the Student’s t value would be 3.143. (This will be referred to as the calculated MDL.) (6) There is an optional step that calls for preparing a material spiked exactly at the calculated MDL and then repeating steps 3-5. This will be referred to as the iterative procedure. Following this procedure, a second or recalculated MDL is produced. If the results of the spiked sample do not allow qualitative identification then steps 3-5 are repeated with a higher spiked concentration. If the spiked concentration is qualitatively identifiable but the standard deviation of the seven replicates of the spike (Sb)is 3.05 times greater than the standard deviation of the calculatedMDL determination (Sa), then another spiked material must be prepared. If the ratio of SdSb is less than 3.05, then the MDL is recalculated by pooling the data following eqs 3 and 4:
Spooled = [(6Sa)2+ (6Sb)2/1211’2
(3)
MDL = 2.681(SPoo1,d) (4) (This will be referred to as the pooled MDL.) It must be emphasized that in following the MDL procedure through to its logical conclusion, there can be as many as four different MDLs: estimated, calculated, recalculated, and pooled.
Experimental Section
(A) Study Design, There were two aspects of any analytical procedure that should be of interest to analytical chemists: (1)How well does the method perform under optimum conditions? (2) How well has the method performed under the actual conditions of day-to-day analysis? To answer both of these questions, the study was setup with two parts. Part 1followed the MDL procedure as closely as possible for five analytes (arsenic, cadmium, molybdenum, selenium, and thallium) in five soils. Each Soil was found to have very small quantities of these analytes. Each analyte was then spiked onto the soil at a different concentration over 4 orders of magnitude plus the “blank” soil (see the Materials section below). Each soil was then digested 7 times and analyzed by two or three instruments. [For three analytes, cadmium, molybdenum,and thallium, three instruments were used, so there were 12 estimated MDLs per analyte. Arsenic and selenium were analyzed using only two instruments, so there were only eight estimated MDLs. Thus, there were a total of 52 estimated MDLs. The instruments used were as follows: a Jobin-Yvon JY50 P simultaneous inductively coupled plasma atomic emission spectrometer (ICP-AES), a Perkin-Elmer PE5500 sequential ICP-AES,and a Thermo Jarrel-Ash Video 12E flame atomic absorption spectrometer (FAAS).] The estimated, calculated, recalculated, and pooled MDLs were determined for each analyte by each instrument. For part 2,the same five soils used in the part 1and five additional soils spiked with Aroclor 1260 (see Materials section below) were analyzed by two sets of laboratories. The first set consisted of 26 environmental laboratories associated with government agencies, who acted as referees for sample validation. These samples were then distributed among 199 private and government laboratories that
were accredited by the Environmental Laboratory Accreditation Program (or ELAP, the California State program) for the analysis of the six analytes described above. A total of 160of these laboratories returned results. Each laboratory analyzed the samples and determined their own MDL value for each analyte, even if the laboratory did not report any less than values. There were no predetermined MDLs for this study. (B)Criteria. The control limits for this study are f50% of the spiked value for concentrations greater than 50 pgl g, rounded to two significant figures. For concentrations of less than 50 pg/g, the limits are &50% with one significant figure. Results outside of these limits are considered “out of control”, Less than values reported by a laboratory are referred to “negatives”. A false positive is a result that reports the presence of an analyte in a sample above the reporting limit when, in fact, the concentration of the analyte is below the reporting limit. For example, in a sample containing less than 1 pg/g thallium (
