13 Development of Analytical Methodology for Assessment of Human Exposure to Pesticides 1
2
ROBERT F. MOSEMAN and EDWARD O. OSWALD
Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch013
Analytical Chemistry Branch, Environmental Toxicology Division, Health Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 Assessment of human exposure to pesticides is important for a variety of reasons. In the occupational situation, it is necessary to know the amount of pesticide that an individual is exposed to in order to protect worker health. Formulators, loaders, pickers and pilots can experience high exposures to pesticides. Humans can be exposed to pesticides through environmental routes. The air we breath and the water we drink are but two sources of environmental exposure. Measurement of human exposure can be done either directly or indirectly. Direct measurement involves determination of the pesticide level in the media through which the exposure occurs. Examples of this are measurement of pesticides in breathing zone air or pesticides adsorbed onto pads or clothing worn by workers (1,2). These techniques provide a direct and calculable measure of human exposure under actual conditions. Most often, however, direct measurement is not possible. In these situations indirect methods of exposure assessment must be used. In the complex process o f development of a n a l y t i c a l methodology f o r the i n d i r e c t assessment of exposure t o p e s t i c i d e s , one of the f i r s t questions t o be addressed, concerns what compound(s) to look f o r and i n what sample type. In most cases the parent p e s t i c i d e w i l l be transformed t o a more p o l a r metabolite. Exposure t o organophosphate p e s t i c i d e s i s o f t e n measured by determination of a l k y l phosphate or phenol metabolites i n the u r i n e . Determination of blood c h o l i n e s t e r a s e a c t i v i t y can be a v a l u a b l e i n d i c a t o r of exposure i f pre-exposure c h o l i n e s t e r a s e a c t i v i t y i s known (_3, 4·, 5). Since normal c h o l i n e s t e r a s e l e v e l s vary over a f a i r l y wide range, post-exposure measurements alone do not always provide u s e f u l information. For the most p a r t , measurement o f u r i n a r y metabolites can provide p o s i t i v e informat i o n without pre-exposure l e v e l s . In a d d i t i o n , a l k y l phosphate Current address: a n a l y t i c a l Chemistry D i v i s i o n ; Radian Corporation; P.O. Box 9948; A u s t i n , Texas 78766 Department of Environmental Health Sciences; School of P u b l i c Health; U n i v e r s i t y o f South C a r o l i n a ; Columbia, S.C. 29208 2
This chapter not subject to U.S. copyright. Published 1980 American Chemical Society Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch013
252
PESTICIDE A N A L Y T I C A L
METHODOLOGY
l e v e l s i n the u r i n e may i n d i c a t e that exposure has occured even though no depression of c h o l i n e s t e r a s e can be detected. As a group, a l k y l phosphate metabolites serve as an i n d i c a t o r of exposure to one or s e v e r a l organophosphate p e s t i c i d e s . Since s e v e r a l d i f f e r e n t p e s t i c i d e s can give r i s e to the same u r i n a r y a l k y l phosphate, some s p e c i f i c i t y may be l o s t . Measurement of u r i n a r y phenols can provide a more p o s i t i v e i n d i c a t i o n of which p e s t i c i d e an i n d i v i d u a l was exposed t o . C e r t a i n halogenated organophosphates such as c h l o r p y r i f o s and ronnel may p o s s i b l y be found i n adipose t i s s u e of h e a v i l y exposed i n d i v i d u a l s . In g e n e r a l , measurement of a l k y l phosphates can serve as a b e t t e r screening technique than measurement of p h e n o l i c metabolites simply because the great m a j o r i t y of organophosphates give r i s e to only about s i x a l k y l phosphate compounds. On the other hand, almost a l l of the organophosphates y i e l d a d i f f e r e n t phenol, making a t r u l y comprehensive multi-phenol a n a l y t i c a l procedure an almost imposs i b l e undertaking. Assessment of exposure to carbamate p e s t i c i d e s can be a complex matter f o r a v a r i e t y of reasons. Measurement of c h o l i n esterase depression can be d i f f i c u l t i n the case of carbamates. Not only i s the methodology n o n - s p e c i f i c , but the determination i t s e l f can be questionable because of the i n s t a b i l i t y of the enzyme-carbamate complex. As with c h o l i n e s t e r a s e determinations f o r organophosphates, the method can be h i g h l y v a r i a b l e even w i t h i n the same l a b o r a t o r y . D i r e c t determination of i n t a c t carbamate p e s t i c i d e s by s e n s i t i v e and s p e c i f i c gas chromatographic procedures can be done. H i s t o r i c a l l y , these procedures employed a d e r i v a t i z a t i o n step to render the carbamates amenable to gas chromatography. Recent developments i n column technology have allowed f o r the gas chromatography of some i n t a c t carbamates at nanogram and subnanogram s e n s i t i v i t y (6, _7, 8). Examination of t i s s u e s and e x c r e t a from humans or animals f o r exposure to carbamate p e s t i c i d e s , w i l l almost never r e s u l t i n d e t e c t i o n of the parent compound. Exposure assessment of t h i s nature r e q u i r e s determination of m e t a b o l i t i c products, except i n extreme s i t u a t i o n s such as acute poisoning. The most widely used i n d i c a t o r of exposure i s probably the determination of u r i n a r y phenols (9). T i s s u e s such as f a t , blood or l i v e r can be examined f o r r e s i dues of the more s t a b l e c h l o r i n a t e d hydrocarbon p e s t i c i d e s . In most cases these t i s s u e s are a v a i l a b l e as a r e s u l t of e l e c t i v e survery, autopsy or biopsy. Exposure to DDT r e s u l t s i n some storage of the parent compound i n body f a t . A l a r g e p o r t i o n , however, i s metabolized and s t o r e d as DDE (9), A l d r i n and heptac h l o r are s i m i l a r l y transformed and s t o r e d as d i e l d r i n and heptac h l o r epoxide. L e v e l s of the u r i n a r y m e t a b o l i t e DDA have been used to assess exposure or body burden of DDT (10, 11, 12). Hexachlorobenzene and the v a r i o u s isomers of h e x a c h l o r o c y c l o hexane are s t o r e d i n f a t as the parent compound but a small
Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch013
13.
M O S E M A N AND OSWALD
Human
Exposure
to
Pesticides
253
percentage i s excreted i n the u r i n e as c h l o r i n a t e d phenols (13,14). Recent work i n our l a b o r a t o r y has i n d i c a t e d that a s i g n i f i c a n t l y greater amount of u r i n a r y phenols and a n i l i n e s can be recovered i f the sample i s hydrolyzed with a c i d p r i o r to e x t r a c t i o n . Appare n t l y many phenols and other p o l a r compounds are found as conjugates i n l i v e r and u r i n e . F a i l u r e to f r e e these compounds before e x t r a c t i o n can l e a d to erroneously low r e s u l t s . In the u r i n e of experimental animals, N-dealkyl metabolites of t r i a z i n e h e r b i c i d e s have been i d e n t i f i e d (15,16). Methodology f o r the determination of these kinds of compounds i s c u r r e n t l y a v a i l a b l e (15) and should be a p p l i c a b l e f o r monitoring human population. S u b s t i t u t e d urea h e r b i c i d e s are a widely used group of compounds. U r i n a r y metabolites of these compounds are halogenated a n i l i n e s . Recent research has lead to the development of analyt i c a l methodology f o r t r a c e l e v e l s of these compounds i n the u r i n e of experimental animals (17). The next l o g i c a l step i n t h i s sequence i s a p p l i c a t i o n to human exposure assessment. A f t e r i s has been determined which compounds are of importance, they must be a v a i l a b l e f o r use as a n a l y t i c a l standards. In the absence of a r e l i a b l e standard, q u a l i t a t i v e and q u a n t i t a t i v e data cannot be obtained. In cases where standards are not a v a i l able commercially, we have s y n t h e s i z e d our own. I f only the t e c h n i c a l m a t e r i a l i s a v a i l a b l e , i t may be p u r i f i e d to a n a l y t i c a l q u a l i t y . A very important part of the e n t i r e q u a l i t y assurance p o r t i o n of methods development i s accurate and r e l i a b l e standards. The e f f i c i e n c y of an e x t r a c t i o n procedure must be determined i n i t i a l l y by f o r t i f y i n g blank ( c o n t r o l ) samples. The f o r t i f i c a t i o n should be done over a range to i n c l u d e the l e v e l s expected i n r e a l samples. I f the u l t i m a t e s e n s i t i v i t y d e s i r e d i s 0.005 ppm, i t does l i t t l e good to e s t a b l i s h recovery data at 0.5 ppm. P l o t t i n g recovery data versus f o r t i f i c a t i o n l e v e l on a l o g - l o g s c a l e prov i d e s a good p i c t u r e of the e x t r a c t i o n e f f i c i e n c y . S e l e c t i n g f o r t i f i c a t i o n l e v e l s of 0.01, 0.03, 0.10 e t c . provides even spacing f o r data p o i n t s and allows coverage of a wide range with a minimum of samples. Quadruplicate runs at each l e v e l provides s u f f i c i e n t data f o r c a l c u l a t i n g the mean and standard d e v i a t i o n . A very important c o n s i d e r a t i o n i n methods development research concerns the vast d i f f e r e n c e between recovery of compounds added to a sample i n the l a b o r a t o r y and recovery of b i o l o g i c a l l y i n c o r porated compounds. For the most part "grown i n " residues can be much more d i f f i c u l t to e x t r a c t from sample matrices. Chemical or enzymatic h y d r o l y s i s i s o f t e n r e q u i r e d to f r e e bound or conjugated r e s i d u e s . Information on recovery of b i o l o g i c a l l y incorpor a t e d residues i s best obtained with the use of r a d i o l a b e l l e d compounds. In the absence of tagged compounds, q u a n t i t a t i o n of e x t r a c t e d r e s i d u e s both before and a f t e r h y d r o l y s i s can y i e l d v a l u a b l e information. Exhaustive e x t r a c t i o n of a sample matrix using d i f f e r e n t s o l v e n t s can give an i n d i c a t i o n of how vigorous the e x t r a c t i o n must be to remove a l l the r e s i d u e present. One
Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch013
254
PESTICIDE
ANALYTICAL
METHODOLOGY
must keep i n mind, however, that the more p o l a r e x t r a c t i o n s o l vents can l e a d to more d i f f i c u l t cleanup and s e p a r a t i o n problems. I d e a l l y , the s o l v e n t chosen w i l l e f f i c i e n t l y e x t r a c t the compounds of i n t e r e s t with a minimum of i n t e r f e r e n c e s . When d e a l i n g with human t i s s u e s , experimental dosing or feeding i s not p o s s i b l e . Determination of p e s t i c i d e s i n human samples taken from i n d i v i d u a l s poisoned or occupâtionally exposed can provide i n f o r m a t i o n u s e f u l i n development of a n a l y t i c a l methodology. These types of samples may contain b i o l o g i c a l l y incorporated p e s t i c i d e s and m e t a b o l i t e s . I f human t i s s u e samples c o n t a i n i n g the p e s t i c i d e s of i n t e r e s t are not a v a i l a b l e , the researcher must r e l y on animal models f o r e s t a b l i s h i n g recovery data f o r p e s t i c i d e s and m e t a b o l i t e s . The choice of the determinative step to be used i n an a n a l y t i c a l procedure i s d i c t a t e d by the nature of the r e s i d u e . The non-polar c h l o r i n a t e d hydrocarbon p e s t i c i d e s are r o u t i n e l y q u a n t i f i e d using gas chromatography (GC) and e l e c t r o n capture(EC) detection. Alternate detectors include e l e c t r o l y t i c conductivity and microcoulometric systems. Organophosphate p e s t i c i d e s which are amenable to GC are responsive to e i t h e r the flame photometric detector (FPD) or the a l k a l i flame detector (AFD). S u l f u r cont a i n i n g compounds respond i n the e l e c t r o l y t i c c o n d u c t i v i t y or flame photometric d e t e c t o r s . Nitrogen c o n t a i n i n g p e s t i c i d e s or metabolites are g e n e r a l l y detected with a l k a l i flame or e l e c t r o l y t i c conductivity detectors. P o l a r compounds or compounds which decompose at elevated temperatures i n gas chromatography, can o f t e n be converted to more s t a b l e d e r i v a t i v e s p r i o r to gas chromatography. P r e p a r a t i o n of d e r i v a t i v e s may a l s o add heteroelements and render c e r t a i n compounds more responsive to element s e l e c t i v e d e t e c t o r s . T h i s l a t t e r technique can sometimes ease cleanup problems by e f f e c t i v e l y reducing the background against which one must q u a n t i f y . S i m i l a r techniques are u s e f u l f o r c o n f i r m a t i o n of r e s u l t s obtained by other methods. The f a s t e s t growing instrumental technique f o r the d e t e r mination of t r a c e organics and p e s t i c i d e s i s high performance l i q u i d chromatography (HPLC). Many compounds which cannot be handled by gas chromatography are e a s i l y chromatographed and separated by HPLC (18, _19, _20, 21). Since ambient temperatures are the r u l e , h e a t - l a b i l e compounds present no p a r t i c u l a r problems. Very p o l a r compounds are subject to chromatography without d e r i v a t i z a t i o n . A wide v a r i e t y of column m a t e r i a l s are a v a i l a b l e as w e l l as reagents f o r p a i r e d i o n chromatography. I s o c r a t i c or s o l v e n t programming may be e f f e c t i v e l y used f o r the d e s i r e d separation. For s e v e r a l years LC detectors were l i m i t e d to r e f r a c t i v e index and u l t r a v i o l e t a b s o r p t i o n systems. Recently introduced systems i n c l u d e the e l e c t r o c h e m i c a l d e t e c t o r and a moving b e l t i n t e r f a c e a l l o w i n g f o r chemical ionization-mass spectrometric d e t e c t i o n . Both of these techniques provide a degree of s e l e c t i v i t y not p r e v i o u s l y a v a i l a b l e .
Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch013
13.
M O S E M A N A N D OSWALD
Human Exposure
to Pesticides
255
L i q u i d chromatography has been e f f e c t i v e l y used i n the residue l a b o r a t o r y f o r cleanup of sample e x t r a c t s p r i o r to the determinative step. T h i s added dimension has allowed f o r detect i o n and c o n f i r m a t i o n of t r a c e substances at much lower l e v e l s than was p o s s i b l e a few years ago. A n a l y t i c a l methodology used f o r the assessment of human exposure to p e s t i c i d e s should always i n c l u d e some form of conf i r m a t i o n analyses. R e l i a n c e on chromatographic peaks generated by a s i n g l e d e t e c t o r i s an open i n v i t a t i o n to c r i t i c i s m . R e s u l t s of analyses i n a monitoring program should be confirmed at a r a t e of 10 to 20 percent. The confirmâtional procedure should provide f o r measurement of the r e s i d u e by some independent physiochemical means. I f the primary a n a l y t i c a l technique i s e l e c t r o n capture gas chromatography, the c o n f i r m a t i o n should be done, at the bare minimum, w i t h gas chromatography and a s e l e c t i v e detector such as e l e c t r o l y t i c c o n d u c t i v i t y . D e r i v a t i z a t i o n of the r e s i d u e i n a sample e x t r a c t and subsequent chromatographic a n a l y s i s provides a f a i r l y r e l i a b l e confirmâtional technique. T h i n - l a y e r and high performance l i q u i d chromatography are good techniques to use i n c o n j u n c t i o n w i t h gas chromatography. I n f r a red or mass s p e c t r o m e t r i c data can provide s o l i d evidence f o r c o n f i r m a t i o n of r e s u l t s . Some p r a c t i c a l a p p l i c a t i o n s of the concepts d i s c u s s e d e a r l i e r have r e c e n t l y been encountered i n our l a b o r a t o r y . During the i n i t i a l stages of the development of a m u l t i - r e s i d u e procedure f o r c h l o r i n a t e d phenols (14), s e v e r a l problems were encountered. At the o u t s e t , we found that h i g h l y p u r i f i e d a n a l y t i c a l standards were not e a s i l y a v a i l a b l e from commercial sources. Some of the compounds were obtained as t e c h n i c a l grade. These were p u r i f i e d by r e c r y s t a l l i z a t i o n . Others d i d not y i e l d good recovery even when c a r r i e d through the a n a l y t i c a l procedure without the i n v o l v e ment of a sample matrix. T h i s l a t t e r problem was t r a c e d to l i g h t and oxygen s e n s i t i v i t y of p e n t a c h l o r o t h i o p h e n o l , t e t r a c h l o r o h y d r o quinone and t e t r a c h l o r o p y r o c a t e c h o l . Decomposition of these compounds was noted during the e x t r a c t i o n and cleanup steps and was circumvented by the a d d i t i o n of sodium b i s u l f i t e and wrapping the glassware i n aluminum f o i l . Once the phenols were methylated, l i g h t s e n s i t i v i t y was no longer a problem and the samples could be handled i n a normal manner. Another major problem i n t h i s work concerned the gas chromatographic overlap of s e v e r a l compounds. The use of a l t e r n a t e columns was i n s u f f i c i e n t f o r complete r e s o l u t i o n of a l l the phenols. An alumina column cleanup and s e p a r a t i o n technique was devised which overcame most of the problem. At the present i t i s known that r e p e t i t i v e thawing and r e f r e e z i n g of u r i n e samples r e s u l t s i n disappearance of phenols C?±). e r to e s t a b l i s h the true l e v e l of u r i n a r y phenols, i t i s important to perform the a n a l y s i s as soon as p o s s i b l e a f t e r the sample i s c o l l e c t e d or to f r e e z e the sample immediately and keep i t f r o z e n u n t i l a n a l y s i s . I
n
o r Q l
Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
PESTICIDE
Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch013
256
ANALYTICAL
METHODOLOGY
Many of the a n a l y t i c a l procedures f o r pentachlorophenol i n the recent l i t e r a t u r e do not c a l l f o r a h y d r o l y s i s step p r i o r t o e x t r a c t i o n of a u r i n e sample. During the course of our research i n the development o f a r e l i a b l e m u l t i - r e s i d u e procedure f o r c h l o r i n a t e d phenols, we found that much more pentachlorophenol could be e x t r a c t e d from the u r i n e i f the sample was hydrolyzed with h y d r o c h l o r i c a c i d (23). A s i m i l a r f i n d i n g was noted during the development of anal y t i c a l methodology f o r the determination of u r i n a r y c h l o r i n a t e d a n i l i n e s . I t was found that much more 3 , 4 - d i c h l o r o a n i l i n e could be e x t r a c t e d from the u r i n e of r a t s f e d diuron and l i n u r o n i f an a c i d h y d r o l y s i s step was used p r i o r t o e x t r a c t i o n (24). When using h y d r o l y s i s i n an a n a l y t i c a l procedure, the chemist must determine i f the compound o f i n t e r e s t i s s t a b l e to the c o n d i t i o n s used. During the development o f a procedure f o r the determination of N-dealkylated t r i a z i n e h e r b i c i d e , i t was found that h y d r o l y s i s of u r i n e samples was unnecessary f o r maximum recovery of these compounds (15). In summary, and at the r i s k of r e p e t i t i o n , i t must be s t r e s s e d that the development o f a n a l y t i c a l methodology f o r the assessment of human exposure t o p e s t i c i d e s i s a complex process. C a r e f u l a t t e n t i o n t o planning of the research i s of utmost importance. As much i n f o r m a t i o n as p o s s i b l e about t r a n s f o r m a t i o n , storage and e x c r e t i o n of the p e s t i c i d e s o f i n t e r e s t should be gathered. P r e l i m i n a r y work should focus on the a n a l y t i c a l behavior of parent compounds and m e t a b o l i t e s . The combination of these aspects with r e l i a b l e a n a l y t i c a l standards and a sound q u a l i t y assurance program should y i e l d v a l i d a n a l y t i c a l methodology.
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.
10.
Wolfe, H.R.; Durham, W.F.; and Armstrong, J.F. Arch. Environ. Health, 14, 622, (1967). Durham, W.F.; and Wolfe, H.R. Bull. World Health Organ., 26, 75, (1962). Morgan, D.P., Editor "Recognition and Management of Pesticide Poisonings", U.S.Environmental Protection Agency, Washington, D.C.,(August, 1976). Ganelin, R.W.; Arizona Medicine, (October, 1964). Hayes, W.J., Arch. Environ. Health, 3, 49, (1961). Lorah, E.J.; Hemphill, D.D., JAOAC, 57, 570, (1974). Moseman, R.F., J. Chromatogr., 166, 397, (1978). Hall, R.C.; and Harris, D.E., J. Chromatogr., 169, 245, (1979). Thompson, J.F., Editor "Manual of Analytical Methods for the Analysis of Pesticide Residues in Human and Environmental Samples", U.S.Environmental Protection Agency, Research Triangle Park, N.C., (June, 1977). Cueto,C.; Barnes,A.G.; and Mettson, A.M., J.Agr. and Food Chem. 4, 943, (1956).
Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
13. MOSEMAN AND
OSWALD
Human Exposure to Pesticides
Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 30, 1980 | doi: 10.1021/bk-1980-0136.ch013
11.
257
Durham, W.F.; Armstrong, J.S.; and Quinby, G.E., Arch. Environ. Health, 11, 76, (1965). 12. Cranmer, M.F.; Carroll, J.J.; and Copeland, M.F., Bull. Environ. Contamin. Toxicol., 4, 214, (1969). 13. Chadwick, R.W.; Freal, J.J. Bull. Environ. Contam. Toxicol., 7, 137, (1972). 14. Edgerton, T.R.; Moseman, R.F.; Linder, R.E.; and Wright, L.H., J. Chromatogr., 170, 331, (1979). 15. Bradway, D.E.; Moseman, R.F., Presented at the 176th National ACS Meeting, PEST #50, Miami, Florida, Sept. 1978. 16. Erickson, M.D.; Frank, L.W.; and Morgan, D.P., J. Agr. and Food Chem., 24, 743, (1979). 17. Lores, E.M. : Bristol, D.W.; and Moseman, R.F., J. Chrom. Sci., 16, 358, (1978). 18. Horgan, D.F.Jr., Chapter 2 in "Analytical Methods for Pesticides and Plant Growth Regulators", Vol. VII, Sherma, J., and Zweig, G., Editors, Academic Press, New York, (1973). 19. Moye, H.A., J. Chromatogr. Sci., 13, 268, (1975). 20. Frei, R.W.; and Lawrence, J.F., J . Chromatogr., 83, 321, (1973). 21. Self, C.; McKerrell, E.H.; and Webber; T.J.N., Proc. Anal. Div. Chem. Soc., 12, 388, (1975). 22. Edgerton, T.R., Personal communication, August, 1979. 23. Edgerton, T.R.: and Moseman, R.F., J. Agr. and Food Chem., 27, 197, (1979). 24. Lores, E.M.; Meekins, R.W.; and Moseman, R.F., submitted to J. Chromatogr., 1979. RECEIVED
February 21, 1980.
Harvey et al.; Pesticide Analytical Methodology ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
