Muiture analytes 0
Procedures for determining detection limits should be dejned by Ann L. Alford-Stevens The ability to detect and measure analytes has frequently been discussed in terms of detection limits (DLs), which can be used to compare analytical methods, instruments, and laboratories. A number of articles give definitions and procedures for calculating DLs for elements or single compounds but not for analytes that are mixtures of organic compounds (1-5). Several pollutants of concern to EPA are mixtures. Examples are chlorinated dibenzo-p-dioxins (CDDs), chlorinated dibenzofurans (CDFs), polychlorinated biphenyls (PCBs), toxaphene (chlorinated camohenes)..,.wlvbrominated biphenyls (PBBs), halowaxes (chlorinated naphthalenes), chlorinated paraffins, and technical chlordane (chlorinated methanoindenes and indans). Most of these mixtures are. or were, commercial products, and although some are well characterized, others are not. For example, toxaphene has been reported to be a complex mixture of at least 202 compounds (6). but structural information is unavailable for many of its components. Although CDDs and CDFs frequently occur as mixtures, they were never deliberately manufactured and sold as commercial products. PCBs, however, occur both as commercial Aroclor formulations and as non-Aroclor pollutants, including those generated as byproducts and incinerator emission components. Nine Aroclors are frequent target analytes, and their compositions are variable, both in number and in level of chlorination of components. Individual Aroclors can contain as many as 100 PCB congeners (7).
cern in this article. Instead, the issue is the particular detector signal that is used to calculate a number reported as a DL for mixture analytes. Current practices vary so much that DLs reported for mixtures are not comparable.
DL determination problems
I
Current situation The issue of appropriate procedures to determine DLs for these mixtures has not been well addressed. Method detection limits (MDLs) have been stated for Aroclor 1242, chlordane, and toxaphene in an article that describes a procedure for estimating MDLs, but the fact that these analytes are mixtures was not discussed (2). Analytical methods have been available for Aroclors, toxaphene, and chlordane for many
years; most methods involve separation of sample components with packed-column gas chromatography (GC) and detection with an electron capture detector. With these methods. identification of mixture analytes requires recognition of the GC peak profile produced by sample components. This peak p r e file then must be compared with that produced by a standard of the mixture of concern. Some methods (for example, Method 608 for organochlorine pesticides and PCBs [@ and Method 625 for base neutrals and acids [9])list DLs for these mixtures but do not specify how they were determined. EPA methods for PCBs (IO) and CDDs and CDFs ( I ] ) provide procedures for identification and measurement by level of chlorination with a mass spectrometer (MS) detector. These methods present an opportunity for a new approach to statements of DLs for those mixture analytes. This article's purpose is to stimulate discussion of the problems with determining DLs for mixtures, but no facile solutions are presented. A DL is the lowest quantity of an analyte that an analytical process can reliably detect (I). Distinctions between instrument detection limits (IDLs), limits of detection (LODs), limits of quantitation (LOQs), and MDLs have been discussed elsewhere (3).Neither these distinctions nor the particular mathematical manipulations required to obtain a reported number, however, are of con-
This article not subject to U.S. copyright. Published 1987 American Chemical Society
Mixtures that are not well characterized must be identified and measured as commercial formulations. Frequently, even well-characterized mixtures are determined as commercial products, because standards of individual components either are unavailable or are scarce and expensive. The problem of DL determinations for mixtures can be illustrated with PCBs, which can be identified and measured by level of chlorination, as commercial Aroclors, or as individual compounds. Determination of a DL for an individual PCB is the same as that for any other single compound analyte and will not be discussed here. When PCBs are determined as Aroclors, a DL cannot be experimentally determined until a particular mixture is identified. Unfortunately, acceptable differences between the GC peak pattern produced by an Aroclor standard and that produced by sample components have not been defined. Because individual Aroclor components vary in relative abundance, the number of components detected will vary with the absolute quantity of Aroclor analyzed. With a given quantity, an analyst can select a small signal (GC peak) that meets stated criteria for comparison to background noise. When a smaller quantity is analyzed, however, that signal will diminish or disappear and another peak will have to be selected to meet signal-to-noise (S/N) criteria. As the Aroclor quantity decreases, fewer components will exceed the minimum detectable quantity, the number of unmeasured components will increase, and the GC peak pattern will change. For example, about 70 PCB congeners were measured in extracts of reagent water fortified with Aroclors 1221 (IOpglL), 1242 (IOOpglL), 1254 Enviran. Sci. Technal.. Vol. 21. No. 2. 1987 137
(Io0pglL), and 1268 (50 pglL) (12). Only about 75% of those congeners were measured, however, when extracts of reagent water containing approximately one-tenth the concentrations of the same Aroclors ( 1 pglL of 1221, IO pg/L of 1242, IO pg/L of 1254, and 6 pgIL of 1268) were analyzed under the same analytical conditions. Obviously, differences were observed in the G C peak patterns produced by these two sets of extracts. At some point, observed GC peaks produce a pattern that no longer resembles that of a given Aroclor or mixture of Aroclors, and identification by pattern recognition is no longer possible. That point, however, is difficult to define. The problem can be illustrated with the total ion current profile (TICP) produced by GClMS analysis of 100 ng of Aroclor 1260 (Figure I). If a smaller quantity were analyzed and only those peaks exceeding 25% relative abundance in Figure 1 were observed, the relationship of the resulting chromato gram to that of Aroclor 1260 would be obscure, even though major components of Aroclor 1260 could still be seen. The question then becomes one of how many components must be observed before an Aroclor is identifiable. This issue has not been addressed in methods that list Aroclors, toxaphene, and chlordane as analytes and state DLs for them (8, 9). Certainly, when sample components can no longer be identified as commercial formulations, DLs cannot be expressed in terms of those products. Even when the sample peak pattern matches the Aroclor peak pattern exactly, DL determination is not straightforward, and selection of appropriate detector signals is left to the analyst’s judgment. An extreme example illustrates the point. An analyst could decide that only the four most abundant peaks (labeled 1252, 1338, 1433, and 1676 in Figure I ) were necessary to identify and measure Aroclor 1260. In that case, a very small quantity of Aroclor 1260 would produce detector signals meeting SIN criteria, and the calculated DL would be absurdly low. When a more reasonable pattern is selected to represent Aroclor 1260, different results will be obtained if one analyst selects a peak of 5% relative abundance and another selects a 20% peak. When PCBs are identified and measured by level of chlorination, analysts may be tempted to express DLs in terms of isomer groups. With that approach, an MS detector response is calibrated by analyses of varying quantities of a single compound selected to represent each level of chlorination. A 138 Environ. Sci. Technol.. Vol. 21. No. 2. 1987
FIGURE 1
Total ion cutrent profile of 100 ng of Aroclor 1260
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Time
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1800
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Retention time (min) concentration is calculated for each level of chlorination, and total analyte mixture concentration is obtained by adding the isomer group concentrations together. This measurement procedure produces results with an inherent error, because an MS detector responds differently to different isomers in a given group. Even if that were not the case, the varying number of isomers in each group negates the validity of DLs stated in terms of level of chlorination. Although a DL for each congener has not been reported, estimated DLs (varying from injected quantity of 0.07 ng for a CI, PCB to 1 ng for Cllo PCB) can be used to illustrate the situation with PCBs (11). All 46 CI5 PCBs could be present in a solution, but none would be seen if the injected quantity of each isomer were below the IDL. Using an MS IDL of 0.5 ng for an individual CIS PCB and assuming it to be valid for all CIS PCBs, one can see that as much as 23 ng of total CIS PCBs can be present without being detected. If only one CIS PCB is present, however, it can be detected when at least 0.5 ng is injected. On the other hand, only one Cllo PCB exists, and as little as I ng of it can be detected. A reported DL for an isomer group, therefore, could vary from 0.2 ng for CI, PCBs (0.07 ng for each of three possible isomers) to 23 ng for CIS PCBs. If a few CIS PCBs were identified and measured to produce a quantity of 6 ng, the result would be quite confusing when the DL for CIS PCBs was stated to be 23 ng. For total PCBs, DLs will depend on the number of individual congeners present and on the
sensitivity of the detector to each.
Possible solutions In the case of determinations by level of chlorination (such as for CDDs, CDFs, PCBs, and PBBs), a possible approach to estimating DLs for mixture analytes is to measure a specific signal produced by each calibration congener and then calculate a DL for each. This value could be used as an estimate of the DL for an individual isomer in a group. One problem, however, is that the variability of MS detector response among isomers creates a level of uncertainty in addition to that usually associated with a DL for an analyte that is an element or single compound. There are different levels of difficulty in determining DLs for commercial products, such as Aroclors, toxaphene, and chlordane. For Aroclors, a particular GC peak at a given relative retention time in a given Aroclor formulation could be used if identification criteria and analytical conditions were standardized. For example, a relatively small GC peak, such as that labeled 1876 in the TICP of Aroclor 1260 (Figure I ) , could be specified as the peak to be used for DL measurements. Then the quantity analyzed to produce a signal of stated characteristics could be used to define a DL with a given procedure when a standard of Aroclor 1260 is analyzed. This approach, however, is not viable for a sample extract that does not contain intact Aroclors. For example, if one or more of the Aroclor 1260 components that produced the selected peak were not extracted from a sample with the same efficiency as other Aroclor
FIGURE 2
Total ion current pmfile of 4 pg of toxaphene 100
amu
I
Time 2 Retention time (min)
1260 components were, its relative abundance would be reduced in the extract chromatogram, and it might not be observed at all. A similar procedure could perhaps be used for toxaphene, hut with more difficulty. The multitude of toxaphene components ( >200) produces a complex GC peak pattern (Figure 2) that contains many unresolved peaks, even with with capillary column separation. The number of observed toxaphene peaks varies with slight variations in GC conditions. Because of the complexity of this mixture, a calculated toxaphene DL would probably have a relatively high level of uncertainty. Analytes that are mixtures of organic compounds of varying concentrations present difficulty for DL determinations because it is not easy to express DLs in terms of isomer group, compound class, or commercial formulation. Until appropriate procedures for determining DLs of mixture analytes are defined and widely accepted, any stated DL must be accompanied by an explanation of how it was determined, including specific citation of the detector signal used for calculations. All mixtures cannot be handled the same way. For example, CDDs, CDFs, PCBs, and PBBs can be determined by level of chlorination with an MS detector, but insufficient information is available to determine toxaphene by this method. On the other hand, CDDs and CDFs can be determined as individual compounds or by level of chlorination, although they cannot be determined as commercial products. Even with analytes that are elements or single compounds, DLs are only sta-
tistical estimates, not determinate values. Certainly, stated DLs for complex miXNreS must be treated only as estimates, and reported numbers ought to be accompanied by sufficient information to allow data users to judge their validity and comparability to other reported DLs. References ( I 1 American Chemical Society Committee an
George W. Ingle. Editor Chemical Manufacturers Association Identifies a n d evaluates major effects of Toxic Substances Control Act (TSCA) on the chemical industry and on society as a whole. Covers detection and analysis of t h e s e effects from a variety of viewpoints. Helps to delineate beneficial and detrimental consequences of this law.
Environmental Improvement. A n d . Chem.
nno, 52, 2242-49. (21 Glaser. J . A. et al. Ewiron. Sri. Techno/. 1 9 ~ 1 . 1 51426-35. , (3) American Chemical Society Committee on
CONTENTS Background. Goals. and Resultant Issues Impact on Market Introduction of New Chemicals Future for lnno~ation Harmonizingthe Regulation of New Chemicals in USandEEC.ContrololExistingChemicats. Overview Aner Five Years * Initiatives of Chemical Industry to Modify TSCA Regulalons .Managemental TSCA-Mandated Information Impact on Corporate Strunure and PrOcedureS Conlidenltalily 01 Chemical Identities Impact on Reactive Polymer Industry * Ellect~01 TSCA on Metalworking Fluids Industw: Increased Awareness of Nitrosamine Contamination Impact on Public Health * Quantitative Analysis as Basis lor DeCisions Under TSCA * Educating the EnvironmentalChemical Professional -Overall Costs and Benefits Summary
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Environmental Improvement. A n d . Chem.
1983,55, 2210-18. (4) Long. G.L.: Winefordner, J . D. Anal. Chem. 1983.55, 712-24 A. (SI Kirchmer. C. J . Environ. Sci. Technol. 19U3.17, 174-81A. (61 Saleh. M. A. J . Agric. Food Chem. 1983, 31. 748-51. (7) Albro. P W.: Carbett. J . T.: Schroeder. J . L. 1. Chmmofogr 1981,205, 103-1 I.
(8) "Guidelines Establishing Test Procedures for the Analysis o f Pollutants under the Clean Water Act," Fed. Regist. 1984, 49(2091,43321-36. (9) "Guidelines Establishing Test Procedures for the Analysis of Pollutants under the Clean Water Act." Fed. Regist. 1984, 49(2091.43385-406. (101 Method 680, "Determination of Peaticides and PCBs in Water and Soil Sediment by Gas ChromatagraphyiMasr Spcctrometry": Environmental Monitoring and Support Laboratory. EPA: Cincinnati. 1985. ( I I) Method 8280. "Method o f Analysis for Chlorinated Dibenzo-p-dioxins and Dibenrofurans"; Environmental Monitoring Systcms Laboratory. EPA: L a s Vcgas. Nev., IO**
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(I21 Alford-Stevens. A . L. et al. Anal. Chem.. 19n6,5~.2022-29.
Ann L. Alford-Stevens is un analytical chemist in EPAs Environmental Monitoring und Support Lnboratory in Cincinnati.
Based on a symposium joinlly sponsored by the ACS Divisions01 Industrialand Engineering Chemislry, Chemical Informarioon, Organic Coatings and Plaslics Chemistry, Small Chemrcal Busioesses, and the Board Commime on Corpora000 Associates
li
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