Environ. Sci. Technol. 1986, 20, 1151-1 157
Halogenated Aromatic Compounds in Automotive Emissions from Leaded Gasoline Additives Markus D. Muller" and Hans-Rudolf Buser
Swiss Federal Research Station, CH-8820 Wadenswil, Switzerland Brominated/chlorinated aromatic compounds were identified in automotive emissions from leaded gasoline containing dibromo- and dichloroethane (DBE and DCE). Analysis of exhaust was carried out by absorption on Tenax, followed by extraction and ethylation of phenolic components prior to high-resolution gas chromatography (HRGC) with electron-capture detection (GC-ECD) and mass spectrometry (GC-MS); [13C]pentachlorophenolwas used as an internal standard. The predominantly brominated aromatic compounds found were phenols, alkylphenols (ring brominated), and alkylbenzenes (side chain brominated). The major compounds were 2-mono-, 2,4-di-, 2,6-di-, and 2,4,6-tribromophenol, monobromo- and dibromomethylphenol, a-bromotoluene, a-bromoxylenes, and (bromomethyl)dimethylbenzenes, and smaller amounts of mixed brominated/chlorinated analogues; the total concentration of these halogenated compounds was more than 25 pg/m3 of exhaust. These compounds were not present in exhaust from unleaded gasoline and arise from the DBE and DCE scavenger additives in premium gasoline, presumably via radical halogenation during or after combustion of fuel in the engine. The preliminary results from these experiments under idling motor conditions indicate conversion of these additives to halogenated aromatic compounds at around 0.142%. With DCE and DBE both present in gasoline, the conversion to brominated aromatic compounds largely exceeds that to chlorinated compounds. 2,4,6-Tribromophenol and other halogenated compounds were also detected on snow collected near a motorway. The halogenated aromatic compounds identified constitute a new group of noxious substances present in automotive emissions due to leaded gasoline.
Introduction Recently, the 100th anniversary of the invention of the automobile by Daimler and Maybach and by Benz in Germany was celebrated. During this century, the automobile has made a remarkable evolution from a curiosity at the beginning to a seemingly indispensable necessity in modern life. The technical development and ever increasing motorization was accompanied by discussions on negative health effects, not only from an increasing number of accidents but also from the emissions of the engines powering the vehicles. In earlier discussions prevailed concern about the presence of noxious gases (CO, NO,.), later that of carcinogenic compounds (polycyclic aromatic hydrocarbons, PAHs), and more recently atmospheric pollution from these automotive emissions and its involvement in photochemical reactions leading to phytotoxic substances damaging plants and trees (Waldsterben) (1).
Parallel with improvements on the internal combustion engine (Otto motor; e.g., increasing power requirements and compression ratios), more and more requirements were placed on the quality of the combustion fuel (2). Since the 1920s, antiknock additives, predominantly organic lead compounds (tetraethyllead and more recently also tetramethyllead, TEL and TML) were added to gasoline as radical scavengers. In Switzerland, premium gasoline 0013-936X/86/0920-1151$01.50/0
contained up to 0.64 g of Pb/L; recently, this level was reduced from 0.4 to 0.15 g of Pb/L, and there is a trend toward the use of unleaded premium gasoline. These Pb additives amount to sizeable quantities, e.g., for Switzerland (present annual premium gasoline consumption 2.6 million tons at 0.15 g of Pb/L) 520 tons of Pb/yr. Not surprisingly,these additives are one of the most important sources of environmental contamination with Pb. Dibromo- and dichloroethane (DBE and DCE) are added to leaded gasoline as scavengers to prevent depositions of P b compounds in the engine. The common understanding was that these additives are completely decomposed during combustion. Recently, however, the presence of DBE in automotive emissions (3, 4) and in ambient air was reported (5). Besides combustion of hydrocarbons of the fuel and their conversion to COz and HzO, numerous side reactions may take place in the engine. Formation of NO, and PAHs and nitration and hydroxylation reactions are probably just a few. Phenol and alkylated phenols were identified in automotive emissions (6, 7), and chlorinated phenols were occasionally observed but never systematically investigated. In this paper, we report the finding of halogenated aromatic compounds in automotive emissions from DBE and DCE in leaded gasoline. The predominantly brominated compounds were identified in exhaust gas sampled by Tenax absorption, followed by extraction, derivatization, and analysis using high-resolution gas chromatography (HRGC) with electron-capture detection (GC-ECD) and mass spectrometry (GC-MS). The results show that halogenation reactions of aromatic hydrocarbons and phenols occur during or after combustion of leaded fuel in the engine.
Experimental Solvents, Reagents, and Adsorbents. Solvents used were from Fluka (Buchs, Switzerland), Merck (Darmstadt, FRG), or Carlo Erba (Milan, Italy). Diazoethane was prepared from N-nitroso-P-(ethy1amino)isobutylmethyl ketone (Fluka) (8). Silica gel was obtained from Merck (Kieselgel60 reinst, 70-230 mesh), Tenax GC from Alltech (Arlington IL), and Sep-Pak CIScartridges from Waters (Milford MA). Cars, Gasoline Types, and Test Conditions. Two Volkswagen Golf CL precatalyst automobiles (common, medium-sized car for Switzerland), equipped with fourcylinder engines (1272 mL) and carburetors, were used. Car A (Model 1985) was used for the experiments with leaded premium gasoline (runs 1and 2) and car B (Model 1984) for the experiment with unleaded gasoline (run 3). The two cars were in good running conditions and both met legal emission control specifications. Gasoline used in runs 1 and 3 was analyzed for additives (courtesy E. Gartenmann, Swiss Federal Laboratory for Materials Testing and Research, EMPA, Dubendorf). Lead was determined by X-ray-spectroscopy, and DCE and DBE were converted to the corresponding halogenides and determined by subsequent titration with silver nitrate solution. Gasoline for run 2 was made up by adding DCE to
0 1986 American Chemical Society
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Table I. Composition of Gasolines Used during Test Runs for Exhaust Sampling" gasoline additive
run Ib
run 2=
run 3d
DBE DCE alkyllead as Pb molar ratio (Br:Cl)
0.112 0.041 0.158 1:0.68
0,112 0.111 0.158 k1.85
nd' nd 0.008 -
oConcentrations are in g/L. Sampling was under idle motor conditions (1000 rpm, 12 m3 of exhaust gas/h) and about 0 "C ambient temperature. Gasoline consumption was 1.2 L per run. *Leaded premium gasoline. Same gasoline as run 1 but 0.07 g/L DCE added. dunleaded gasoline (Euro Quality, 95% octane). end, not detected (detection limit 0.001 g/L).
leaded gasoline from run 1. Details on the composition of the gasolines used and on further test conditions are given in Table I. Exhaust Sampling. Sampling of exhaust gases was carried out under idle motor conditions via a glass tube (0.5-m length, 6-mm o.d., 4-mm i.d.) with a glass wool plug at the back end positioned close at the outlet of the muffler inside the exhaust pipe of a parked car, leading the exhaust gas into two absorption cartridges in series, each filled with 0.6 g of preconditioned Tenax (9). The first cartridge was spiked with the internal standard [20 pL of a toluene solution containing l ng/pL [13C,]pentachlorophenol ([13C]PCP),courtesy P. Schmid, Institute of Toxicology, University of Zurich and Federal Institute of Technology, Schwerzenbach] prior to sampling. After passing through the Tenax traps, the gases were cooled in a gas wash bottle immersed in icewater; an oil-free diaphragm pump (Capex MK I, Austen Pumps, Byfleet, U.K.) maintained a constant flow rate of about 3 L/min (about 1.3 times isokinetic conditions). The total amount of gas sampled (typically 200 L) was measured by means of a gas meter (Wohlgroth, Zurich, Switzerland). The total sampling time of 70 min was divided in 15-min intervals, after which the probe was removed and the pump was turned off. The car was driven for about 20 km prior to the sampling periods to thoroughly warm up the motor and exhaust system and for 2 km in the sampling intervals to simulate a stop and go situation and to maintain medium motor block temperatures. The sampling train was disassembled after a test run and the cartridges were stored until analyzed in preconditioned test tubes with screw caps covered with aluminum foil in a refrigerator. Extraction of Cartridges, Derivatization, and Cleanup. The probe was rinsed with 10 mL of a mixture of acetone and hexane (1:l). Cartridges I and I1 were extracted separately in the same way (10 mL of solvent each); the washes from the probe and cartridge I were combined. The extracts were carefully reduced in volume (rotary evaporator, vacuum), dried with anhydrous sodium sulfate, and ethylated with diazoethane after addition of a few drops of methanol. After careful reduction in volume, sample purification was carried out by passing the extract (about 0.5 mL) through 0.5 g of silica gel; 10 mL of hexane eluted the compounds of interest. Collection and Sample Preparation of Snow Samples. Snow samples were collected near Wadenswil in February 1986,2 days after a light snowfall, from shady places at 20- and 3000-m distances from a busy four-lane motorway (N3 Zurich-Chur). Snow from a surface area of 1m2and a depth of 3 cm was collected into a precleaned stainless steel vessel. The samples were covered with aluminum foil after adding 20 ng (20 pL) of internal standard ([13C]PCP) onto the snow surfaces. After 1152
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melting, the samples (about 1.5 L) were acidified (pH 3) with dilute sulfuric acid, and the neutral and acidic components were adsorbed on a Sep-Pak CIBcartridge and eluted with 1.5 mL of acetone (10). A blank sample was run with the same procedure but with 0.5 L of mineral water in place of snow. The resulting acetone extracts were dried with anhydrous Na2S04after addition of n-hexane (3 mL), reduced in volume, and ethylated with diazoethane. The derivatized extracts were purified on 0.5 g of silica gel, eluting the compounds of interest with 10 mL of n-hexane, and then carefully concentrated to 100 pL. Reference Compounds. The halogenated phenols used were 2- and 4-mOnO-, 2,4- and 2,6-di-, and 2,4,6-tribromophenol and their chlorinated analogues (Fluka). Methyl- and dimethylphenols used for the preparation of brominated derivatives were 2-, 3-, and 4-methylphenol (Aldrich, Steinheim, FRG) and 2,4-, 2,6-, and 3,5-dimethylphenol (Fluka). Further reference compounds were 2-, 3-, and 4-bromotoluene and 2-, 3-, and 4-bromobenzoic acid (Fluka). Mixed brominated/chlorinated phenols with up to three halogen substituents in ortho/para positions were prepared by bromination of various mono- and dichlorophenols. They were 4-bromo-2-chloro-, 6-bromo-2-chloro-, and 4,6-dibromo-2-chlorophenol, 2-bromo-4-chloro- and 2,6dibromo-4-ch1oropheno1, and 6-bromo-2,4-dichloro- and 4-bromo-2,6-dichlorophenol. Electron-impact (EI) mass spectra of their ethyl derivatives gave strong molecular ions (M+) with the expected ion clustering due to the Br and C1 isotopes; intense (usually base peak) fragment ions at M+ - C2H4 (M+- 28) were characteristic. The same EI-MS behavior was observed for the ethylated parent bromo and chloro compounds. Elution temperatures and EI-MS data of these compounds are listed in Table 11. Isomeric mixed halogenated compounds showed earlier elution of isomers with o-Br than with o-C1 substitution (reduced retention of isomer with bulkier ortho substituent). Bromination of 2- and 4-methylphenol gave mono- and dibromo derivatives; bromination of 3-methylphenol gave additionally a small amount of a tribromo derivative Bromination (presumably2,4,6-tribromo-3-methylphenol). of dimethylphenols gave both ring- and side-chain-substituted products (later eluting). Ring-brominated compounds (ethylated mono- and dibromo derivatives) show strong M+, base-peak M+ - 28, and M+ - 107 (M+ - CzH4 - Br) ions; side-chain-brominated products show M', base-peak M+ - 79 (M+- Br), and M+ - 107 ions (M+ 28 ions small or absent). The two types of compounds were thus distinguishable by EI-MS. Isomer assignments of the various mono- and dibromomethylphenols/mono- and dibromodimethylphenols were not attempted since the HRGC system was not optimized for best isomer resolution. 3,5-Dimethylphenol also formed a tribromo ringsubstituted derivative (presumably 2,4,6-tribromo-3,5-dimethylphenol). Bromination of toluene and xylenes (mixture of isomers) lead to mono- and dihalogenated, side-chain-substituted products. These compounds show weaker M+ ions, but more intense M+ - Br ions than the earlier eluting, ringsubstituted compounds. Brominations were carried out as follows: 5-10 mg of a phenol was dissolved in 0.2 mL of CC4and Br2 solution (0.5 mL of Br2 in 20 mL of CC4)dropwise added at room temperature until the red color remained (0.5-1 mL). The solutions were kept at room temperature for about 1 h, occasionally Br2solution being added if required. Solvent and byproducts (HBr, remaining Br2) were evaporated in a stream of nitrogen at 60-80 "C, the residue was taken
Table 11. Halogenated Aromatic Compounds Identified in Automative Emissions compd. no.
compdD
elution temp, OC
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 32 33 34 35 36
bromobenzene (bromomethyl)benzene (bromomethy1)methylbenzene dibromobenzene 2-bromophenol (Et) 4-bromophenol (Et) dibromomethylbenzene (bromomethyldimethylbenzene (bromobenzofuran) (bromomethy1)dimethylbenzene 6-bromo-2-chlorophenol (Et) bromomethylphenol (Et) bromomethylphenol (Et) tribromobenzene 2,6-dibromophenol (Et) 4-bromo-2-chlorophenol (Et) bromodimethylphenol (Et) bromodimethylphenol (Et) (dibromodimethylbenzene) 6-bromo-2,4-dichlorophenol (Et) 4-bromo-2,6-dichlorophenol (Et) dibromophenol (Et) 2,4-dibromophenol (Et) 4,6-dibromo-2-methylphenol (Et) dibromomethylphenol (Et) (dibromobenzofuran) 2,6-dibromo-4-chlorophenol (Et) 4,6-dibromo-2-chlorophenol (Et) dibromomethylphenol (Et) dibromomethylphenol (Et) 2,4,6-tribromophenol (Et) dibromoethylphenol (Et) dibromochloromethylphenol (Et) tribromophenol (Et) dibromochloromethylphenol (Et) dibromodimethylphenol (Et) pentachlorophenol (Et) [13C]pentachlorophenol (Et) ([13C]PCP) tribromophenol (Et) (dibromoethylmethylphenol (Et)) tribromomethylphenol (Et) (dibromoethylmethylphenol (Et)) (tribromoethylphenol (Et)) tribromodimethylphenol (Et) 2,3,4,6-tetrabromophenol(Et) (tribromomethylmethylphenol (Et))
73.0 87.7 96.6 97.1 101.7 102.7 106.8 106.9 108.7 109.6 110.4 112.1 113.5 119.2 119.8 120.4 121.2 121.7 121.7 125.0 126.0 127.9 130.0 131.7 132.3 133.3 134.8 136.0 141.0 142.5 145.5 148.3 149.0 149.6 149.8 153.8 156.7 156.7 156.9 158.8 159.2 160.4 164.2 172.7 176.7 177.3
37 38 39 40 41 42 43 44
EI-MS datab frag ion M+ 156 (Br) 170 (Br) 184 (Br) 234 (Br,) 200 (Br) 200 (Br) 248 (Br,) 198 (Br) 196 (Br) 198 (Br) 234 (BrCl) 214 (Br) 214 (Br) 312 (Br3) 278 (Br,) 234 (BrCl) 228 (Br) 228 (Br) 262 (Br2) 268 (BrC1,) 268 (BrCl,) 278 (Br2) 278 (Br,) 292 (Br2) 292 (Br,) 274 (Br2) 312 (Br,Cl) 312 (Br2Cl) 292 (Br,) 292 (BrJ 356 (Br3) 306 (Br2) 326 (Br,Cl) 356 (Br3) 326 (BqCl) 306 (Br,) 292 (Cl,) 298 (Cl,) 356 (Br3) 320 (Br,) 370 (Br3) 320 (Br2) 384 (Br3) 384 (Bra) 434 (BrJ 398 (Br3)
M+ - 28 M+ - 28
Mt- 28 M+ - 28 Mt - 28
M+- 28 MC - 28 M+ - 28 Mt - 28 M+ - 28 M+ - 28 M+ - 28 M+ - 28 M+ - 28 Mf - 28 Mt - 28 Mt - 28 Mt- 28 M+ - 28 M+- 28 Mt - 28/43 Mt - 28 Mt - 28 M+ - 28 M+ - 28 Mt - 28 M+ - 28 Mt - 28 M+ - 28/43 M+ - 28 Mt - 28/43 Mt - 28/43 M+ - 28 M+ - 28 M+ - 28/43
run 1
++ +++ ++++ + ++++ ++ + ++++ ++ +++ + +++ + + ++++ nd + + + + nd + ++++ ++ +++ + + ++ +++ +++ ++++ ++ + + + ++ + d + + +++ + + + + +
concn rangec run 2 run 3
++
+++ ++++
+ ++++ ++ + +++ ++ ++ + +++ + + ++++ ++ + + + + + + ++++ ++ +++ + ++ +++ ++++ +++ ++++ ++ + + ++ +++ + d + + ++++ + + + + +
nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd d nd nd nd nd nd nd nd nd
Phenolic compounds analyzed as ethyl (Et) derivatives; compounds tentatively identified are in parentheses. Exact isomerism is given where available. * m / z values; halogenation given in parentheses. Concentration ranges given semiquantitatively as (nd) not detected (50, >9, and >12 ng/m2, respectively, with recoveries unknown; E1 mass spectrum of dibromobenzoicacid ethyl ester in Figure 3). The latter compounds may also be related to automotive emissions although they were not detected so far in our exhaust samples. Conclusions
A series of brominated/chlorinated aromatic compounds were observed in automotive emissions from leaded gasoline containing DBE and DCE. Surprisingly, the compounds (phenols, methyl- and dimethylphenols, benzenes, and alkylbenzenes) were predominantly brominated and much less chlorinated despite the fact that both additives were present in similar molar quantities and in an experiment (run 2) more DCE was added. In case of the phenols, all possible mixed halogenated combinations were observed. Halogenated compounds were not present when unleaded gasoline was used. Since premium gasoline in Switzerland contains DBE or a mixture of DBE and DCE (5),brominated and to some degree also mixed halogenated products have to be expected, but not the exclusively chlorinated compounds. This, however, might have been different in earlier years when supposedly only DCE was used as an additive. Formation of these compounds likely proceeds via radical halogenation of aromatic hydrocarbons and phenols during or after combustion of fuel in the engine. Aromatic hydrocarbons (benzene, alkylbenzenes) are common constituents in automotive emissions (15,16), and phenols can be formed by OH' attack on these hydrocarbons during combustion. Thus, phenol, all isomeric methyl- and ethylphenols, certain dimethylphenols, and others have been identified in such emissions (6, 7). Our study now revealed that these compounds can be further substituted by halogen and that Br' seems to react much more efficiently than Cl' toward formation of halogenated aromatic compounds, though C1' generally is regarded as the more reactive species. This finding may result from preferred formation of Br' from the precursor additive or from a more efficient trapping of C1' by another mechanism. Ring substitution of phenols and alkylphenols and side chain substitution of alkylbenzenes were the predominant reactions observed although the actual amount 1156
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of halogen in these products was only a minute fraction (about 0.1-0.2%) of the total amount of halogen emitted. Halogenation is a further example of a substitution reaction occurring in internal combustion engines. Nitration reactions with formation of nitrated aromatic compounds have already been reported (6,17). The total concentration of the brominated compounds observed in exhaust was at least 25 pg/m3; it is probably higher because of the insufficient collection of the more volatile compounds and losses in the isolation procedure. The conversion level of DBE and DCE into these aromatic compounds is in the order of 0.1-0.2% (calculable from gasoline consumption). Because the experiments were carried out with the engines running idle, the results are difficult to extrapolate to actual driving conditions and to the motoring situation in Switzerland. Further studies are required for a final assessment and should also cover the influence of different motor conditions on qualitative and quantitative changes in the composition of these products. Furthermore, the brominated phenols observed are ortho-substituted and therefore have a potential for polyhalogenated dibenzo-p-dioxin (dioxin) formation. Brominated dioxins, very likely of similar toxicity as the chlorinated analogues (well-documented environmental contaminants) (18-20), have so far not been identified in the environment. The environmental significance of this source of halogenated aromatic compounds is unknown. Presently, it is premature to extrapolate and estimate emission rates for these compounds from our preliminary automotive emission rates. Additional studies will be required to confirm our results and the presence of these compounds in the environment. In such studies, however, care has to be taken because environmental levels and the nature of the compounds may change due to chemical reactions (oxidation, photolysis, others) between emission sources and final deposition. The brominated compounds thus far detected (2,4,6-tribromophenol,bromo-, bromochloro-,and dibromobenzoic acid) have structural similarities to herbicidal compounds. Acknowledgments
We thank P. Schmid for the [13C]PCPinternal standard material and P. Gartenmann for analysis of the gasoline additives. Registry No. DBE, 25620-62-6; DCE, 1300-21-6;bromophenol, 32762-51-9; 2,4-dibromophenol, 615-58-7; 2,6-dibromophenol, 608-33-3; 2,4,6-tribromophenol, 118-79-6; bromomethylphenol, 55909-73-4; dibromomethylphenol, 86006-42-0;a-bromotoluene, 100-39-0;bromobenzene, 108-86-1; (bromomethyl)benzene, 10039-0; (bromomethyl)methylbenzene, 28777-60-8; dibromobenzene, 26249-12-7; dibromomethylbenzene, 29732-82-9; (bromomethyl)dimethylbenzene, 104155-11-5; bromobenzofuran, tribromobenzene, 104155-12-6;6-bromo-2-chlorophenol,2040-88-2; 28779-08-0; 4-bromo-2-ch1oropheno1, 3964-56-5; bromodimethylphenol, 58170-30-2; dibromodimethylbenzene, 28805-90-5; 6-bromo-2,4-dichlorophenol, 4524-77-0; 4-bromo-2,6-dichlorophenol, 3217-15-0; dibromophenol, 28514-45-6; 4,6-dibromo-2methylphenol, 609-22-3; dibromobenzofuran, 104155-13-7; 2,6dibromo-4-ch1oropheno1,5324-13-0; 4,6-dibromo-2-chlorophenol, 4526-56-1; dibromochloromethylphenol, 86006-44-2; pentachlorophenol, 87-86-5; dibromoethylmethylphenol, 104155-14-8; tribromomethylphenol, 65436-87-5; tribromodimethylphenol, 58170-32-4; 2,3,4,6-tetrabromophenol, 14400-94-3; tribromoethylmethylphenol, 104155-15-9; bromobenzoic acid, 25638-04-4; bromochlorobenzoic acid, 25638-14-6; dibromobenzoic acid, 65436-55-7.
Literature Cited (1) Rehfuess, K. E. Allg. Forst Z. 1983, 38, 601-610.
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Rossenbeck, M. In UllmannsEnzyklopadie der technischen Chemie, 4th ed.; Verlag Chemie Weinheim: New York, 1979; Vol. 17, pp 51-57. Leinster, P.; Perry, R.; Young, R. J. Atmos. Enuiron. 1978, 12, 2383-2387. Sigsby, J. E.; Dropkin, D. L.; Bradow, R.; Lang, J. M.; "AutomotiveEmissions of Ethylene Dibromide";Technical Paper 820786; Society of Automotive Engineers (SAE): New York, 1982. Zingg, M. Thesis, Federal Institute of Technology Nr. 7592, Zurich, 1984. Mulawa, P. A.; Cadle, S. H. Anal. Lett. 1981, 671-678. Roumeliotis, P.; Liebald, W.; Unger, K. K. Int. J. Environ. Anal. Chem. 1981,9, 27-43. Baumgarten, H. E., Ed. Organic Syntheses, Collection V; Wiley: New York, 1973; p 351. Ligocki, M. P.; Pankow, J. F. Anal. Chem. 1985, 57, 1138-1144. Renberg, L.; Lindstrom, K. J. Chromatogr. 1981, 214, 327-334. Krost, J. K.; Pellizzari, E. D.; Walburn, S. G.; Hubbard, S. A. Anal. Chem. 1982, 54, 810-817. Leuenberger, Ch.; Pankow, J. F. Anal. Chem. 1984, 56, 2518-2522.
Paasivirta, J.; Sarkka, J.; Leskijarvi, T.; ROOS,A. Chemosphere 1980,9, 441-456. Xie, T.-M. Chemosphere 1983,12, 1183-1191. "Effect of Gasoline5 Aromatics Content on Exhaust Emissions";Report 3/73; Concawe Foundation: Den Haag, 1973. Merian, E. Chem. Rundsch. 1974,42, 5-11. Wang, Y. Y.; Rappaport, St. M.; Sawyers, R. F.; Talcott, R. E.; Wei, T. E. Cancer Lett. 1978,5,39-47. Stalling, D. L.; Smith, L. M.; Petty, J. D.; Hogan, J. W.; Johnson, J. L.; Rappe, C.; Buser, H. R. In Human and Environmental Risks of Chlorinated Dioxins and Related Compounds;Tucker, R. E.; Young, A. L.; Gray, A. P., Eds.; Plenum: New York, 1983; pp 221-240. Buser, H. R.; Rappe, C.; Bergqvist, P. A. Environ. Health Perspect. 1985, 60, 293-302. Czuczwa, J. M.; Hites, R. A. Enuiron. Sci. Technol. 1986, 20, 195-200.
Received for review April I , 1986. Revised manuscript received June 23, 1986. Accepted July 10, 1986. This work was part of a project for monitoring organic pollutants in the environment of the Swiss Federal Office for Environmental Protection and its support is gratefully acknowledged.
Analytical Method Comparisons by Estimates of Precision and Lower Detection Limit David M. Holland*,+and Frank F. McElroyz Monitoring and Assessment Division and Quality Assurance Division, Environmental Monitoring Systems Laboratory, Office of Research and Development, U.S.Environmental Protection Agency, Research Triangle Park, North Carolina 27711
Principal component analysis (PCA) can be used to estimate the operating precision of several different analytical instruments or methods simultaneously measuring a common sample whose actual value is unknown. This approach is cost-effective when none of the analytical techniques is sufficiently superior to serve as a reference and obviates the need for experimental designs requiring duplicate instruments. PCA provides composite reference values from sample measurements to approximate the true analytical values. From these composite values, estimates of the operating precision can be obtained. This technique is used to estimate the operating precision of six commercial chemiluminescence analyzers used to measure 1-h average ambient nitrogen dioxide concentrations. Precision obtained alternatively from duplicate data available for one of the analyzer types agreed closely with the PCA estimate. For each analyzer, a measure of a detection limit, defined as the lowest concentration that is detectable with a given degree of certainty, is also provided. Introduction Frequently, the need arises to compare the performance of a number of different analytical instruments or methods on the basis of simultaneous measurements of common or identical samples of a material whose true value is unknown. The process of obtaining data is such that, for any given sample, there is only one opportunity for measurement, but it is possible for several instruments to make simultaneous measurements of that sample. The property of the material to be measured is variable over time so that the same instrument cannot make replicate observations. Monitoring and Assessment Division.
* Quality Assurance Division.
Duplicate measurements may be unobtainable due to high costs of testing duplicate measurement systems. A number of instruments or methods can be compared under these circumstances for a variety of operational and performance characteristics, one of which would likely be measurement errors caused by various operational factors. Measurement errors are usually described in terms of precision and accuracy, which can also be described by random and systematic errors, respectively. Random analytical error (precision) is a measure of method repeatability, and systematic error (accuracy) is usually described as being composed of additive (fixed) and/or multiplicative (proportional) biases. As an aid in determining relative operating performance, principal component analysis (PCA), as outlined by Lawton et al. (I), may be advantageously used to estimate the operating precisions of several analytical instruments under the conditions described above. This approach involves all portions of the entire measurement process and estimates precision of actual or typical analytical measurements. Confidence intervals may be constructed for each precision estimate to facilitate comparison of the instruments on the basis of the precision associated with each one. A general discussion of PCA and its use in estimating operating precisions and subsequent determination of detection limits is provided. The procedures are then applied to estimate these performance parameters for six different models of commercial atmospheric nitrogen dioxide analyzers. Background When an error-free reference method is available, the precision of p methods simultaneously measuring common
Not subject to US. Copyright. Published 1986 by the American Chemical Society
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