Analytical Methods Used To Identify Nonmethane Organic

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Nonmethane Organic Compounds in Ambient Atmospheres H a l Westberg and Pat Z i m m e r m a n 1

2

Laboratory for Atmospheric Research, Washington State University, Pullman, WA 99164 Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307

1

2

Nonmethane hydrocarbons are important participants in the atmospheric chemical reactions that cause photochemical smog, acid deposition, and greenhouse gases. In order to understand their involvement in these processes, researchers are using sophisticated air-quality simulation models. In most cases, these models require individual species information. Consequently, considerable effort is currently being directed toward the development and validation of analytical methods for measuring the concentration of nonmethane organic species in various ambient environments. Analytical methods currently used to define ambient concentrations of hydrocarbons and their oxygenated derivatives (carbonyls and organic acids) are summarized. This review emphasizes strengths and weaknesses of the various analytical techniques and shows where new or improved methods are needed.

THE METHODOLOGIES USED

to d e t e r m i n e the c o n c e n t r a t i o n of n o n m e t h a n e organic c o m p o u n d s ( N M O C s ) i n a m b i e n t atmospheres are r e v i e w e d i n this c h a p t e r . T h e s y m p o s i u m from w h i c h this chapter e v o l v e d was d e s i g n e d to p r o v i d e analytical chemists w h o are not d i r e c t l y i n v o l v e d i n atm o s p h e r i c m e a s u r e m e n t s w i t h a b r i e f s u m m a r y o f m e a s u r e m e n t technologies c u r r e n t l y u s e d for d e t e r m i n i n g n o n m e t h a n e organic c o m p o u n d s , w i t h the hope that interest i n d e v e l o p i n g i m p r o v e d m e a s u r e m e n t m e t h o d s c o u l d b e 0065-2393/93/0232-0275$06.00/0 © 1993 American Chemical Society

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

276

MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

generated. T h e volatile organic c o m p o u n d s of interest i n c l u d e t r u e h y d r o carbons (i.e., t h e y c o n t a i n C a n d H only) as w e l l as various families of oxygenated h y d r o c a r b o n s . O v e r the past 10 to 15 years, the t r u e h y d r o c a r bons have r e c e i v e d the most attention a n d c o n s e q u e n t l y are best charac t e r i z e d . O f the oxygenated h y d r o c a r b o n s , those c o n t a i n i n g the a l c o h o l f u n c t i o n a l g r o u p are least u n d e r s t o o d . C a r b o n y l s a n d organic acids have attracted c o n s i d e r a b l e interest i n recent years; the result is a g r o w i n g data base of c o n c e n t r a t i o n i n f o r m a t i o n i n various a m b i e n t atmospheres. V o l a t i l e organic c o m p o u n d s are i m p o r t a n t a t m o s p h e r i c constituents f r o m b o t h a c h e m i c a l a n d b i o l o g i c a l standpoint. B i o l o g i c a l l y , they serve as (1) a c a r b o n source for t e r r e s t r i a l Microorganisms, (2) p l a n t h o r m o n e s (e.g., e t h ylene), (3) p h e r o m o n e s ,

andV(4) a c o n t r i b u t i n g factor i n c o n t r o l l i n g p l a n t

selection a n d g r a z i n g pressure t h r o u g h vegetation palatability. H y d r o c a r b o n s play an i m p o r t a n t role i n t r o p o s p h e r i c c h e m i s t r y . F i g u r e 1 s u m m a r i z e s some

ΐ/RBAN ATMOSPHERE PHOTOCHEMICAL

SMOG

N M O C + NOx+SUNLIGHT

•

SMOG

REGIONAL ATMOSPHERE PHOTOCHEMICAL

SMOG

ACID DEPOSITION N M O C +OXIDANT

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PEROXY COMPOUNDS •

H2SO4 AND HNO3

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ORGANIC ACIDS

P E R O X Y COMPD'S + SO2/NO2/O2

GLOBAL ATMOSPHERE GLOBAL WARMING N M O C +OXIDANT

•

N M O C + NOx + SUNLIGHT Figure 1. The role of hydrocarbons

CO + C 0 •

2

OZONE

in atmospheric

chemistry.


10.

WESTBERG & ZIMMERMAN

Nonmethane Organic

Compounds

277

of the significant a t m o s p h e r i c functions o f n o n m e t h a n e organic species. H y drocarbons are o n e o f the p r i m a r y i n g r e d i e n t s i n the c h e m i c a l process that produces s m o g o n the u r b a n a n d r e g i o n a l scale. O r g a n i c acids c o n t r i b u t e to the l o w e r i n g o f p H i n acidic d e p o s i t i o n processes. H y d r o c a r b o n s i n f l u e n c e a t m o s p h e r i c acidity because products o f t h e i r o x i d a t i o n , such as peroxy r a d icals, facilitate t h e oxidation o f sulfur a n d n i t r o g e n oxides to s u l f u r i c a n d n i t r i c a c i d . O n a global scale, h y d r o c a r b o n o x i d a t i o n leads to p r o d u c t s s u c h as C 0 a n d 0 , w h i c h absorb o u t g o i n g I R r a d i a t i o n a n d thus can c o n t r i b u t e to c l i m a t e w a r m i n g . C a r b o n m o n o x i d e , w h i c h is a p r o d u c t o f h y d r o c a r b o n oxidation, is not a p r i m a r y greenhouse gas; h o w e v e r , i t can affect c l i m a t e change i n d i r e c t l y t h r o u g h its reaction w i t h a t m o s p h e r i c h y d r o x y l radical. Increases i n C O w i l l r e d u c e O H levels, w h i c h i n t u r n w i l l l e a d to an increase i n a t m o s p h e r i c m e t h a n e concentrations, because O H is t h e major sink for m e t h a n e . M e t h a n e is o n e o f the m o r e i m p o r t a n t greenhouse gases i n t h e troposphere. 2

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I n o r d e r to b e t t e r u n d e r s t a n d n o n m e t h a n e h y d r o c a r b o n i n v o l v e m e n t i n the processes o u t l i n e d i n F i g u r e 1, researchers are u s i n g sophisticated a i r q u a l i t y s i m u l a t i o n m o d e l s . T h e s e models u s u a l l y r e q u i r e i n d i v i d u a l species information. C o n s e q u e n t l y , considerable effort is c u r r e n t l y b e i n g d i r e c t e d t o w a r d the d e v e l o p m e n t a n d v a l i d a t i o n o f analytical methods for m e a s u r i n g the concentrations o f vapor-phase organic c o m p o u n d s i n various e n v i r o n ments.

Determination of Atmospheric Concentrations A l l o f the methods u s e d to d e t e r m i n e a t m o s p h e r i c h y d r o c a r b o n c o n c e n t r a tions i n c l u d e the three distinct steps o f • collection, • speciation, a n d • detection. T h e t h r e e phases are o f r o u g h l y e q u a l i m p o r t a n c e . I f sample i n t e g r i t y is n o t m a i n t a i n e d d u r i n g c o l l e c t i o n , t h e result w i l l not reflect t r u e a m b i e n t c o n ditions. Separation o f the air matrix into i n d i v i d u a l c o m p o n e n t s is c l e a r l y a r e q u i r e m e n t for m e a n i n g f u l analysis, a n d w i t h o u t sensitive a n d p r e c i s e d e tectors, quantitation is not possible. Collection. P r o c e d u r e s c o m m o n l y u s e d to collect vapor-phase organic c o m p o u n d s i n c l u d e w h o l e - a i r , cryogenic, a d s o r p t i o n , a b s o r p t i o n , a n d d e rivatization methods. W h o l e - a i r s a m p l i n g involves the capture o f an air p a r c e l i n a container. Stainless steel canisters o r plastic bags c o n s t r u c t e d f r o m a n i n e r t m a t e r i a l , such as Teflon o r T e d l a r , are most c o m m o n l y u s e d . E a c h o f


278


these container types has advantages a n d disadvantages. R i g i d m e t a l c a n isters are easier to c l e a n , less p r o n e to leakage, a n d b e t t e r for s h i p p i n g samples f r o m f i e l d sites to an analytical laboratory. H o w e v e r , canisters are expensive. I n a d d i t i o n , because of t h e i r rigid s t r u c t u r e , cans are not as useful as bags for the c o l l e c t i o n of t i m e - i n t e g r a t e d samples. I n several r e c e n t field studies, a c o m b i n a t i o n of bags a n d canisters was u s e d . F o r e x a m p l e , Teflon bags, because of t h e i r m i n i m a l w e i g h t , can b e attached to a t e t h e r e d b a l l o o n l i n e a n d f i l l e d at various altitudes i n o r d e r to define v e r t i c a l h y d r o c a r b o n profiles. C o n t e n t s of the bags are t h e n transferred to stainless steel canisters for storage a n d s h i p m e n t to the laboratory. I f storage t i m e s are short, a l l of the true hydrocarbons present i n a m b i e n t air can be r e c o v e r e d at h i g h efficiency f r o m canisters a n d bags. T h i s is not t r u e , h o w e v e r , for the m o r e polar oxygenated hydrocarbons. O r g a n i c acids a n d alcohols are r e c o v e r e d at v e r y l o w efficiencies; carbonyls are i n t e r m e d i a t e b e t w e e n these c o m p o u n d s and the hydrocarbons. C a n i s t e r losses appear to b e d u e to w a l l losses a n d not c h e m i c a l degradation. T h e usual p r o c e d u r e for filling canisters involves flushing the container for a few m i n u t e s a n d t h e n c l o s i n g the d o w n s t r e a m valve a n d p r e s s u r i z i n g to about 2 a t m (200 kPa). Passage of the airstream t h r o u g h a p u m p removes most of the ozone, a n d w h a t remains is q u i c k l y d e s t r o y e d o n contact w i t h the container walls. H u m i d i t y i n the canister reduces N M O C adsorptive losses (I). A p p a r e n t l y w a t e r occupies active w a l l sites m o r e efficiently than N M O C s . L a b o r a t o r y studies d e s i g n e d to quantify storage lifetimes o f N M O C s i n canisters m u s t i n c l u d e the a d d i t i o n o f w a t e r i n o r d e r to simulate r e a l atmosphere conditions. T h e i n c l u s i o n of h u m i d i f i e d d i l u t i o n air is essential w h e n N M O C standards are p r e p a r e d i n stainless steel canisters. C r y o g e n i c collection utilizes a glass, T e f l o n , o r stainless steel trap that is c o o l e d to s u b a m b i e n t temperatures. A l i q u i d argon o r l i q u i d oxygen c o o l e d trap (-185 °C) w i l l q u a n t i t a t i v e l y r e t a i n a l l o f the n o n m e t h a n e organic c o m p o u n d s b u t p e r m i t passage of n i t r o g e n a n d most of the oxygen i n a m b i e n t air. T h i s separation is i m p o r t a n t because a b i g p l u g o f n i t r o g e n or oxygen flushed onto a c a p i l l a r y c o l u m n w i l l d i s r u p t the i n i t i a l p o r t i o n of the c h r o m a t o g r a m . T h i s is a good collection m e t h o d for h y d r o c a r b o n s , i n c l u d i n g m a n y of the oxygenates. H o w e v e r , ozone a n d w a t e r v a p o r i n a m b i e n t air can cause p r o b l e m s . O z o n e w i l l be concentrated i n the c r y o g e n i c t r a p , a n d w h e n the l o o p is w a r m e d to transfer its contents to the gas c h r o m a t o g r a p h , ozonolysis reactions w i l l occur. T h u s , i f olefins are to b e m e a s u r e d , the ozone m u s t be r e m o v e d f r o m the airstream before the sample enters the trap. A short p r e c o l u m n of potassium carbonate or s o d i u m sulfite w i l l scavenge the ozone w i t h o u t affecting most of the organic c o m p o u n d s . Ice w i l l restrict the a m o u n t of air that can be passed t h r o u g h the cryogenic trap. T h e most c o m m o n sorbent u s e d to collect organic c o m p o u n d s i n air is Tenax. It has the d e s i r e d p r o p e r t y of not r e t a i n i n g significant amounts of w a t e r , a n d the organic c o m p o u n d s that are adsorbed can be e l u t e d b y heating. T h e m a i n advantage of the adsorbent m e t h o d is that large v o l u m e s of


10.


Nonmethane Organic

Compounds

279

a m b i e n t air can be processed; these large samples l e a d to v e r y l o w d e t e c t i o n levels for m a n y organic c o m p o u n d s . C a r e m u s t be exercised, h o w e v e r , w h e n i n t e r p r e t i n g data a c q u i r e d w i t h Tenax or o t h e r adsorbents. C o l l e c t i o n a n d d e s o r p t i o n efficiencies must be established for each organic species. F o r example, the b r e a k t h r o u g h v o l u m e is a few h u n d r e d c u b i c c e n t i m e t e r s or less for h i g h l y volatile hydrocarbons (fewer t h a n six carbons) w h e n Tenax is used. O t h e r sorbents have b e e n u s e d that w i l l r e t a i n the l o w e r m o l e c u l a r w e i g h t organic c o m p o u n d s , b u t w i t h m a n y of these, quantitative r e c o v e r y of the h i g h e r m o l e c u l a r w e i g h t species is not possible. A single adsorbent that provides an acceptable m e d i u m for c o l l e c t i n g h y d r o c a r b o n s o v e r the e n t i r e C - C range is not yet available. S o l i d adsorbents w i l l p r o d u c e some b l e e d peaks d u r i n g the heat d e s o r p t i o n process. T h e r e f o r e , " b l a n k " analyses must b e p e r f o r m e d o n a regular basis, a n d these m u s t b e i n t e g r a t e d into the final data i n t e r p r e t a t i o n . T h e adsorbent m e t h o d o l o g y is g o o d for i n d i v i d u a l species i f q u a l i t y assurance studies show that interferences are absent. 2

1 2

D e r i v a t i z a t i o n is c o m m o n l y u s e d to collect polar, oxygenated h y d r o c a r bons. M o s t carbonyls can be extracted from an air sample b y passage t h r o u g h a m e d i u m containing 2,4-dinitrophenylhydrazine ( D N P H ) . T h e usual m e t h odology involves coating a chromatographic m a t e r i a l s u c h as silica g e l , F l o r i s i l , or C w i t h an acidic solution of the D N P H . W h e n a m b i e n t air c o n t a i n i n g carbonyls is passed t h r o u g h the d e r i v a t i z i n g m e d i u m , hydrazones are f o r m e d . T h e hydrazones can b e e l u t e d w i t h a n a p p r o p r i a t e organic solvent and q u a n t i f i e d b y c o n v e n t i o n a l analytical methods. O r g a n i c acids can b e c o l l e c t e d as salts u p o n passage of an airstream t h r o u g h a filter i m p r e g n a t e d w i t h a base. D é n u d e r tubes coated w i t h a basic m a t e r i a l s u c h as s o d i u m carbonate are an effective c o l l e c t i o n m e d i u m for organic acids as w e l l . 1 8

U s e o f the d e r i v a t i z a t i o n methods r e q u i r e s that c o l l e c t i o n a n d r e c o v e r y efficiencies be established for each species. I n a d d i t i o n , e x t r e m e care m u s t be exercised i n o r d e r to exclude c o n t a m i n a t i o n d u r i n g storage before a n d after exposure to a m b i e n t air. A series of blanks m u s t b e i n c l u d e d that represents the various sources of c o n t a m i n a t i o n — s o l v e n t s , transportation to c o l l e c t i o n site, a n d storage p r i o r to e l u t i o n for analysis. I n a d d i t i o n to positive interferences d u e to c o n t a m i n a t i o n b y the species o f interest, negative i n terferences are also possible. F o r example, i f ozone is p r e s e n t at c o n c e n t r a tions i n excess of 50 parts p e r b i l l i o n b y v o l u m e (ppbv), the ozone w i l l cause a negative interference w i t h f o r m a l d e h y d e w h e n the latter is c o l l e c t e d b y passing air t h r o u g h a cartridge c o n t a i n i n g silica that is i m p r e g n a t e d w i t h D N P H (2). Studies have s h o w n that f o r m i c a c i d can b e f o r m e d o n filters i m p r e g n a t e d w i t h base (3). T h i s formation p r e s u m a b l y occurs t h r o u g h the oxidation of f o r m a l d e h y d e , w h i c h is t h e n t r a p p e d as f o r m i c a c i d . T h i s reaction doesn't appear to be a p r o b l e m w i t h dénuder tubes. A b s o r p t i o n methods are c u r r e n t l y b e i n g u s e d to collect p o l a r a i r c o n taminants that are soluble i n water. L o w - m o l e c u l a r - w e i g h t organic acids a n d peroxides are examples of species that can b e r e m o v e d f r o m a m b i e n t air that contacts a w a t e r surface. F o r m i c a n d acetic a c i d can b e c o l l e c t e d b y


280


i n j e c t i n g a m b i e n t air into a c h a m b e r c o n t a i n i n g a w a t e r v a p o r m i s t . W i t h this m e t h o d o l o g y , the acids are r e p o r t e d to b e s c r u b b e d f r o m the a i r s t r e a m w i t h 100% efficiency (4). A n o t h e r m e t h o d i n w h i c h air is a l l o w e d to diffuse past a condensation plate that is c o o l e d b e l o w the d e w p o i n t has b e e n u t i l i z e d for the c o l l e c t i o n of organic acids i n a m b i e n t atmospheres (5). T h e gaseous acids are absorbed i n the w a t e r layer that condenses o n the c o o l e d plate. L a z r u s et a l . c o l l e c t e d a i r b o r n e peroxides b y passing air t h r o u g h a w a t e r trap (6). Various trap designs have b e e n u s e d for c o l l e c t i n g the peroxides. T h e p r e f e r r e d p r o c e d u r e a n d that c u r r e n t l y u s e d i n v o l v e s s c r u b b i n g the peroxides from an airstream that is slowly p u m p e d t h r o u g h a c o i l c o n t a i n i n g l i q u i d water. C o l l e c t i o n technology for vapor-phase organic c o m p o u n d s can b e s u m m a r i z e d as follows: • T r u e hydrocarbons can be c o l l e c t e d a n d stored i n passivated stainless steel canisters w i t h o u t significant losses. • C r y o g e n i c c o l l e c t i o n of t r u e hydrocarbons as w e l l as some oxygenated hydrocarbons works w e l l i f precautions are t a k e n to r e m o v e ozone. • A d s o r p t i o n of organic c o m p o u n d s o n materials s u c h as Tenax is best done o n an i n d i v i d u a l species basis i n w h i c h the c o l l e c t i o n a n d r e c o v e r y efficiencies are established a n d b l a n k a n a l yses show no interferences f r o m the adsorbent m a t e r i a l . • D e r i v a t i z a t i o n a n d absorption are b e i n g u s e d for c o l l e c t i o n o f polar a n d reactive organic species that do not b e h a v e w e l l w h e n c o l l e c t e d b y the w h o l e - a i r , cryogenic, or a d s o r p t i o n t e c h niques. A s i d e f r o m w h o l e - a i r c o l l e c t i o n i n stainless steel canisters, a d d i t i o n a l analytical studies are n e e d e d w i t h the other c o l l e c t i o n methodologies. F o r e x a m p l e , the cryogenic c o l l e c t i o n p r o c e d u r e p o t e n t i a l l y offers a n excellent w a y o f a u t o m a t i n g the c o l l e c t i o n of h y d r o c a r b o n s . H o w e v e r , m e t h o d s for r e m o v i n g ozone a n d w a t e r m u s t be d e v e l o p e d that are a p p l i c a b l e for c o n tinuous operation. A single s o l i d adsorbent m a t e r i a l that w i l l q u a n t i t a t i v e l y retain a n d elute organic species w o u l d b e h i g h l y d e s i r a b l e . Speciation. T h e m e t h o d u s e d to resolve a c o m p l e x air m a t r i x i n t o i n d i v i d u a l species is d e p e n d e n t o n the c o l l e c t i o n p r o c e d u r e that was u s e d . Gaseous samples are separated into the i n d i v i d u a l c o m p o n e n t s w i t h gas c h r o m a t o g r a p h y , whereas samples i n l i q u i d m e d i a ( d e r i v a t i z e d a n d absorbed) are usually r e s o l v e d o n a l i q u i d or i o n c h r o m a t o g r a p h . T h e favored m e t h o d w i l l always incorporate a gas c h r o m a t o g r a p h i c separation w h e n possible because of the m u c h b e t t e r r e s o l u t i o n achievable o n


10.


281 Nonmethane Organic

Compounds

gas c h r o m a t o g r a p h ( G C ) c o l u m n s as c o m p a r e d to the l i q u i d systems, L o w m o l e c u l a r - w e i g h t oxygenated c o m p o u n d s that don't behave w e l l (low r e sponse or h i g h polarity) i n gas c h r o m a t o g r a p h i c systems are best r e s o l v e d w i t h l i q u i d chromatographic conditions. Gas Chromatography. F u s e d silica capillary c o l u m n s are u s e d to s e p arate gaseous m i x t u r e s . A p r e c o n c e n t r a t e d (cryogenic or adsorbent) sample is i n j e c t e d onto the h e a d of a capillary c o l u m n that has b e e n c o o l e d to s u b a m b i e n t t e m p e r a t u r e ( ~ - 5 0 °C). T h i s setup p r o v i d e s a m e c h a n i s m for focusing most of the hydrocarbons i n a t h i n b a n d at the h e a d of the c o l u m n . The G C o v e n is t h e n p r o g r a m m e d to increase the t e m p e r a t u r e to a p p r o x i m a t e l y 100 °C. A t y p i c a l r u n w o u l d go f r o m - 5 0 to 100 °C at 4 ° C / m i n , w i t h a h o l d at the final t e m p e r a t u r e for an a d d i t i o n a l 15 m i n . U n d e r these c o n ditions, hydrocarbons c o n t a i n i n g u p to 10 carbons can b e e l u t e d i n 45 m i n . F i g u r e 2 (a t h r o u g h c) shows examples of the separation that can b e a c h i e v e d w i t h this p r o c e d u r e . T h e u r b a n sample c h r o m a t o g r a m , s h o w n i n F i g u r e 2a, contains o v e r 100 peaks. T h e C h y d r o c a r b o n s (ethane, e t h y l e n e , a n d acet ylene) are not r e s o l v e d , a n d r e s o l u t i o n is m a r g i n a l for the C h y d r o c a r b o n s . D o m i n a n t hydrocarbons i n the C to C m o l e c u l a r w e i g h t range are w e l l resolved. Separation is acceptable for anthropogenic h y d r o c a r b o n s i n t h e C and C range, b u t q u a n t i f y i n g trace amounts of m o n o t e r p e n e s that m i g h t be present i n u r b a n atmospheres w o u l d b e difficult. 2

3

4

8

9

1 0

C o m p l e x i t y of the c h r o m a t o g r a m is r e d u c e d i n r u r a l samples; about 25 major species are usually present. T h e use of fused silica capillary c o l u m n s allows separation of locally e m i t t e d b i o g e n i c h y d r o c a r b o n s f r o m the b a c k g r o u n d m i x of l o n g - l i v e d anthropogenic species. A s s h o w n i n F i g u r e 2 b , isoprene a n d the p i n e n e s can b e r e a d i l y q u a n t i f i e d . Samples r e p r e s e n t a t i v e of the free troposphere ( F i g u r e 2c) contain v e r y few peaks because h y d r o carbons i n the C a n d above m o l e c u l a r w e i g h t range are p r e s e n t at c o n c e n trations near or b e l o w the detection l i m i t of most G C systems. 4

H y d r o c a r b o n s c o n t a i n i n g two a n d three carbons are g e n e r a l l y separated o n p a c k e d c o l u m n s . C h e m i c a l l y b o n d e d materials s u c h as η-octane or p h e n y l isocyanate o n Porasil have p r o v e n to b e good separator systems for these h i g h l y volatile n o n m e t h a n e hydrocarbons. H o w e v e r , these systems r e q u i r e a separate analysis f r o m that e m p l o y e d for the C - C h y d r o c a r b o n s (7). R e c e n t d e v e l o p m e n t s i n c l u d e the use of c a p i l l a r y - t y p e c o l u m n s [e.g., A l 0 porous layer o p e n t u b u l a r ( P L O T ) ] for separation of the l o w e r m o l e c u l a r w e i g h t hydrocarbons (8). 4

1 2

2

3

Liquid-Ion Chromatography. O x y g e n a t e d h y d r o c a r b o n s , s u c h as c a r b o n y l s , that have b e e n c o n v e r t e d to derivatives a n d that absorb l i g h t i n the U V range can be q u a n t i f i e d b y l i q u i d c h r o m a t o g r a p h i c p r o c e d u r e s . R e versed-phase chromatographic t e c h n i q u e s w i t h gradient e l u t i o n are n o r m a l l y u s e d . W h e n c o m p a r e d to gas c h r o m a t o g r a p h y separations, the l i q u i d c h r o -


282


TIME

•

Figure 2. Chromatograms typical of a, an urban environment. GC conditions: 30-m DB-1 fused silica capillary column; oven temperature -50 to 100 °C at 4 °C/min. Continued on next page.

matography ( L C ) separations are poor. H o w e v e r , L C c o l u m n e l u e n t systems have b e e n d e v e l o p e d that p r o v i d e adequate r e s o l u t i o n o f the l o w - m o l e c u l a r w e i g h t c a r b o n y l derivatives (9). F i g u r e 3 illustrates t h e t y p e o f separation achievable for several o f the m o r e volatile c a r b o n y l species p r e s e n t i n u r b a n atmospheres. O r g a n i c acids have r e c e i v e d considerable interest r e c e n t l y because o f t h e i r p o t e n t i a l for r e d u c i n g t h e p H i n p r e c i p i t a t i o n (10). A n i o n e h r o m a t o g r a p h (IC) w i t h a c o n d u c t i v i t y detector is best s u i t e d for d e t e r m i n i n g organic a c i d concentrations. A s i n d i c a t e d i n t h e " C o l l e c t i o n " section, t h e acids are r e m o v e d from a m b i e n t a i r e i t h e r t h r o u g h d e r i v a t i z a t i o n to a n alkali m e t a l salt (by a filter-denuder) or b y absorption i n water. I n e i t h e r case, a w a t e r m i x t u r e is i n j e c t e d onto the I C c o l u m n , w h e r e separations as d e p i c t e d i n F i g u r e 4 can b e expected. I o n chromatography, l i k e l i q u i d c h r o m a t o g r a p h y , has m u c h greater p o t e n t i a l for interference than gas c h r o m a t o g r a p h y because of m u c h p o o r e r c o l u m n r e s o l u t i o n .


10.


Nonmethane Organic

283

Compounds

(b)

5

ill

tn or-. I

uEJ

*

©

ΓΟΓΟ

TIME (c)

TIME Figure 2. Continued. Chromatograms typical of b, a rural forested environ ment, and c, a clean continental environment. GC conditions: 30-m DB-1 fused silica capilhry column; oven temperature -50 to 100 °C at 4 °Cimin.


284


7

10

»

20

30

Time

40

Τ 50

"Ί 60

(min.)

Figure 3. LC chromatogram of carbonyls collected on a DNPH-impregnated silica gel cartridge. Peak identities: 1, DNPH; 2, formaldehyde; 3, acetaldehyde; 4, acrolein; 5, acetone; 6, propionaldehyde; 7, x-acrolein; 8, crotonaldehyde; 9, butyraldehyde; and 10, benzaldehyde. (Reproduced with permission from reference 8. Copyright 1992.)

Detection. N e a r l y a l l of the vapor-phase organic c o m p o u n d s w i l l r e s p o n d w h e n a d d e d to a flame i o n i z a t i o n detector. C o n s e q u e n t l y , this d e tector is most c o m m o n l y u s e d . O t h e r special-purpose detectors i n c l u d e p h o t o i o n i z a t i o n , mass s p e c t r o m e t r y , a t o m i c e m i s s i o n , i o n m o b i l i t y , m e r c u r y oxide r e d u c t i o n , a n d c h e m i l u m i n e s c e n c e detectors.

Flame Ionization. T h i s has p r o v e n to b e t h e best d e t e c t i o n system for most organic c o m p o u n d s . Its n e a r l y e q u a l c a r b o n response for the t r u e h y d r o c a r b o n s greatly facilitates calibration. F o r e x a m p l e , a single h y d r o c a r b o n (or a m i x t u r e of a few hydrocarbons) can b e u s e d to d e t e r m i n e t h e response-versus-concentration c u r v e for c a l i b r a t i o n o f a G C s y s t e m . It is t h e n a s i m p l e matter to d e t e r m i n e the concentrations o f a l l the h y d r o c a r b o n s i n a c o m p l e x m i x t u r e , s u c h as that r e p r e s e n t e d b y the c h r o m a t o g r a m i n F i g u r e 2a. I n a d d i t i o n to b e i n g easy to calibrate, flame i o n i z a t i o n detectors ( F I D s ) are robust systems that can b e t r a n s p o r t e d to a n d o p e r a t e d at r e m o t e field sites. T h e w i d e l i n e a r response range a n d l o w d e t e c t i o n l i m i t m a k e t h e F I D an i d e a l detector for q u a n t i f y i n g h y d r o c a r b o n s i n u r b a n atmospheres. T h e p r a c t i c a l d e t e c t i o n l i m i t for a t y p i c a l flame system is a p p r o x i m a t e l y 10 parts p e r t r i l l i o n b y v o l u m e (for an air v o l u m e of 1 L ) ; h o w e v e r , p r e c i s i o n deteriorates r a p i d l y as concentrations fall b e l o w 100 p p t r v . T h u s , t h e F I D is m a r g i n a l l y acceptable for the analysis o f free t r o p o s p h e r e samples i n w h i c h most of the hydrocarbons are present at l o w p a r t s - p e r - t r i l l i o n l e v e l s . A n e n h a n c e m e n t of F I D sensitivity b y a factor of 10 to 100 w o u l d b e v e r y beneficial for the d e t e r m i n a t i o n of h y d r o c a r b o n concentrations i n c l e a n at mospheres.


10.


Nonmethane Organic

Compounds

285

Figure 4. IC chromatogram showing resolution of organic acids. A polystyrene-divinylbenzene (HPIC-AS4A) separator column was used; the eluent solution was 0.0013 M sodium tetraboratedecahydrate at a flow rate of 2 mL/min.

Mass Spectrometry. T h e use of a q u a d r u p o l e mass s p e c t r o m e t e r as a G C detector for n o n m e t h a n e h y d r o c a r b o n analysis has c o m e of age i n r e c e n t years. D e v e l o p m e n t of capillary c o l u m n s w i t h l o w c a r r i e r gas flows has greatly facilitated the i n t e r f a c i n g o f the G C a n d mass s p e c t r o m e t e r ( M S ) . The e n t i r e capillary c o l u m n effluent can b e d u m p e d d i r e c t l y i n t o the M S ion source to m a x i m i z e system sensitivity. G C - M S d e t e c t i o n l i m i t s are c o m p o u n d - s p e c i f i c b u t i n most cases are s i m i l a r to those o f t h e flame i o n i zation detector. Q u a n t i t a t i o n w i t h a mass spectrometer as detector r e q u i r e s i n d i v i d u a l species calibration curves. H o w e v e r , the N M O C response p a t t e r n as r e p r e s e n t e d b y a G C - M S total i o n c h r o m a t o g r a m is usually v e r y s i m i l a r to the e q u i v a l e n t F I D c h r o m a t o g r a m . C o n s e q u e n t l y , the M S d e t e c t o r can


286

M E A S U R E M E N T C H A L L E N G E S IN ATMOSPHERIC CHEMISTRY

b e u s e d to establish h y d r o c a r b o n identities a n d an F I D system for q u a n t i tation. T h e c o u p l i n g o f two mass spectrometer systems has r e c e i v e d a t t e n t i o n i n recent years. T h i s system can be o p e r a t e d i n an a t m o s p h e r i c p r e s s u r e m o d e b y passing the air m a t r i x d i r e c t l y i n t o the i o n i z a t i o n source (II). T h i s m e t h o d m i n i m i z e s sample c o n t a m i n a t i o n a n d degradation p r o b l e m s . D e t e c t i o n l i m i t s are c o m p o u n d - d e p e n d e n t a n d can v a r y o v e r m o r e t h a n an o r d e r o f m a g n i t u d e for different families of h y d r o c a r b o n s . F o r e x a m p l e , an aromatic h y d r o c a r b o n such as t o l u e n e cannot be d e t e c t e d at levels b e l o w 5 p p b v , whereas most aldehydes are detectable at levels as l o w as 50 p p t r v . T h e t a n d e m M S - M S system has the p o t e n t i a l to be a useful d e t e c t i o n system for organic c o m p o u n d s that do not store w e l l i n c o l l e c t i o n containers. Atomic Emission. C o u p l i n g an atomic e m i s s i o n detector ( A E D ) w i t h a gas c h r o m a t o g r a p h has the p o t e n t i a l for selective a n d sensitive d e t e r m i nation o f n o n m e t h a n e hydrocarbons. O r g a n i c c o m p o u n d s e x i t i n g the G C c o l u m n are e x c i t e d i n a m i c r o w a v e p l a s m a that y i e l d s l i g h t emissions at wavelengths characteristic o f the elements present. B y m e a s u r i n g the i n tensity of light e m i t t e d , chromatograms selective for various e l e m e n t s (e.g., C , H , O , a n d so forth) are o b t a i n e d . A G C - A E D system is n o w c o m m e r c i a l l y available, b u t no reports have appeared d o c u m e n t i n g its use i n the d e t e r m i n a t i o n o f trace organic c o m p o u n d s i n the atmosphere. L i t e r a t u r e p r o v i d e d b y the manufacturer ( H e w l e t t - P a c k a r d ) indicates that d e t e c t i o n l i m i t s for c a r b o n a n d h y d r o g e n are l o w e r than those o f the flame i o n i z a t i o n detector. T h e system appears to have an acceptable l i n e a r d y n a m i c range (~10 ) as w e l l . T h e a b i l i t y to verify the presence of C , H , a n d Ο i n a single analysis s h o u l d p r o v e to b e v e r y beneficial for the i d e n t i f i c a t i o n of trace organic c o m p o u n d s i n the atmosphere. Q u a n t i t a t i o n w i l l r e q u i r e i n d i v i d u a l species calibration. 4

Special-Purpose Detectors. Ion mobility, H g O reduction, and c h e m i l u m i n e s c e n c e are u s e d as special-purpose d e t e c t i o n systems that can be u t i l i z e d for N M O C analysis. H i l l a n d co-workers (12) have d e s c r i b e d the use of a c a p i l l a r y c o l u m n gas c h r o m a t o g r a p h c o u p l e d to an i o n m o b i l i t y detector for trace organic analysis. E f f l u e n t f r o m the G C c o l u m n enters the detector, w h e r e the organic molecules are i o n i z e d b y a proton-transfer m e c h a n i s m i n v o l v i n g ( H 0 ) H . T h e i o n i z e d organic c o m p o u n d t h e n passes t h r o u g h a drift t u b e at a p a r t i c u l a r v e l o c i t y that d e p e n d s o n factors s u c h as c o l l i s i o n f r e q u e n c y w i t h drift-gas m o l e c u l e s , t e m p e r a t u r e , charge o n the i o n , a n d so forth. T h e i o n m o b i l i t y detector is r e p o r t e d to have p i c o g r a m s e n sitivity a n d can p r o v i d e v e r y selective d e t e c t i o n . 2

n

+

T h e m e r c u r i c oxide r e d u c t i o n detector was o r i g i n a l l y d e s c r i b e d for m o n i t o r i n g carbon m o n o x i d e i n clean atmospheres (13). H o w e v e r , because the p r i n c i p l e o f d e t e c t i o n relies o n l y o n the transformation of H g O to H g v a p o r ,


10.


Nonmethane Organic

Compounds

287

any species that w i l l effect this r e d u c t i o n can b e d e t e c t e d . O r g a n i c m o l e c u l e s that c o n t a i n unsaturated b o n d s w i l l r e s p o n d . O ' H a r a a n d S i n g h u s e d the gas r e d u c t i o n c e l l to measure acetaldehyde a n d acetone concentrations (14). T h e y r e p o r t a w i d e l i n e a r range (10 ) a n d s e n s i t i v i t y 20 to 40 t i m e s that o f a flame i o n i z a t i o n detector. Z i m m e r m a n a n d G r e e n b e r g have u s e d a gas c h r o m a t o g r a p h e q u i p p e d w i t h a gas r e d u c t i o n c e l l for analysis o f b i o g e n i c h y d r o c a r b o n s i n forest e n v i r o n m e n t s (15). Isoprene a n d m o n o t e r p e n e s can be d e t e c t e d at l o w p a r t s - p e r - b i l l i o n levels w i t h a 1-cm i n j e c t i o n . T h i s s e n sitivity is advantageous for r u r a l studies because it e l i m i n a t e s the n e e d for c r y o g e n i c p r e c o n c e n t r a t i o n . T h e contents of a 1-cm s a m p l e l o o p can b e transferred d i r e c t l y to a megabore-type c a p i l l a r y c o l u m n . A n a d d i t i o n a l p o s itive feature of this detector is that it doesn't r e q u i r e flame s u p p o r t gases (hydrogen a n d oxygen). 3

3

3

T h e reaction b e t w e e n olefins a n d ozone p r o d u c e s l i g h t that can b e m e a s u r e d a n d r e l a t e d to the c o n c e n t r a t i o n o f the reactants. O n e o f t h e p r e f e r r e d methods for m e a s u r i n g a m b i e n t ozone concentrations u t i l i z e s the c h e m i l u m i n e s c e n c e generated i n the o z o n e - e t h y l e n e reaction for d e t e c t i o n . R e c e n t l y , H i l l s a n d Z i m m e r m a n (16) d e s c r i b e d the use of this d e t e c t i o n p r i n c i p l e for d e t e r m i n i n g h y d r o c a r b o n concentrations. T h e y u t i l i z e d the c h e m i l u m i n e s c e n c e created w h e n ozone reacts w i t h i s o p r e n e for d e v e l o p m e n t of a c o n t i n u o u s , fast-response isoprene analyzer. T h i s r e a l - t i m e isop r e n e system is r e p o r t e d to be l i n e a r o v e r t h r e e orders o f m a g n i t u d e a n d to have a d e t e c t i o n l i m i t of about 1 p p b v . Because the system doesn't i n c l u d e a preseparation o f h y d r o c a r b o n s , interferences f r o m o t h e r olefins (ethylene, p r o p y l e n e , a n d so forth) c o u l d occur. T h u s far the c h e m i l u m i n e s c e n t detector has b e e n u s e d to m o n i t o r isoprene emissions u n d e r c o n d i t i o n s i n w h i c h the concentrations of olefins that c o u l d interfere are n e g l i g i b l e c o m p a r e d to those of the b i o g e n i c h y d r o c a r b o n .

Future Directions State-of-the-art m o n i t o r i n g systems for n o n m e t h a n e h y d r o c a r b o n s are c u r r e n t l y available a n d v e r y adequate for d e f i n i n g the q u a l i t a t i v e a n d q u a n t i tative N M O C c o m p o s i t i o n i n u r b a n e n v i r o n m e n t s . F u t u r e d e v e l o p m e n t efforts for u r b a n m o n i t o r i n g n e e d to be d i r e c t e d t o w a r d a u t o m a t i o n a n d i m p r o v e d speciation o f oxygenated h y d r o c a r b o n s . R e c e n t m o n i t o r i n g efforts in A t l a n t a have u t i l i z e d an a u t o m a t e d system that collects a m b i e n t h y d r o carbons o n an adsorbent trap a n d t h e n causes a t h e r m a l d e s o r p t i o n a n d automatically transfers the sample to a c a p i l l a r y G C c o l u m n . I n f o r m a t i o n c o n c e r n i n g the success of this analytical p r o c e d u r e s h o u l d b e available i n the near future. Scientists i n the N a t i o n a l O c e a n i c a n d A v i a t i o n A d m i n i s tration ( N O A A ) A e r o n o m y L a b o r a t o r y are p e r f e c t i n g an a u t o m a t e d system for d e t e r m i n a t i o n of C - C hydrocarbons i n c l e a n , r u r a l e n v i r o n m e n t s (R G o l d a n , u n p u b l i s h e d data). T h e a i m is to d e v e l o p a system that w i l l p r o v i d e 3

1 0


288


h o u r l y speciated h y d r o c a r b o n concentrations o n a continuous basis. A c a p i l l a r y c o l u m n - F I D gas c h r o m a t o g r a p h i c system is b e i n g u t i l i z e d a l o n g w i t h cryogenic c o l l e c t i o n - p r e c o n c e n t r a t i o n methods. T h e real challenge comes i n d e s i g n i n g a c o l l e c t i o n system that w i l l retain the organic c o m p o u n d s b u t c o n t i n u o u s l y r e m o v e water, ozone, a n d any o t h e r i n t e r f e r i n g species f r o m an a m b i e n t air sample. T h e procedures u s e d to d e t e r m i n e a m b i e n t c a r b o n y l concentrations i n v o l v e a c o l l e c t i o n step w i t h silica or C cartridges i m p r e g n a t e d w i t h 2,4d i n i t r o p h e n y l h y d r a z i n e . C o n t a m i n a t i o n is i n e v i t a b l e w i t h this s y s t e m , a n d blanks m u s t b e u s e d to compensate for the degree o f c o n t a m i n a t i o n . S e l e c t i o n o f the appropriate b l a n k values to subtract is a difficult a n d u n c e r t a i n process. C o n s e q u e n t l y , d e v e l o p m e n t of a gas c h r o m a t o g r a p h i c system that w i l l resolve a n d r e s p o n d to the l o w - m o l e c u l a r - w e i g h t aldehydes a n d ketones is n e e d e d . T h e m e r c u r i c oxide a n d atomic e m i s s i o n detectors s h o u l d p r o v i d e adequate response for the carbonyls. 1 8

S i m p l i f i e d analytical procedures for d e t e r m i n a t i o n o f gas-phase organic acids w o u l d b e v e r y beneficial. C u r r e n t l y , the acids are c o l l e c t e d b y u s i n g i m p r e g n a t e d filters, dénuder tubes, or w a t e r absorption t e c h n i q u e s a n d t h e n an i o n chromatographic analysis. N o r m a l l y , the c o l l e c t i o n a n d analysis steps are d e c o u p l e d i n t i m e (i.e., samples c o l l e c t e d at a field site are r e t u r n e d to a h o m e laboratory for I C analysis). O n c e again, b l a n k samples m u s t b e u t i l i z e d to compensate for c o n t a m i n a t i o n d u r i n g transport a n d storage p r i o r to analysis. D e v e l o p m e n t of fast-response t e c h n i q u e s for m e a s u r e m e n t of N M O C fluxes is b a d l y n e e d e d . D e t e c t o r s w i t h specificity for a c o m p o u n d a n d the s p e e d to b e n e t w o r k e d w i t h fast e d d y c o r r e l a t i o n m i c r o m e t e o r o l o g i c a l t e c h n i q u e s w o u l d b e v e r y useful. P r e s e n t d e v e l o p m e n t activities i n this area are a i m e d at c o u p l i n g existing fast m i c r o m e t e o r o l o g i c a l sensors w i t h s l o w a n a lytical methods for the N M O C s . F i g u r e 5 illustrates a c o n d i t i o n a l s a m p l i n g system d e s i g n e d to p r o v i d e h y d r o c a r b o n flux i n f o r m a t i o n . T h e i n l e t for h y d r o c a r b o n s a m p l i n g is colocated w i t h the sensor u n i t o f a sonic a n e m o m e t e r . A c o m p u t e r - c o n t r o l l e d s o l e n o i d n e t w o r k is d e s i g n e d to c h a n n e l h y d r o c a r b o n sample l i n e flow into one of three c o l l e c t i o n containers, d e p e n d i n g o n the d i r e c t i o n of air m o v e m e n t . W h e n eddies are m o v i n g u p w a r d as d e t e r m i n e d b y the a n e m o m e t e r , a m b i e n t air flows i n t o the u p c o n t a i n e r w h i l e n e u t r a l air m o t i o n fills the stagnant collector; d u r i n g d o w n w a r d m o t i o n , air is c h a n n e l e d to the r e m a i n i n g container. T h e flux is p r o p o r t i o n a l to the difference i n concentrations i n the u p a n d d o w n collectors times the v e r t i c a l w i n d s p e e d fluctuations (a ). T h e N M O C concentrations are m e a s u r e d w i t h a c o n v e n t i o n a l G C - F I D system. w

M e t h o d s are also n e e d e d for e s t a b l i s h i n g accuracy o f N M O C analysis. A t present, each research g r o u p m a k i n g N M O C m e a s u r e m e n t s m u s t p r e p a r e its o w n c a l i b r a t i o n standards. M e a s u r e m e n t accuracy is t h e n j u d g e d b y i n t e r c o m p a r i n g the results o b t a i n e d w h e n two or m o r e laboratories analyze the same samples. W h e n the results of u r b a n samples have b e e n i n t e r c o m -


10.


sonic ometers U

anem

Nonmethane Organic

289

Compounds

computer

valve and pump control

sample inlet

Figure 5. Block diagram of a conditional sampling system for NMOC fluxes.

quantifying

p a r e d , agreement b e t w e e n laboratories has b e e n generally q u i t e good. T h e relative standard d e v i a t i o n for a l l o f the d o m i n a n t hydrocarbons was 2 0 % o r less for samples c o l l e c t e d i n several northeastern U n i t e d States cities a n d analyzed b y three i n d e p e n d e n t laboratories (17). H y d r o c a r b o n c o n c e n t r a tions i n the u r b a n i n t e r c o m p a r i s o n studies g e n e r a l l y v a r i e d b e t w e e n 10 a n d 100 p p b C . L a b o r a t o r y i n t e r e o m p a r a b i l i t y for C - C h y d r o c a r b o n s i n samples c o l l e c t e d i n t h e r e m o t e atmosphere is n o t v e r y good. T h e p e r c e n t relative standard d e v i a t i o n for ethane (—2 p p b v ) from 10 laboratories was about 2 0 % b u t increased to greater than 1 0 0 % for m a n y hydrocarbons p r e s e n t at 5 0 0 p p t r v o r less (18). 2

5

References 1. Pate, B . ; Jayanty, R. Κ. M.; Peterson, M. R.; Evans, G . F. J. Air Waste Man agement Assoc. 1992, 42, 460-462. 2. Arnts, R. R.; Tejada, S. B . Environ. Sci. Technol. 1989, 23, 1428. 3. Keene, W. C . ; Talbot, R. W.; Andreae, M. O . ; Beecher, K . ; Berresheim, H.; Castro, M.; Farmer, J. C . ; Galloway, J. N.; Hoffman, M. R . ; Li, S. M.; Maben, J. R.; Munger, J . W.; Norton, R. B . ; Pszenny, A . A . P.; Puxbaum, H.; Westberg, H.; Winiwarter, W. J. Geophys. Res. 1989, 94, 6457-6460.


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4. Cofer, W. R., III; Collins, V. G.; Talbot, R. W. Environ. Sci. Technol. 1985, 19, 557-560. 5. Dawson, G . Α.; Farmer, J. C. J. Geophys. Res. 1988, 93, 5200-5206. 6. Lazrus, A. L.; Kok, G . L.; L i n d , J . Α.; Gitlin, S. N.; Heikes, B. G.; Shetter, R. E . Anal. Chem. 1986, 58, 594-597. 7. Greenberg, J . P.; Zimmerman, P. R. J. Geophys. Res. 1984, 89(D), 4767-4778. 8. Rudolph, J.; Khedim, Α.; Bonsang, B. J. Geophys. Res. 1992, 97(D), 6181-6186. 9. Tejada, S. B. Int. J. Environ. Chem. 1986, 26, 167-185. 10. Talbot, R. W.; Beecher, K . M.; Harriss, R. C . ; Cofer, W. R., III J. Geophys Res. 1988, 93(D), 1638-1652. 11. Dumdei, B. E.; Kenny, D . V.; Shepson, P. B . ; Kleindienst, T. E.; Nero, C. M.; Cupitt, L . T.; Claxton, L. D . Environ. Sci. Technol. 1988, 22, 1493-1498. 12. St. Louis, R. H.; Siems, W. F.; H i l l , H . H., Jr. LC-GC 1987, 6, 810-814. 13. Robbins, R. C . ; Borg, Κ. M.; Robinson E . J. Air Pollut. Control Assoc. 1968, 18, 106-110. 14. O'Hara, D.; Singh, H. B. Atmos. Environ. 1988, 22, 2613-2615. 15. Zimmerman, P. R.; Greenberg, J . P. E O S , Trans., Am. Geophys. Union 1988, 69, 1056. 16. Hills, A. J.; Zimmerman, P. R. Anal. Chem. 1990, 62, 1055-1060. 17. Westberg, H.; Lonneman, W.; Holdren, M. Identification and Analysis of Or ganic Pollutants in Air; Keith, L . H., Ed.; Butterworth Publishers: Boston, 1984; pp 323-337. 18. Carsey, T. P.; Bachmann, K . ; Blake, D . R.; Blake, N. J.; Bonsang, B . ; Dalluge, B.; Greenberg, J.; Harvey, G . R.; Kanakidou, M.; Laird, C . K . ; Lightman, P.; Penkett, S.; Rasmussen, R. Α.; Rowland, S.; Rudolph, J.; Westberg, H.; Z i m merman, P. R. J. Atmos. Chem. 1990, submitted. RECEIVED

1992.

for review March 20, 1991.

ACCEPTED

revised manuscript September 16,


Analytical Methods Used To Identify Nonmethane Organic

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