10 Laser-Induced Fluorescence of Polycyclic Aromatic Hydrocarbons in a Flame
Laser Probes for Combustion Chemistry Downloaded from pubs.acs.org by YORK UNIV on 12/07/18. For personal use only.
DONALD S. COE and JEFFREY I. STEINFELD Department of Chemistry and Chemical Engineering and Center for Health Effects of Fossil Fuel Utilization, Massachusetts Institute of Technology, Cambridge, MA 02139
Laser-induced fluorescence spectroscopy (LIF) is being developed as an in-situ, real time diagnostic for polycyclic aromatic hydrocarbons (PCAH) in combustion systems. PCAH are known to be formed in sooting flames and are of interest both for their carcinogenic properties and possible role in the soot formation process. Gas chromatography and mass spectrometry have provided probe measurements of PCAH in flames; however, there is a need for a real time, non-intrusive technique for measurement of PCAH in combustion systems. Probe measurements have indicated individual PCAH concentrations in the 10 ppb to 10 ppm range . LIF has been shown to be capable of detection of flame radicals at these concentrations and is expected to give similar limits for PCAH. The individual PCAH of interest include naphthalene, pyrene, fluoranthene, phenanthrene, anthracene, benzpyrene, and others. In a combustion environment many PCAH will be present in varying concentrations, so that detection of an individual species requires deconvoluting complex spectra from the multicomponent mixture. This requires a detailed knowledge of excitation and fluorescence spectra for individual species under flame conditions. A literature search indicated that in most cases the available vapor phase spectra are insufficiently detailed for this purpose, even at near-room-temperature conditions. 1,2
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Thus i n t h e i n i t i a l s t a g e s o f t h e s t u d y , e x c i t a t i o n a n d f l u o r e s c e n c e s p e c t r a were measured f o r i n d i v i d u a l s p e c i e s i n a c e l l ( h e a t e d t o a p p r o x i m a t e l y 100 C t o p r o v i d e s u f f i c i e n t v a p o r p r e s s u r e ) t o d e t e r m i n e t h e i r ( n e a r ) room t e m p e r a t u r e s p e c t r a . I n d i v i d u a l PCAH were t h e n i n j e c t e d i n t o a f l a m e t o d e t e r m i n e ' t h e e f f e c t s o f flame temperatures on t h e s p e c t r a and t o determine sensitivities. T h e s e s p e c t r a w i l l t h e n be u s e d a s a d a t a b a s e t o a t t e m p t t o d e c o n v o l u t e t h e c o m p l e x s p e c t r a o b s e r v e d upon e x c i t a t i o n o f t h e flame i t s e l f .
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The e q u i p m e n t i n s t a l l e d f o r t h i s p r o j e c t c o n s i s t s o f a t u n a b l e dye l a s e r and t h e f l u o r e s c e n c e d e t e c t i o n s y s t e m . Both t h e l a s e r and t h e d a t a a c q u i s i t i o n a r e u n d e r c o m p u t e r c o n t r o l . The a p p a r a t u s s c h e m a t i c i s shown i n F i g u r e 1. The l a s e r i s a Phase-R C o r p o r a t i o n DL-1400 f l a s h l a m p - p u m p e d t u n a b l e dye l a s e r . B e c a u s e o f t h e h i g h g a i n a f f o r d e d by t h e c o a x i a l f l a s h l a m p , t h i s u n i t has t h e c a p a b i l i t y o f l a s i n g d i r e c t l y t h r o u g h o u t t h e 350-760 nm w a v e l e n g t h r a n g e . A KDP d o u b l i n g c r y s t a l has b e e n i n s t a l l e d , w h i c h c a n p r o d u c e t u n a b l e u.v. r a d i a t i o n i n t h e 290-350 nm r a n g e by f r e q u e n c y - d o u b l i n g . Additional crystals a r e a v a i l a b l e , w h i c h c a n e x t e n d t h i s r a n g e down t o 200 nm. W i t h t h e p r e s e n t a r r a n g e m e n t , h o w e v e r , we a r e a b l e t o c o v e r t h e f l u o r e s c e n c e e x c i t a t i o n r e g i o n s o f most o f t h e PCAH s p e c i e s of i n t e r e s t . The l a s e r p r o d u c e s a p u l s e a p p r o x i m a t e l y 300 n s e c l o n g w i t h peak powers o f 1 Megawatt. P u l s e r e p e t i t i o n r a t e s up t o 10 p e r s e c o n d a r e p o s s i b l e . The o u t p u t b a n d w i d t h i s n a r r o w e d t o 0.2 nm w i t h a d o u b l e p r i s m o r i e n t e d a t B r e w s t e r ' s a n g l e t o p r o d u c e a h o r i z o n t a l l y p o l a r i z e d beam. By t u n i n g t h e p r i s m s ( i . e . , c h a n g i n g t h e r e f l e c t i o n a n g l e — see F i g u r e 1 ) , t h e o u t p u t w a v e l e n g t h can be s c a n n e d o v e r 30-50 nm f o r a g i v e n dye solution. The t u n i n g o f t h e p r i s m s , f r e q u e n c y d o u b l e r ( i f u s e d ) , and e t a l o n s ( i f u s e d ) a r e a l l c a r r i e d o u t by s t e p p i n g m o t o r s c o n t r o l l e d by t h e o n - l i n e c o m p u t e r . The o u t p u t w a v e l e n g t h i s f u r t h e r s t a b i l i z e d by a f e e d b a c k s y s t e m i n t h e f l u i d pump l i n e w h i c h m a i n t a i n s a c o n s t a n t t e m p e r a t u r e d i f f e r e n c e between the c o o l i n g w a t e r and t h e dye s o l u t i o n , t h u s m i n i m i z i n g t u r b u l e n c e i n t h e dye. The l a s e r - i n d u c e d f l u o r e s c e n c e i s o b s e r v e d p e r p e n d i c u l a r t o t h e beam a x i s w i t h a l o w - d i s p e r s i o n monochromator and p h o t o m u l t i p l i e r (PMT). The l a s e r p u l s e e n e r g y i s m o n i t o r e d w i t h a p h o t o d i o d e (PD) c a l i b r a t e d a g a i n s t a S c i e n t e c h M o d e l 3600 power m e t e r . The 300 n s e c p u l s e s f r o m t h e PMT and t h e PD a r e a v e r a g e d w i t h d u a l g a t e d i n t e g r a t o r s on a p u l s e - t o - p u l s e b a s i s . The i n t e g r a t o r o u t p u t s a r e t h e n r e a d i n t o t h e PDP-8/L c o m p u t e r , t h r o u g h a p a i r o f 1 0 - b i t A/D c o n v e r t e r s . The c o m p u t e r t h e n p e r f o r m s p u l s e - t o - p u l s e a v e r a g i n g and n o r m a l i z a t i o n and s t a t i s t i c a l a n a l y s i s o f t h e d a t a , as w e l l as s c a n n i n g t h e o p t i c a l components o f t h e l a s e r i t s e l f . T y p i c a l l y , averaging 10 p u l s e s y i e l d s s t a t i s t i c a l l y a c c e p t a b l e d a t a . Results F i g u r e 2 shows f l u o r e s c e n c e s p e c t r a f o r p y r e n e and f l u o r a n t h e n e i n a t m o s p h e r i c p r e s s u r e c e l l s pumped a t e s s e n t i a l l y t h e same w a v e l e n g t h . T h e s e s p e c t r a a r e t y p i c a l o f t h e t y p e s o f p r o f i l e s o b t a i n e d f o r PCAH. The s p e c t r a a r e b r o a d band w i t h no s i g n i f i c a n t f i n e s t r u c t u r e . C o m p a r i s o n o f t h e two s p e c t r a
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Figure 2. Fluorescence spectra—atmospheric pressure cell (air): upper figure, pyrene at 78°C,\ lower figure, fluoranthehe at 90°C; fluorescence-analyzing bandpass of 4 nm,
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shows t h a t , a t l e a s t i n a l o w t e m p e r a t u r e (100 C) two component m i x t u r e o f p y r e n e and f l u o r a n t h e n e , t h e i n d i v i d u a l c o n c e n t r a t i o n s c o u l d be a s c e r t a i n e d by d e c o n v o l u t i n g t h e f l u o r e s c e n c e s p e c t r u m alone. Similar r e s u l t s hold f o r the e x c i t a t i o n spectra. F i g u r e 3 shows t h e f l u o r e s c e n c e s p e c t r a f o r p y r e n e and f l u o r a n t h e n e i n j e c t e d i n t o t h e p o s t - r e a c t i o n zone o f an e t h y l e n e a i r flame. I t was f o u n d t h a t t h e f l u o r e s c e n c e s i g n a l d i d n o t a t t e n u a t e a p p r e c i a b l y a s t h e sample volume was moved downstream of t h e i n j e c t o r . This i n d i c a t e s that the species are not being consumed i n t h e f l a m e . The t e m p e r a t u r e o f t h e i n j e c t e d s t r e a m i s n o t known; h o w e v e r , t h e i n j e c t o r f l o w r a t e was k e p t l o w t o a l l o w more e f f i c i e n t h e a t i n g o f t h e s t r e a m . T h i s t e m p e r a t u r e w i l l b e m o n i t o r e d i n f u t u r e measurements. C o m p a r i s o n o f t h e s e s p e c t r a w i t h those i n F i g . 2 i n d i c a t e s the e f f e c t o f increased temperature. I n b o t h c a s e s , t h e s p e c t r u m i s b r o a d e n e d somewhat b u t t h e q u a l i t a t i v e f e a t u r e s a r e s t i l l d i s t i n g u i s h a b l e and c a n be a t t r i b u t e d t o t h e s p e c i e s i n j e c t e d . T h u s , i t a p p e a r s t h a t the e f f e c t o f h i g h t e m p e r a t u r e s on the s p e c t r a i s n o t so gross that i t precludes i d e n t i f i c a t i o n of i n d i v i d u a l species. Further, the s p e c t r a a t flame t e m p e r a t u r e s c a n be measured by i n j e c t i o n i n t o f l a m e s , p r o v i d i n g a means o f c a l i b r a t i n g t h e L I F method f o r b o t h s p e c t r a l s i g n a t u r e and s e n s i t i v i t y f o r i n d i v i d u a l PCAH. The d e t e c t i o n l i m i t s o f i n d i v i d u a l PCAH w i l l b e d e t e r m i n e d by i n j e c t i n g known amounts i n t o a f l a m e . An e s t i m a t e o f t h e l i m i t f o r p y r e n e c a n be o b t a i n e d f r o m t h e a t m o s p h e r i c p r e s s u r e c e l l measurements. P y r e e n e was d e t e c t e d i n t h e c e l l a t 50°C and 1 atm o f a i r , where i t s v a p o r p r e s s u r e i s a b o u t 0.1 m i l l i t o r r , o r a b o u t 0.1 ppm. W h i l e t h i s l i m i t c a n be i m p r o v e d b y o p t i m i z a t i o n o f t h e o p t i c s and e l e c t r o n i c s , i t i s s u f f i c i e n t t o d e t e c t pyrene i n t h e c o n c e n t r a t i o n s expected from probe measurements. Conclusion PCAH have been o b s e r v e d i n a f l a m e u s i n g l a s e r i n d u c e d f l u o r e s c e n c e s p e c t r o s c o p y by i n j e c t i n g i n d i v i d u a l s p e c i e s i n t o the p o s t - r e a c t i o n zone. W h i l e t h e s p e c t r a a r e broadened by the e l e v a t e d temperature, the s p e c t r a a r e q u a l i t a t i v e l y s i m i l a r to l o w t e m p e r a t u r e (100 C) s p e c t r a and a r e i n d i c a t i v e o f t h e p a r t i c u l a r s p e c i e s i n j e c t e d . Thus, t h e i n j e c t i o n p r o c e d u r e a p p e a r s t o be a f e a s i b l e method o f d e t e r m i n i n g PCAH s p e c t r a a t f l a m e t e m p e r a t u r e s . T h e s e s p e c t r a w i l l be u s e d a s a d a t a b a s e t o d e t e r m i n e i n d i v i d u a l PCAH c o n c e n t r a t i o n s i n f l a m e s f r o m t h e i r LIF spectra. Acknowledgement T h i s work i s s u p p o r t e d G r a n t No. 5P01-ES01640.
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Figure 3. Fluorescence spectra—sample injected in an ethylene-air flame: upper figure, pyrene 8 mm downstream of injection point; lower figure, fluoranthene 13 mm downstream of injection point; fuel equivalence ratio of 1.7; fluorescenceanalyzing bandpass of4nm.
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References 1.
Bittner, J.D. and Howard, J.B., "Role of Aromatics in Soot Formation" Alternative Hydrocarbon Fuels: Combustion and Chemical Kinetics (C.T. Bowan and J. Birkeland, eds.) Progress in Aeronautics and Astronautics, Vol. 62, American Institute of Aeronautics & Astronautics, New York, 1978 p. 335-358.
2. Wagner, H.G., "Soot Formation in Combustion" Seventeenth Symposium (International) on Combustion 1979, p. 3. 3.
Crittenden, B.D., and Long, R., "Formation of Polycyclic Aromatics in Rich, Premixed Acetylene and Ethylene Flames" Comb, and Flame, 20, 359-368, 1973.
4. Numerous examples may be found in the present volume. RECEIVED
February 5, 1980.
