Spin Trapping Organic Radicals - Analytical Chemistry (ACS

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Spin Trapping Organic Radicals play an important role of toxicologically sig-

y pyrolytic processes, e reactivity associa association with smoke or polluted air. Thus the development of methods for ecies has been an imporlaboratories.

gas-ph

radic Its principal advantage over EPR spectroscopy is sensitivity;its detection limit is at least 4-5 orders of magnitude lower than

that are sensitive and sel data and identification, ho cially of unknown organi cals are not drastically affect ture of the matrix. As a re are electron can sometimes it becomes increas

(EPR) spectroscopy ap suited for the determination of centrations of free radic gaseous media. EPR spe highly selective; it detec that are paramagnetic

Both EPR and LMR t additional disadvantages use for directly detecting and stud gas-phaseradicals, especially mental work. The complex an

curate measurermation using nitroso spin possible using nitrones. nitroso spin traps generally an the rates for spin trapping substantially better initial

adducts of similar radic

the vast majority of spin-trapping applications, numerous gas-phase studies have been published.

used-is that an un

be studied convenientlyby EPR spectroscopy. The most common spin traps are nitrone and nitroso compounds, both of which react with radicals to form stable nitroxide (aminoxyl) radicals, as shown in Equations 1 (nitrone spin trap) and 2 (ni-

somewhat photochemically and thermally labile and therefore can be awkward to handle, especially over the longer sampling periods required for trapping species in environmental samples. Largely because of the limitations of the nitroso spin traps, nitrones are more commonly used. In this article, use of nitrone spin traps and, in particular, a-phenyl-Ntert-butylnitrone (PBN), can be assumed whenever the trap is not explicitly identified. The identities of the trapped radicals occasionally can be determined unambiguously from the EPR spectra of the spin adducts; more typically, only the general type(s) of radicals present (alkoxyl, per- y l , alkyl, etc.) can be determined by the d R spectrum alone. Figure 1shows a

The radical adds directly to the na scent nitroxide nitrogen of the nitroso reas radicals add to the nt to the nitro . ' 420

cal Chemistry, Vol. 66, No. 7,April

ally quantitatethese two spin add they were present in the same mixture. Even when the HFSCs of two spi are different (e.g., those for the me and methoxyl radicals), it is often diffic to obtain accurate resolution of ping spectra.

of the spin adduct spectra and spe simulation (7,8). However, even when these techniques are effective, the best one can typically expect from HFSC data alone is the correct classification of a spintrapped radical (i.e., as carbon- or oxyge centered); more detailed structural e dation requires other techniques. The preceding discussion reveals two principal weaknesses of spin trapping with detection by conventional EPR spectroscopy: It does not by itself all

Figure 1. EPR spin adduct spectrum

.HFSC paramet

ctual signals because it mi

fication and canrelatively

cts can be obtaine ination of the soli merous processes (I1

Simple gas-phase syst

isotropic. Better spe

ple collection in gas-

sition to both methy

should only be necessary

1 A

However, when the gas phase resulting from the photolysis of acetone was passed over solid PBN, only the methyl radical spin adduct was observed. This result can be explained by assuming that the unimolecular decomposition of acetyl radicals to carbon monoxide and methyl radicals (Equation 4) is too fast for the acetyl radicals to be trapped by PBN (19).

This conclusion is consistent with later estimates of rate constants for radical trap ping by PBN (6).The photochemical decomposition of acetone was repeated and everything was kept the same except that MNP was used as the spin trap. The spin adduct in this latter experiment was attributed to the acetyl radical; the methyl adduct was not observed. This result is consistent with the observation that nitroso traps react more rapidly with carboncentered radicals (both alkyl and acyl) than do nitrones (6). It is not clear, however, why the methyl radical is not also trapped by MNP Maeda and Ingold have shown that both acyl and alkyl radicals react with MNP with similar rate constants (6). One likely possibility is that the methyl spin adduct of MNP is much less stable than the spin adduct derived from the acetyl radical. It is well known that the nitroxides with an alpha hydrogen atom are prone to undergo a bimolecular disproportionation to nonradical products, namely a new nitrone and a hydroxylamine (Equation 5).

I-

The preceding results make it clear that neither of these two spin traps alone can provide a meaningful measure of even the relative gas-phase concentrations of the two radical species present in this simple gas-phase system.

borne free radicals in air samples from a plant during the processing of polyethylene and polystyrene plastics. Air samples were collected into a spin trap solution. Gas-phase organic free radicals were formed in air samples colldcted during injection molding, extruding, seam welding, and wire-cutting of the plastics. In all Combustion and pyrolytic processes of these processes the materials are subMost combustion and many pyrolytic pro- jected to high temperatures and mechanicesses involving organic materials occur cal stress. There are undoubtedly many other high-temperature industrial provia radical-mediated mechanisms (22). Indeed, the spontaneity of most combuscesses that also produce high gas-phase tions arises from the fact that they are radical concentrations; however, there are branched radical chain oxidations. Moreno other reports of comparable studies. over, the high temperatures to which orThe results of this simple experiment ganic materials are exposed during either raise a fundamental question about the spontaneous combustion or pyrolyses at nature of radicals in the more complex temperatures higher than - 400 "C are environmentallyrelated systems. The radsufficient to cause the homolysis of many icals detected by spin trapping typically of the bonds in organic molecules. are characterized as highly reactive small In recent years researchers have ques- organic species that would not be extioned whether organic radicals might in pected to have liietimes of more than a some way contribute to the toxicity of the few seconds under ambient conditions smoke or fumes produced by combustion (28).Consistent with this expectation, and/or pyrolytic processes (22,23).The results from early gas-phase spin-trapping key premise is that organic free radicals experiments using simple chemical sysare highly reactive and, if inhaled, would tems demonstrated that a short time pebe able to react with critical target biomol- riod between radical formation and trap ecules in the lung, a process that would ping was needed to obtain maximal spin lead to their oxidative destruction. The adduct concentrations (16). One could toxicological interest in combustionpredict that as gas-phase mixtures bederived gas-phase free radicals relates come increasingly complex, there will be primarily to the exposure of individuals to additional pathways for the destruction of the gases produced during structural reactive radicals, and effective radical lifefires; there are also industrial hygiene times will become even shorter. However, concerns for workers exposed to fumes gas-phase radicals in combustion systems produced by the high-temperature procan often be detected for many seconds or cessing of various materials. even minutes after the combustion event in which they were formed. A key observation is that some of the pathology resulting from smoke inhalaThese observations constitute a paration resembles the pathology associated dox that can best be resolved if it is aswith acute respiratory distress syndrome sumed that the reactive radicals actually (24),a condition known to involve radical- being spin trapped are in fact being conmediated damage to lung tissue (25,26). tinuously formed in the gas phase and are In this context, a number of studies using present in a steady state. These radical EPR spin trapping have been designed to steady states may be formed through a identify and quantitate gas-phase radicals variety of mechanisms; the particular formed by combustion or pyrolytic promechanism is obviously a function of the cesses in order to confirm their presence, material burned (or pyrolyzed) and the characterize them chemically, and thus nature of the combustion process itself. gain a better understanding of the mechaEPR spin trapping has been used to nism(s) for their toxicity. probe for the presence of gas-phase radiOne of the first applications of spin cals in the smoke produced by the comtrapping to an environmental gas-phase bustion of various natural and synthetic problem was studied by Westerberg, M- materials in an effort to understand the fli, and Sundholm (27),who detected air- toxicity of the smoke produced when

422 A Analytical Chemistry, Vol. 66, No. 7, April I, 1994

these materials burn. In particular, many deaths attributed to smoke inhalation occur several hours after exposure and cannot be explained by high levels of identifiable toxins such as carbon monoxide or hydrogen cyanide. Lowry et al. were among the first to propose that free radicals might play a role in this sort of “inexplicable”smoke toxicity (29).These workers conducted a full-scaletest burn of a furnished room and sampled the combustion gases produced in this controlled fire into a solution of the spin trap PBN. They showed that reactive free radicals were present in s u b stantial concentrations. Furthermore, they proposed that the radical concentrations reached levels that were sufficiently h g h to account for otherwise inexplicable toxicity associated with the smoke from fires of this type. The radicals produced in this experimental fire were not identified conclusively; on the basis of the published spectra of the spin adducts,they appear to result from the trapping of alkoxyl radicals. There is, however, a serious question about the interpretation of the spin adducts detected in this study. Toluene was used as the solvent to dissolve the PBN for sampling. This solvent is itself highly oxidizable,and the observed spin adducts might actually have been derived from oxidation of this solvent. Although nearly all materials burn or oxidatively degrade with heat to form gasphase radicals that can be detected by spin trapping (8,30),radical production, as measured by spin trapping, varies widely. For example, various materials were heated to a fixed temperature in a quartz combustion tube in a constant velocity air stream that carried the combustion products into a solution of PBN (8). Polyethylene and butyl rubber gave the highest concentrations of spin adducts observed under these conditions; cellulosic materials such as wood gave two to three times lower spin adduct concentrations. Other materials, such as nylon, gave nearly a 50-fold decrease in spin adducts compared with polyethylene. Some materials, such as perfluoropolymers (PFPs), did not give any detectablespin adducts. The striking variations in these spin adduct yields can be attributed to several factors. First, different materials may burn

to produce gas-phaseradicals with significantly different stabilities. Second, if different radicals are produced, there may be significant differences in their trapping rates. Third, gas-phase radicals are produced by different mechanisms during the combustion and/or pyrolysis of the different materials. Wood smoke has been thoroughly studied with respect to gas-phase radical lifetimes. The smoke from burning wood or any other cellulosic material can be aged for many minutes, either in the gas phase or after bubbling the smoke through and allowing it to dissolve in a suitable solvent. The aged gas phase can then be bubbled through a solution of spin trap, or spin trap can be added to the aged solution of wood smoke. Substantial levels of spin adducts are observed when wood smoke is aged by either method (30).Possibly the combustion of wood produces a gas phase that contains s u b

constructionmaterials (31).This toxicity is typified by massive lung damage that is consistent with what would be expected to result from oxidative damage (31).On the basis of this characterization,it has been hypothesized that the pyrolysis of PFPs results in the formation of gas-phase free radicals and that these radicals might contribute to the toxicity of PFP pyrolysis products (32). As discussed above, initial experiments in which the fumes from oxidatively pyrolyzed PFPs were spin-trapped did not result in the formation of observable spin adducts. However, subsequent work demonstrated that with sufficiently high pyrolysis temperatures, longer sampling times, and minimum gas-phase residence time (aging), the spin adducts of many radical species-including those of an organic oxy radical (alkoxyl or peroxyl) and the fluorine atom-could be observed when unfiltered smoke was b u b bled through a solution of spin trap (8).In addition, the spin trap (PBN) was oxidized to benzoyl-tert-butylnitroxide(PBNO,) . Of special note in these experiments was the observation and unambiguous characterization of a spin adduct attributable to the trapping of chlorine atoms. The source of the chlorine is not certain, although it is most likely related to the presence of chlorine-containing impuritiesin the PFPs, even though chlorine was not detectable by traditional methods for determining the elemental composition of PFP. The fact that comparable concentrations of the spin adducts of the fluorine and chlorine atoms were observed as their PBN spin adducts can be attributed to one or more of the following factors. First, the somewhat less reactive chlorine atoms may have substantially longer gasphase lifetimes than do fluorine atoms. In stantial concentrationsof exceptionally stable free radicals. However, it is unlikely addition, chlorine atoms may be trapped more efficiently than fluorine atoms. The that such stable free radicals would react chlorine atom spin adducts may also be with spin traps. It is more likely that the radicals giving rise to spin adducts in aged much more stable than fluorine atom spin adducts. Finally, the chlorine atoms may wood smoke are derived from the contibe produced during the pyrolysis by a nous decomposition of metastable nonmore efficient mechanism than fluorine radical species (30). Another smoke of particular toxicologi- atoms, which may lead to higher steadystate concentrations.This observation of cal interest is formed when PFPs are oxichlorine atom spin adducts clearly demondatively pyrolyzed. Under certain conditions, these PFP pyrolysis gases are more strates the caution necessary when using spin adduct yields to assess steady-state toxic than the gases produced by the pyrolysis or combustion of other common radical concentrations.

Wood smoke has been thoroughly studied with respect to gas-phase radical lifetimes

Analytical Chemistry, Vol. 66, No. 7, April I , 1994 423 A

Tobacco smoke contai verse radical populatio lates and the animals were ments were not reported in

between the pyrolysis chamber d the spin-trappingsolution (35). Examination of the particulate collected on the filter by EPR spectroscopy revealed a relatively stable population of end-chain peroxyl radicals (33,36,37). Although they are stable for many hours

cal species; the principal tar radical s cies has been characterize of quinone, semiquinone, an quinone functionalitiesheld in meric matrix. In marked co extremely stable tar-phase r gas phase of tobacco smok passes through the Cambrid tains radicals that are too re detected directly; however resear have used spin trapp

lkyl peroxyl species

(38).A "puff' (35mL volume) i through the burning cigarette i syringe, and the resulting gas subsequently bubbled throug of the spin trap. This protocol Bluhm, Weinstein, and Sousa

of the data reveals that the intensitie the spin adduct signals from cigar tobacco are substantially s the case for cigarettes based on si sampling volumes for the respect Menzel, Vincent, and Wasson imilar approach to trap radical hase of the smoke from r rettes (44). These workers observed sub stantial changes in the spin addu when the samples were aged; the1 suggested the presence of at least ng-lived spin adducts. This example illustrates the difficu sociated with using spin trapping fo uantitation;different spin adducts may ave different stabilities,and the relative spin adducts may chang riod of time. This can a1 limitations on the use of ve long sampling times in studies of en radicals that occur at very lo

tects radicals associ late fraction of PFP

combustion process, an between its freetion. What distingu from the gases produce tion and/or pyrolysis of and many natural maten plexity of the smoke. itself has a complex c poses the tobacco, it also results in volatilization of many of the compo the tobacco. Many thousands of com pounds have been identified in smoke, and an even larger number remain un known. Although the nature of tobacco smoke is complex, spin-trappingstudies

Figure 2. EPR spin adduct

through or (b) P owed bv the spin adducts witkbenzene

424 A Analytical Chemistry, Vol. 66, No. 7, April I , 1994

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bon-centered species (aN= 1.44m'l = 0.20 mT). On the basis of these o rved HFSCs, researchers proposed that this carbon-centered radi able to a cyclohexadienyl by the addition of radicals in the cigarette

atic solvent, as shown

ing occurs in the spectrum obtained bubbling through PBN broadening is almost c interaction of some of the nitroxide radicals. With the other protocol, the s not only serves as a support fo trap, it also helps to trap and r of the more polar compounds

that the radicals s much more convenient trapping sys especially in field studies, because neither fragile bubblers nor toxic solvents are required. changing the trapping techssible to demonstrate the echanism. F i e 2b t” observed when

silica gel with benzene; shown was obtained g benzene solution. Only als were observed in this 20 mT) and carbonaN = 1.43 mT and a H =

clear understand oteworthy aspects of

chemistry

ing comb intensity

smoke (38). Ana

1. 66, No. 7, April 1,

gas-phase free radicals, one might think that this technique would have been widely applied to air sampling in pollution studies. Aside from the report of air sampling in a plastics-fabricatingfactory reported above (27), however, there is just a single example of such an application. Watanabe et al. coated glass-fiber filters with PBN and then exposed these coated filters to the upper atmosphere (14).The filters were then washed with benzene for analysis by EPR spectroscopy, which showed the presence of the spin adduct of the hydroxyl radical. This identification was confirmed by reducing the spin adduct to the hydroxylamine, preparing the trimethylsilyl derivative, and detection with GC/MS. This experimentsuggests that spin trapping may well be useful for studying radicals in the atmosphere. However, the results raise a notable question. Why is the hydroxyl radical the only radical o b served?This highly reactive species typically occurs at concentrationsthat are

336 337 338 339 340 7eld (mT)

as Figure 4. EPR study of the gas-phase reaction of ozone and propylene. (a) PBN spin adduct spectra. (b) Time profiles for the total spin adducts (alkoxyl plus alkyl), PBNO,, and ozone. The initial concentrations of ozone and propylene were 40 and 2100 ppm, respectively.

orders of magnitude lower than those generally reported for organic speciesespecially relatively stable ones such as alkyl peroxyl radicals. This could reflect the reactivity difference between hydroxyl and peroxyl radicals toward PBN. Alternatively, the spin adducts of organic radicals may be substantially less stable than those of the hydroxyl radical. Finally, nitrones are known to be hydrolyzed to a-hydroxy-hydroxylaminesthat are airoxidized to the same nitroxide formed by spin trapping the hydroxyl radical. These experiments were not repeated, nor were adequate control experiments performed to ensure that the observed nitroxide radical was not an artifact. Unpublished model experiments demonstrate that other atmospheric processes form radicals that are detectable by spin trapping. It is known that ozone reacts with alkenes in solution to produce spin adducts that are detectable by spin trapping (46).When gaseous mixtures of ozone and propylene are allowed to react for various periods of time before the gas mixture passes into a solution of PBN, strong spin adduct spectra are observed. The HFSCs determined for the spin adducts are consistent with trapping primarily alkoxyl radicals (aN= 1.38 mT and aH= 0.20 mT), along with a lower concentrationof alkyl radical adducts (aN= 1.48 mT and aH= 0.31 mT), as well as the ubiquitous PBNO,. Figure 4a shows the PBN spin adduct spectra obtained after allowing the ozone/ propylene gas mixture to age for various lengths of time. Figure 4b plots the spintrapping data and measured ozone levels on the same time axis. Note that spin adduct production clearly builds to a maximum, substantially lagging the ozone concentration. In fact, at the point where the ozone has just become undetectable,the total spin adduct yield is - 70%of the a p parent maximum value. In addition, this plot demonstrates that the levels of the PBN oxidation product PBNO, parallel the levels of ozone, a result suggesting that PBNO, may be derived primarily from direct oxidation of the PBN by ozone. Conclusions

There appear to be three principal reasons that spin trapping has not been more

426 A Analytical Chemistry, Vol. 66, No. 7, April 7 , 1994

widely applied to environmental problems. First, the currently available spin-trapping compounds are not entirely satisfactory. Important classes of radicals such as peroxyls do not readily react with nitrones. Furthermore, the spin adducts formed by currently available traps are often quite labile. More robust traps that react readily with a wide range of radical types and that form more stable spin adducts are needed. Second, there is a lack of specific structural information provided by the EPR spectrum of the spin adduct. The marriage of HPLC to both EPR spectroscopy and MS has already begun to address this shortcomingand is likely to be used increasingly in coming years. However, currently available MS technology does not readily lend itself to the detection of nitroxide radicals at the levels of spin adducts formed in many biological or environmental systems of interest. Finally, application of spin trapping is subject to many artifacts. In spite of these limitations, spin trapping has advantagesthat can be exploited in studies of radicals in gas-phase systems. EPR spin trapping can be used effectively to study radical intermediates in complex gas-phase systems. EPR spectroscopy is highly selective, detects only paramagnetic species, and is relatively insensitive to the matrix in which the radicals occur. Spin trapping appears to be suitable for studies of radical intermediates in the atmosphere, especially when remote sampling techniques are employed. Some of the gas-phase work is the result of collaborative efforts with William A. Pryor; the author is grateful for his support and advice over the years. Some of the EPR spectra were acquired on a MicroNow EPR spectrometer purchased with funds provided by the Louisiana Education Quality Support Fund, grant number LEQSF (1991-92)-ENH-15.

References (1) Hack, W. Int. Rev. Phys. Chem. 1 9 8 5 , 4 , 165. (2) Wertz, J. E.; Bolton, J. R Electron Spin Resonance: Elementary Theory and Practical Applications; Chapman and Hall: New York, 1986; p. 345. (3) Janzen, E. G.; Blackbum, B. J. J. Am. Chem. SOC.1969,91,4481. (4) Janzen, E. G.Acc. Chem. Res. 1 9 7 1 , 4 , 31.

-(5) Rosen, G. M.; Cohen, M. S.; Britigan, B. E.; Pou, S. Free Radical Res. Commun. 1990,9,187. (6) Maeda, Y.; Ingold, K. J. Am. Chem. Soc. 1979,101,4975. (7) Triolet, J.; Raf3, J.; Agnel, J. P.; Battesti, C.; Thiery, C.; Vincent, P. Magn. Reson. Chem. 1992,30,1051. (8) Lachocki, T. M.; Nuggehalli, S. K.; Scherer, K. V.; Church, D. F.; Pryor, W. A. Chem. Res. Toxicol. 1989,2,174. (9) Albro, P. W.; Knecht, K. T.; Schroeder, J. L.; Corbett, J. T.;Marbury, D.; Collins, B. J.; Charles, J. Chem. Bid. Interact. 1992,82,73. (10) Kieber, D. J.; Johnson, C. G.; Blough, N. V. Free Radical Res. Commun. 1992, 16,35. (11) Janzen, E. G.; Krygsman, P. H.; Lindsay, D. A.; Haire, D. L. J. Am. Chem. SOC. 1990,112,8279. (12) Iwahashi, H.; Parker, C. E.; Mason, R. P.; Tomer, K. B. Anal. Chem. 1992,64, 2244. (13) Parker, C. E.; Iwahashi, H.; Tomer, K. B. J. Am. SOC. Mass Spectrom. 1991,2,413. (14) Watanabe, T.; Yoshida, M.; Fujiwara, S.; Abe, K.; Onoe, A.; Hirota, M.; Igarashi, S. Anal. Chem. 1982,54,2470. (15) Pou, S.; Hassett, D. J.; Britigan, B. E.; Cohen, M. s.;Rosen, G. M. Anal. Biochem. 1989,177,l. (16) Janzen, E. G.; Gerlock, J. L. Nature 1969, 222, 867. (17) Janzen, E. G.; Lopp, I. G.J. Phys. Chem. 1972, 76,2056. (18) Janzen, E. G.; Kasai, T.;Kuwata, K. Bull. Chem. Soc. Jpn. 1973,46,2061. (19) Janzen, E. G.; Lopp, I. G.; Morgan, T. V. J. Phys. Chem. 1 9 7 3 , 77, 139. (20) Chandra, H.; Davidson, I.M.T.; Symons, M.C.R J. Chem. Soc. Faraday Trans. 1 1983, 79,2705. (21) Migita, C. T.; Chaki, S.; Nakayama, M.; Ogura, K J. Chem. Soc. Perkin Trans. 2 1990,1965. (22) Cullis, C. F.; Hirschler, M. M. The Combustion of Organic Polymers;Oxford University Press: New York, 1981. (23) Birky, M.; Halpin, B. M.; Caplan, Y. H.; Fisher, R. S.; McAllister, J. M.; Dixon, A. M. Fire Mater. 1979,3,211. (24) Crapo, R. 0.;Nellis, N. Management of Smoke Inhalation Injuries; Intermountain Thoracic Society: Salt Lake City, UT, 1980. (25) Tate, R. M.; Repine, J. E. In Free Radicals in Biology;Pryor, W. A., Ed.; Academic Press: New York, 1984;Vol. 6; p. 199. (26) Morganroth, M. L.; Till, G. 0.;Kunkel, R. G.; Ward, P. A Lab. Invest. 1 9 8 6 , 5 4 , 507. (27) Westerberg, L. M.; Pfaffli, P.; Sundholm, F. Am. Ind. Hyg. Assoc. J. 1982,43,544. (28) Batt, L.; Milne, €3. T. Int. J. Chem. Kinet. 1976,8,59. (29) Lowry, W. R; Peterson, J.; Petty, C. S.; Badgett, J. L. J. Forensic Sci. 1985,30, 73. (30) Lachocki, T. M.; Church, D. F.; Pryor, W. A. Free Radical Biol. Med. 1989,7, 17. (31) Lee, K. P.; Zapp, J. A.; Sarver, J. W. Lab. Invest. 1976,35,152. (32) Williams, S. J.; Clarke, F. B. Fire Mater. 1983,7,96.

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(33) Seidel, W. C.; Scherer, K. V.; Cline, D.; Olson, A. H.; Bonesteel, J. K.; Church, D. F.; Nuggehalli, S.; Pryor, W. A. Chem. Res. Toxicol. 1991,4, 229. (34) Baker, B. B.; Kaiser, M. A. Anal. Chem. 1991,63,79. (35) Iachocki, T. M. Ph.D. Dissertation, I m i siana State University, 1988. (36) Metcalfe, E.; Harman, A. R. Fire Mater. 1991,15,53. (37) Pryor, W. A.; Nuggehalli, S. K.; Scherer, K. V.; Church, D. F. Chem. Res. Toxicol. 1990,3,2. (38) Church, D. F.; Pryor, W. A. Environ. Health Perspect. 1985,64,111. (39) Zhou, B.; Yan, I,.; Hou, J.; Xin, W. Chin. Med. J. (Beijing, Engl. Ed.) 1991,104, 591. (40) Pryor, W. A.; Tamura, M.; Church, D. F. J. Am. Chem. Soc. 1984,106,5073. (41) Ohkubo, Y.; Kadosima, C.; Kaneko, T.; Chuchiya, J.; Akutsu, Y.; Tamura, M.; Yoshida, T. J. Jpn. Soc. Air Pollut. 1991, 26, 171. (42) Halpern, A.; Knieper, J. NBS Spec. Publ. (US.) 1986, 716,306. (43) Bluhm, A. L.; Weinstein, J.; Sousa, J. A. Nature 1971,229,500. (44) Menzel, E. R.; Vincent, W. R.; Wasson, J. R. J. Magn. Reson. 1976,21,321. (45) Pryor, W. A.; Church, D. F.; Evans, M. D.; Rice, W. Y.; Hayes, J. R. Free Radical Biol. Med. 1990,8,275. (46) Church, D. F.; McAdams, M.; Pryor, W. A. In Oxidative Damage and Repair: Chemical, Biological and Medical Aspects; Davies, K.J.A., Ed.; Pergamon Press: New York, 1991; p. 517. (47) Haire, D. L.; Oehler, U. M.; Krygsman, P. H.; Janzen, E. G. J. Org. Chem. 1988, 53,4535. (48) Ohto, N.; Niki, E.; Kamiya, Y. J. Chem. Soc. Perkin Trans. II 1977, 1770. (49) Rehorek, D.; Winkler, W.; Wagener, R.; Hennig, H. Inorg. Chim.Acta 1981,64, L7.

Daniel F. Church earned his Ph.D. in chemistry from Oregon State University in 1975. He was a postdoctoral research associate at Louisiana State University, where he worked under the direction of William Pryor. He is currently an associate professor of chemistry at LSU (Baton Rouge, LA 70803), where his research focuses on the application of EPR spectroscopy and spintrapping techniques to study free-radical mediated processes in chemistry, environmental toxicology, and biology.

So if you typically do ground-breaking research one day and routine measurements the next, say hello to total electrochemistry. Call for information today at 1-609-530-1000. n

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