Ultra-trace dioxin and dibenzofuran analysis: 30 years of advances

Ultra-trace dioxin and dibenzofuran analysis: 30 years of advances. Ray E. Clement. Anal. Chem. , 1991, 63 (23), pp 1130A–1139A. DOI: 10.1021/ac0002...
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ULTRATRACE DIOXIN AND DIBENZOFURAN ANALYSIS: 30 YEARS OF ADVANCES Ray E. Clement Ontario Ministry of the Environment Laboratory Services Branch 125 Resources Road (P.O. Box 213) Rexdale, Ontario, Canada M9W 5L1

Enough newspaper headlines have appeared around the world that “dioxins” and “furans” are now house hold words; among the sources they are associated with are incinerators, the pulp and paper industry, the manufacture of chlorophenols and re lated products, and accidental PCB fires. Those not working directly in the field of the chlorinated dibenzofi-dioxins (dioxins) and chlorinated dibenzofurans (furans) may think that these contaminants are relatively new to environmental r e search. But, in fact, these compounds have been with us since at least 1872 when their synthesis was first re-

ported by German chemists ( I ) . According to scientists at Dow Chemical Company who have performed extensive studies, dioxins and furans may be produced a t trace level amounts by any combustion source (2). If this “trace chemistry of fire” hypothesis i s t r u e , dioxins a n d furans may have been with us since the first forest fire. Despite tremendous research efforts to ascertain the effects of these chemicals on biological systems, there is still much we don’t know. Until such issues are resolved (and they may never be!), it is clear that the controversy surrounding dioxins and furans will remain. In fact, new sources of these compounds are still being found. At the 1989 International Dioxin Symposium, Travis used modeling techniques to show t h a t four of the major sources of 2,3,7,8-TCDD (i.e., municipal incin -

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erators, automobiles, residential wood combustion, and hospital waste incinerators) accounted for only 13% of the total input to the environment (3).He made a convincing case that a few major sources of dioxins-or many minor sources-have yet to be discovered. The concerns and issues raised above have one thing in common: their resolution requires a tremendous number of analytical measure ments, at very low detection limits, in a wide variety of sample types. This is a difficult task: The dioxins and furans in all of the sample types under investigation must be extracted from the original matrix; separated from a complex mix of hundreds or thousands of other organic co-extractives, some of which are potential interferences present at concentrations many times greater than that of the analytes; and then mea0003-2700/91/0363-1130A/$02.50/0 0 1991 American Chemical Society

sured accurately at part - per - trillion (pptr) or lower concentrations. Accomplishing this task has challenged the abilities of even the most proficient analytical laboratories. In this REPORT, I will show that analytical science has made tremendous a d vances in determining dioxins and furans over the past 30 years, but that further refinements are still required. What are dioxins and furans? The structures of the chlorinated dibenzo-#-dioxins (CDDs) and the chlorinated dibenzofurans (CDFs) are shown in Figure 1. Up to eight chlorine atoms can be placed on the basic structure, giving rise to 75 dioxin congeners and 135 furan congeners. All of the 75 dioxins are congeners of one another, or members of a like group, and congeners having the same number of chlorines are isomers of one another. Thus there are 22 tetrachlorinated dioxin isomers. A group of dioxin or furan isomers is often referred to as a congener group. Dioxins or furans containing different numbers of chlorines are often also called homologues. This is, however, incorrect because consecutive members of a homologous group differ by a fixed s t r u c t u r a l u n i t , whereas dioxin or furan congener groups differ by the number of hydrogens that have been replaced by chlorine atoms. Although the terms “dioxins” and “furans” are technically incorrect, these short-form names are now so widely used t h a t they have become standard terminology. Some of the difficulties inherent in determining these compounds in environmental samples can be predicted by considering their structures. They are hydrophobic; thus their determination in water samples must include a consideration of suspended particulates. They are not, however, particularly soluble in common solvents. Compounds with seven or eight chlorine atoms are difficult to dissolve even in toluene, making the preparation and storage of standards difficult. Dioxins and furans are also lipophilic, and are thus subject to bioaccumulation and biomagnification in the environment. Therefore low detection limits are essential to tracking down sources. In addition, dioxins and furans are very stable, so their long-term transport in the environment is assured once they are emitted into air or water. Early detection methods In 1957 millions of broiler chickens in the United States died of “chick edema” disease (4).By 1959 the cause was traced to toxic components

in feed fats, and in 1966 X-ray crystallography techniques led to the identification of one of the toxic substances as 1,2,3,7,8,9-hexachlorodibenzo-$-dioxin. By 1972 the source of the dioxin was traced to fleshing grease from hides contaminated with commercial pentachlorophenol ( 4 ) . Before 1970, detection limits were in the part-per-million to part-perbillion (ppb) range, and were obtained using packed-column GC with electron capture detection (ECD). Most work centered on the determination of the most toxic dioxin-2,3,7,8TCDD-in industrial chemicals. A typical GC-ECD chromatogram is illustrated in Figure 2. From 1970 to 1980 the explosive increase in the use of G C N S methods for trace determination of environmental pollutants made GC-ECD somewhat obsolete for use in dioxin determination. The problems with GC-ECD analysis were with specificity as much as with detection limits: Mixtures obtained from environmen-

tal sources were so complex t h a t highly specific as well as highly sensitive detection systems were needed to determine dioxins at low ppb concentrations. In addition, it was be-

coming more important to study many different sample types in order to determine the sources and environmental distribution of dioxins and furans. Even G C N S by itself was insufficient to produce the desired results, and a great deal of front-end chemistry was required to separate trace analytes from potential inter ferences. All of the groundwork for today’s state-of-the-art methods was established in the early 1 9 7 0 ~and ~ it can be argued t h a t most subsequent work has simply optimized these early methods. Evolution of current methods The development of analytical methods was complicated by another pressing need, that of the separation of all toxic dioxins and furans from those not considered to be toxic. Until about 1980 the main challenge consisted of the determination of total congener groups (e.g., total tetrachlorinated dioxins and total pentachlorinated dioxins) in addition to the isomer- specific determination of 2,3,7,8-TCDD. However, results from a number of toxicological and biochemical studies showed that all dioxins and furans with chlorine substitution at the 2, 3, 7, and 8 ring positions (Figure 1)were of toxicological concern, not just 2,3,7,8-TCDD. Thus, for purposes of risk and health assessments, those congeners not substituted a t the 2, 3, 7, and 8 positions can be ignored; all but 17 of the 75 dioxins and 135 furans fall into this category. Those not considered toxic are serious interferences for the 17 that are. For example, of the 22 tetrachlorinated dioxin isomers, only 2,3,7,8-TCDD is of concern. But because the 22 isomers are so similar in structure, it is difficult

hgure 2. GC-ECD chromatogram of dioxins extracted from toxic fleshing grease. Figure 1. Structures of the chlorinated dibenzo-pdioxins and chlorinated dibenzof urans.

The numbers above the peaks represent the numbers of halogen atoms in the individual dioxin molecules. (Adapted with permission from Reference 4.)

ANALYTICAL CHEMISTRY, VOL. 63, NO. 23,DECEMBER 1,1991

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REPORT to differentiate 2,3,7,8-TCDD from other TCDD isomers in the sample. Conventional MS detection did not help, because the electron ionization mass spectra of dioxin and furan isomers are almost identical. Other techniques such as FT-IR and NMR spectroscopy, noted for their ability to differentiate among isomers, could not solve this problem because the necessary detection limits were too low. The importance of the correct assignment of identities to dioxin and

furan congeners is further illustrated by Table I, which lists the relative toxicities of the 17 toxic dioxins and furans (as determined by toxicologi cal studies) in terms of International Toxicity Equivalency Factors ( I TEFs). More than one set of factors exists, but those shown in Table I are those most commonly used by the international community. The 2,3,7,8TCDD toxicity- equivalent concentration for each compound is calculated by multiplying the concentration of the congener by its I-TEF. Summing

Figure 3. Chromatograms of TCDD isomers from fly ash extract using (a) pachau and (b) capillary columns. (Adapted from Reference 5.)

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the 17 I-TEF-corrected concentrations generates a single number that is equivalent (in terms of toxicity) to the concentration of all 17 toxic dioxins and furans in the sample if all were present as 2,3,7,8-TCDD. In practice, the determination of isomer - specific concentrations of all 17 toxic dioxins and furans is a difficult analytical challenge. To address this need, methods for chromatographic separations have been developed and improved. A combination of HPLC and high-resolution capillary GC (HRGC) h a s made possible the required separation. The commercial development of the fused-silica HRGC column, beginning around 1980, was one of the most important steps in environmental dioxidfuran analysis. The flexible fused - silica capillary column made it possible for even relatively inexperienced users to routinely employ advanced HRGC technology. Figure 3 illustrates the importance of HRGC technology. Figure 3a shows a gas chromatogram from a packed - column GC/MS determination of TCDD isomers in municipal incinerator fly ash whereas Figure 3b is a chromatogram of the same sample, performed on a homemade glass capillary column (5).The largest peak in Figure 3a is in the elution region of 2,3,7,8TCDD. The HRGC analysis clearly shows that this large peak is the sum of at least three smaller peaks, only one of which can be 2,3,7,8-TCDD. In fact, 2,3,7,8-TCDD is one of the smaller peaks in the chromatogram. Advances in sample cleanup and in HRGC and GC/MS techniques during the 1980s brought dioxidfuran analysis to the present state of the art: the ability to determine all 17 toxic dioxins and furans in virtually any air, water, solid, or biological sample type at detection levels of pptr to ppq (part per quadrillion). The key to successful analysis is the number and efficiency of analytical separation steps. For example, the use of a sulfuric acid - on- silica cleanup column, sodium hydroxide-on- silica cleanup column, dual alumina cleanup column, carbon cleanup column, HPLC cleanup, HRGC separation, and mass spectrometric ion separation in the same analysis has been reported. In addition, complete methods may include acid or base washes of the sample extract, drying of the extract (i.e., with sodium sulfate), and other techniques such as tandem or chemical ionization MS. Most laboratories now use high-resolution MS (HRMS) at a resolving power of 9000-12,000 for highest detection limits and ex-

cellent specificity. However, without the many chemical cleanup steps prior to detection, even the most advanced and expensive instrumentation will not achieve the desired analytical results. Many laboratories have found that the development of trace dioxin/furan analytical capabil ity requires much more than the purchase of an expensive high -resolu tion GC/MS system. The improvement i n analytical performance between the years 1971 and 1988 is illustrated in Figure 4. Figure 4a is a chromatogram of two picograms of 2,3,7,8-TCDD published in 1971 (6).Note that the signal is barely visible above background and the peak shape is poor. Figure 4b, published in 1988 ( 7 ) , is a chromatogram of 0.13 picograms of 2,3,7,8-TCDD and has good signal-to-noise ratio a n d excellent peak shape. Also, three ions characteristic of TCDD were detected in

Figure 4b but only one was monitored in the 1971 determination. Whereas the 1971 analysis was truly state of the art at that time (only one or two laboratories worldwide could have achieved this performance), more laboratories today can boast of subpicogram detection on a routine basis-at least for some sample types. In a 1988 survey conducted by NATO, 108 laboratories in member nations claimed to have some dioxin/ furan analysis capability. Of these labs, 48 were in the United States, 23 in Germany, and 13 in Canada. This survey did not include non-NATO countries such as Japan that also have several dioxin laboratories. The number of laboratories that perform this determination has increased since 1988; there may now be close to 150 worldwide. Of course, not all these labs are equal in capability. There are still no more than 20 labs,

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tlyure 4. State-of-the-art determinations of 2,3,/,8I wL) pumshed in (a) I Y I I (adapted from Reference 6) and (b) 1988 (adapted with permission from Reference 7).

perhaps as few as a half-dozen, that can consistently demonstrate sub pptr analytical detection levels for the 17 toxic dioxindfurans in a wide range of sample types including air, water, soil, biota, and industrial stack emissions. Any laboratory with sufficient financial resources can purchase an HRMS system, but relatively few analysts are expert in the application of a variety of complex sample cleanup procedures. How real are the numbers? To an analytical chemist, one of the most amazing aspects of dioxidfuran analytical work is the long time it took to develop calibration s t a n dards. It still is not possible to purchase all 210 dioxins and furans from a single commercial source. However, all 17 of the 2,3,7,8-substituted compounds are now available in either crystalline form (for the adventuresome) or solution. Also available are 13C-labeled standards of the 17 toxic dioxins/furans. The evolution of these standards parallels the evolution of the analytical methodology. The first commercial isotopically labeled dioxin standards were made by KOR Isotopes in the early 1970s. These were radioactive dioxins made for the U.S. Air Force. Between 1973 and 1981 environmental incidents such as those a t Seveso, Italy, and Times Beach, MO, led to a demand for a wider variety of labeled and unlabeled dioxin standards. The increased u s e of isotope dilution GC/MS methods during this period resulted in a n expanding market for these standards. New standards that were developed included 37Cl- and 13C-labeled 2,3,7,8-TCDD and octachlorodioxin. In 1981 the specialists who made these and other stand a r d s left KOR to found a new company, Cambridge Isotope Laboratories (CIL). This new company was so successful that in the mid-1980s it purchased KOR. In the 1980s a new concern about CDFs produced by PCB fires and an especially controversial incident in a Binghamton, NY,office building cat alyzed CIL’s production of both furan and dioxin standards. I n 1987 a round-robin study was initiated, involving many of the leading dioxin/ f u r a n analytical laboratories in North America. The goal was to develop consensus values for all 2,3,7,8substituted dioxindfurans provided by CIL (8). These results were also used by CIL to characterize the corresponding 13C-labeled standards used for isotope dilution GUMS analysis.

ANALYTICAL CHEMISTRY, VOL. 63, NO. 23, DECEMBER 1, 1991

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Although excellent calibration standards for dioxidfuran analysis are now available, the same cannot be said for certified reference materials (CRMs). In fact, there are currently no real-matrix dioxidfuran CRMs available. The efficiency of extraction and cleanup procedures is largely determined by the recovery of 13C-labeled standards added to the sample before extraction. Unfortunately, good recovery of standards spiked onto the surface of a solid matrix, or spiked into an aqueous sample (into which the dioxindfurans will not dissolve), does not guarantee good recovery of corresponding ana lytes. Excellent within-laboratory repro ducibility can be obtained for difficult dioxidfuran analyses. For example, reproducibility when analyzing am bient air samples can be difficult because concentrations of analytes are generally very low, and sampling variations may be greater for collocated Hi-Vol samplers than for many other sample types. Despite this problem, comparison of results from collocated ambient air samplers shows only small differences. Our laboratory recently completed a round-robin study of dioxindfurans in ambient air extracts. Solvent extracts of ambient air samples were used because they are easy to homogenize and apportion equally to the participating labs. A total of 13 laboratories that employed high - resolu tion GC/MS procedures participated. Results showed that the percent relative standard deviations (% RSDs) were greater for low-level than for high-level samples. With one exception, the % RSDs were also greater for the determinations of total tet rachlorodibenzo-fi- dioxins and tet rachlorodibenzofurans than for the corresponding octachlorinated con gener, which means simply that it is more precise to determine a single analyte than the total of several ana-

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Figure 5. Number of cumulative dioxidfuran references cited in Chemical Abstracts, 1967-1 989.

lytes. The average RSD of -20% is good for trace organic analyses, especially for difficult determinations such as the dioxins/furans. There is certainly some room for improve ment in interlaboratory precision. The development of a range of realmatrix CRMs is of great importance. We have found that even without CRMs the use of effective sample cleanup followed by isotope dilution high-resolution GC/MS can produce believable results even at pptr (or lower) concentrations. However, more improvement is still desirable.

Where do we go from here? Aside from the pressing need for dioxidfuran CRMs, it may appear that analytical methods are so advanced that there is little need for additional improvement. However, many countries are considering regulations to further limit the input of dioxins and furans to the environment. Depending on the regulatory need, required detection limits may have to be lowered even more. On the other hand, evidence exists t h a t the potential hazards from exposure to dioxins/ furans may be less severe than previ-

The overall growth in dioxidfuran research in the past 20 years is illustrated in Figure 5, which shows the number of cumulative dioxin references found in a year-by-year computer search of the Chemical Abstracts (CAI database by using as key words the CA codes for all 2,3,7,8-substi-

ously thought (9). There is still considerable disagreement on this issue (10, 21). Effective analytical methods are needed not only to support regulatory actions but also to serve major ongoing scientific investigations that are attempting to resolve these controversies.

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REPORT tuted dioxins and furans, and the dioxin congener groups. Because not all publications concerning dioxins/ furans include CA numbers for specific dioxins/furans, the 3253 refer ences found as of 1989 do not include all relevant papers-but the trend is obvious. These reports are coming from laboratories all over the world. The increasing numbers of papers show that dioxin and furan research is an ongoing international activity. Even more powerful analytical methods are needed to support this international effort. Is there room for improvement in dioxidfuran methodologies? What are the principal needs? First of all, even lower detection limits are desired. Because of large bioaccumulation factors, very low detection limits in receiving waters are required to protect aquatic organisms. Some PCBs have been shown to have appreciable toxicities, and a few researchers have suggested t h a t 2,3,7,8-TCDD TEFs be developed for them. Regulations based on total TEFs that include dioxins, furans, and PCBs may also require lower detection limits and would present other challenges to established meth-

ods. Many regulatory bodies a r e the ability to detect 1fg of an analytadopting multimedia approaches to ical standard does not mean we could environmental regulation. This sim see such a quantity in a real environmental sample. ply means that total exposure from There a r e important concerns all sources must be considered to other than detection limits. The cost protect human and ecosystem health. of analysis (from about $1000 to Therefore a calculated maximum to1erable exposure to dioxins and furans $2000 per sample) inhibits further would have to be apportioned among research and encourages investigathe various exposure pathways. The tors to pare the number of quality detection limits required when esticontrol samples to a minimum. Ofmating exposure from any single ten, too few replicate samples (or even too few samples) are taken in pathway would be much lower than if the entire tolerable exposure were field investigations, and too few allocated to that exposure pathway. round- robin and interlaboratory It is very likely that detection camethod validation studies have been pability will be lowered considerably conducted to date. Improved automaby the end of this decade. Because tion and simpler yet effective cleanup scientists have only recently been schemes would help to lower costs. working at pptr and ppq levels, such Cost reduction will be difficult to concentrations seem particularly achieve because of the expensive inlow. One pptr is 10-12g/g or ~ O - ~ ~ / Lstrumentation , and highly trained and one ppq is 10-15g/g or 10-12g/L. and experienced staff needed. And of The approximate number of 2,3,7,8course, all dioxidfuran analysts TCDD molecules in 10-l2g is 2 x lo9. would love to have available a range of real-matrix environmental CRMs. Thus, even at extremely low concenNotwithstanding the above, the trations of pptr and ppq, millions of improvements in dioxidfuran anamolecules are still being detected. Within a few years we should be getlytical capability over the past 30 ting close to 1-fg detection, and it years have resulted in significant adshould be possible within a decade to vances in measurement science. Imclose in on 1-ag detection. Of course, provements in the areas of GC and

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MS instrumentation, the use of isotope dilution, quality control procedures, and other techniques are applicable to the study of other trace contaminants. The impact of dioxin/ furan analytical development on the commercial manufacture of mass spectrometer systems illustrates these points very well: Demand for high-resolution systems for dioxin/ furan determinations h a s driven manufacturers to greatly improve scan speed and stability, software, and general ruggedness of available instruments. Tough competition between various companies has caused the cost of these systems to fall dramatically, enabling many additional laboratories to gain access to this advanced technology. Thirty years of dioxidfuran research has benefited many fields in addition t o significantly advancing analytical science. Studies in toxicology, e pi d emi o 1o gy , environment a1 transport and fate, and many other fields would be difficult or impossible without the support of analytical chemists and their sensitive, specific methodologies. Although advances to date in dioxidfuran measurement capability have been impressive, it

ley, J. S.; Tondeur, Y. G.; Wehler, J. R. Chemosphere 1990,20, 487-94. (9) Hanson, D. J. Chem. Eng. News 1991, Jan. 28, 7. (10) Hanson, D. J. Chem. Eng. News 1991, Apr. 29, 13-14. (11) Hanson, D. J. Chem. Eng. News 1991, Aug. 12, 7-14.

would not be surprising if those in the next decade are equally so. Thanks are extended to Tom Tiernan of Wright State University and Joel Bradley of Cambridge Isotope Laboratories for their valuable comments in the preparation of this manuscript. This paper is based on a presentation given a t the 42nd Pittsburgh Conference and Exposition.

References (1) Long, J. R.; Hanson, D. J. Chem. Eng. News 1983,June 6, 25. (2) Bumb, R. R.; Crummett, W. B.; Cutie, S. S.; Gledhill, J. R.; Hummel, R. H.; Kagel, R. 0.; Lamparski, L. L.; Luoma, E. V.; Miller, D. L.; Nestrick, T. J.; Shadoff, L. A.; Stehl, R. H.; Woods, J. S. Science 1980,210, 385-90. (3) Travis, C. A.; Hattemer-Frey, H. A. Chemosphere 1990,20(7-91, 729-42. (4) Firestone, D. Environ. Health Perspect. 1973,5,59-66. ( 5 ) Clement, R. E. Ph.D. Dissertation, University of Waterloo, 1981, p. 202. (6) Baughman, R.; Meselson, M. In ACS Advances in Chemistry Serzes; American Chemical Society: Washington, DC, 1973; Vol. 120, pp. 92-104. (7) Karasek, F. W.; Clement, R. E. Basic Gas Chromatography-Mass Spectrometry: Principles and Techniques; Elsevier Science: Amsterdam, 1988, p. 126. ( 8 ) Bradley, J. C.; Nichols, A. W.; Bonaparte, K.; Campana, J. E.; Clement, R. E.; Czuczwa, J. M.; DeRoos, F. L.; L a m p a r s k i , L. L.; N e s t r i c k , T. J.; Patterson, D. G.; Phillips, D. L.; Stan-

Ray E. Clement was awarded a Ph.D. from the University of Waterloo in 1981. He joined the Ontario Ministry of the Environment in 1982 and is currently a senior research scientist in the Laboratory Services Branch. Clement has published 90 scientific papers, mostly in the dioxin/ b r a n field, and has published or edited four books. He has a strong interest in teaching environmental and analytical chemistry and holds appointments at Carleton University and the University of Western Ontario. He was recently apCHEMISTRY pointed to the ANALYTIC' Instrumentation Advisory Panel.

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Base-catalyzed esterification of Benzoic Anhydride. Two hour run at 4 cm-l resolution with MIR fiber optic probe.

C 3-D and 2-D contour run display. ]

Time-base programming to match time to reaction rate decay. --

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