LASER-ENHANCED IONIZATION SPECTROMETRY WITH A TOTAL CONSUMPTION BURNER

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LASER-ENHANCED IONIZATION SPECTROMETRY WITH A TOTAL CONSUMPTION BURNER

R. Green, Janet Hall

To cite this version:

R. Green, Janet Hall. LASER-ENHANCED IONIZATION SPECTROMETRY WITH A TOTAL CONSUMPTION BURNER. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-319-C7-326.

�10.1051/jphyscol:1983729�. �jpa-00223286�

JOURNAIL DE PHYSIQUE

Collocjue C 7 , suppldxent au n O 1 l , Tome 44, novernbre 1933 page C7-319

LASER-ENHANCED IONIZATION SPECTROMETRY WITH A TOTAL CONSUMPTION BURNER

R . B . Green and J . E . Hall

DeparLment o f Chemistry, Universi tg oj' Arkansas, Faayettevi l Le, Arkansas 7 2 7 0 1 , U.S.A.

RGsum6 - Cet expos6 d C c r i t l ' u t i l i s a t i o n d ' u n b r Q l e u r B consommation complbte comme r e s e r v o i r d'atomes pour l ' a n a l y s e de t r a c e s par s p e c t r o n 6 t r i e l a s e r B augmentation de l ' i o n i s a t i o n . Les l i m i t e s de d C t e c t i o n e t d 1 i n t e r f 6 r e n c e s de m a t r i c e s sont comparees pour l e b r Q l e u r B consommation complbte e t l e brGleur B c o m b u s t i b l e s pr6m6lang6s. Ces r 6 s u l t a t s montrent que des mesures 5 grande s e n s i b i l i t 6 par a c c r o i s s e m e n t d ' i o n i s a t i o n l a s e r s o n t p o s s i b l e s avec d e s e c h a n t i l l o n s dans des m i l i e u x d g f a v o r a b l e s pour l e s q u e l s l e s mgthodes de spec- t r o m s t r i e o p t i q u e s ' 6 t a i e n t r6v616es i n s u f f i s a n t e s .

A b s t r a c t

-

U n t i l now, o n l y premixed burners have been used f o r a n a l y t i c a l laser-enhanced i o n i - z a t i o n ( L E I ) s p e c t r o m e t r y . T h i s paper r e p o r t s t h e u s e o f a t o t a l consumption burner f o r LE1 s p e c t r o m e t r y . Trace m e t a l d e t e r m i n a t i o n s u s i n g LE1 s p e c t r o m e t r y employ a dye l a s e r tuned t o a d i s c r e t e a b s o r p t i o n t r a n s i t i o n o f an a n a l y t e atomized i n a f l a m e . The l a s e r e x c i t a t i o n enhances t h e thermal ( c o l l i s i o n a l ) i o n i z a t i o n r a t e o f t h e a n a l y t e a t o m s , producing a measurable c u r r e n t i n t h e flame. The l a s e r - r e l a t e d c u r r e n t i n t h e flame i s d e t e c t e d w i t h biased e l e c t r o d e s and i s a measure o f t h e c o n c e n t r a t i o n o f t h e absorbing s p e c i e s (1).

In a premixed b u r n e r , t h e f u e l and o x i d a n t a r e mixed w i t h t h e a s p i r a t e d sample s o l u t i o n prior t o combustion. The l a r g e s o l v e n t d r o p l e t s which remain a f t e r nebu- l i z a t i o n a r e condensed on b a f f l e s w i t h i n t h e premixing chamber and e l i m i n a t e d . N i n e t y percent or more o f t h e sample may be l o s t , b u t t h i s i s n e c e s s a r y t o a c h i e v e a q u i e t laminar f l o w flame amenable t o o p t i c a l s p e c t r o m e t r y . Due t o t h e p o t e n t i a l f o r e x p l o s i v e m i x t u r e s i n t h e premixing chamber, t h e c h o i c e o f f u e l and o x i d a n t i s l i m i t e d .

Although e x c e l l e n t l i m i t s o f d e t e c t i o n have been reported f o r LE1 s p e c t r o m e t r y w i t h premixed b u r n e r s , t h e t o t a l consumption burner has s e v e r a l p o t e n t i a l advantages.

In t h e t o t a l consumption b u r n e r , t h e e n t i r e sample i s a s p i r a t e d i n t o t h e flame.

T h i s should r e s u l t i n more atoms i n t h e f l a m e , e v e n c o n s i d e r i n g t h e unvaporized s o l u t i o n p r e s e n t , producing a n e t g a i n i n s e n s i t i v i t y . LE1 d e t e c t i o n i s not s e n s i - t i v e t o s c a t t e r e d source l i g h t as are purely o p t i c a l t e c h n i q u e s and so should not be l i m i t e d by s c a t t e r i n g from unvaporized s o l u t i o n d r o p l e t s . S i n c e t h e f u e l and o x i d a n t are mixed a t t h e n o z z l e o f t h e t o t a l consumption burner t h e r e i s no possi- b i l i t y o f f l a s h b a c k . r e g a r d l e s s o f t h e choice o f f u e l and o x i d a n t . Rapid-propagat- i n g , h i g h - t e m p e r a t u r e , oxygen-based f l a m e s w i l l be a v a i l a b l e f o r LE1 s p e c t r o m e t r y w i t h t h e t o t a l consumption b u r n e r , p e r m i t t i n g more e f f e c t i v e u t i l i z a t i o n o f atomic t r a n s i t i o n s o r i g i n a t i n g i n e x c i t e d s t a t e s . High t e m p e r a t u r e flames w i l l a l s o be u s e f u l f o r d e t e r m i n a t i o n o f m e t a l s which form r e f r a c t o r y o x i d e s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983729

C7-320 JOURNAL DE PHYSIQUE

T h i s paper d e s c r i b e s t h e f i r s t use o f a t o t a l consumption burner as an a n a l y t i c a l atom r e s e r v o i r f o r LE1 s p e c t r o m e t r y . In a d d i t i o n , t h e t o t a l consumption burner and t h e premixed burner a r e compared f o r l i m i t s o f d e t e c t i o n and m a t r i x i n t e r f e r e n c e s . P o t e n t i a l improvements t o t h e t o t a l consumption burner are suggested f o r LE1 spec- t r o m e t r y .

Experimental Sect i o n

Apparatus: The b a s i c e x p e r i m e n t a l apparatus i s described i n d e t a i l elsewhere ( 2 ) . A Chromatix CMX-4 flashlamp-pumped dye l a s e r w i t h f r e q u e n c y doubling c a p a b i l i t y was used as t h e e x c i t a t i o n source f o r some experiments (Chromatix, Inc., Sunnyvale, CA). I t was operated a t 10 Hz w i t h a l ps pulse d u r a t i o n w i t h rhodamine 590 l a s e r dye ( E x c i t o n Chemical Co., Dayton, O H ) . In o t h e r e x p e r i m e n t s , a frequency-doubled DCM ( E x c i t o n Chemical Co.) dye l a s e r (PDL-1, Quanta-Ray, Mountain V i e w , CA) whichl was pumped by t h e second harmonic (532 m , 225 m j l p u l s e ) o f a Nd:YAG l a s e r (DCR-1, Quanta-Ray) was t h e e x c i t a t i o n source. The dye l a s e r o u t p u t was doubled t o provide t u n a b l e u l t r a v i o l e t e m i s s i o n w i t h a 58' KD*P c r y s t a l i n a wavelength e x t e n s i o n s y s t e m ( W E X , Quanta-Ray). The l a s e r was operated a t 10 Hz w i t h a 6 ns pulse dura- t i o n . A 5X t e l e s c o p e ( O r i e l Corporation. S t a m f o r d , CT) was used t o expand t h e dye l a s e r beams f o r some e x p e r i m e n t s .

The sample was a s p i r a t e d i n t o both a Beckman atomizer burner (No. 4020 f o r hydro- genloxygen and h y d r o g e n l a i r or No. 4030 f o r a c e t y l e n e l o x y g e n and a c e t y l e n e l a i r ) and a commercial atomic a b s o r p t i o n premixed burner (Model 370, Perkin-Elmer Corp., Norwalk, CT) w i t h a 10-cm s l o t burner head ( a c e t y l e n e l a i r ) . Flame c u r r e n t measure- ments were made w i t h water-cooled immersed e l e c t r o d e s c o n s t r u c t e d from s t a i n l e s s s t e e l t u b i n g ( 3 ) , connected t o a grounded t a p - w a t e r l i n e . The l a s e r beam was posi- t i o n e d near t h e c a t h o d e ( s ) i n each o f t h e c o n f i g u r a t i o n s used. Enough clearance was maintained t o prevent t h e l a s e r beam from s t r i k i n g t h e e l e c t r o d e s u r f a c e . E i t h e r t h e burner head or a second water-cooled e l e c t r o d e served as t h e anode, depending on t h e e l e c t r o d e c o n f i g u r a t i o n . The c a t h o d e ( s ) was r o u t i n e l y maintained a t -1000 V (Model 4150 HV Power S u p p l y , Fluke Mfg. I n c . , S e a t t l e , WA) w i t h r e s p e c t t o ground. The s i g n a l pulses corresponding t o t h e enhanced i o n i z a t i o n s i g n a l were f i l t e r e d , a m p l i f i e d , d i s p l a y e d on an o s c i l l o s c o p e (Model 1741A, Hewlett-Packard Co.. Palo A l t o , C A ) , and processed w i t h a boxcar s i g n a l averager (Model 162 w i t h Model 164 gated i n t e g r a t o r , P r i n c e t o n . NJ) w i t h a 0.5-ps gate and a l - s time c o n s t a n t . The o s c i l l o s c o p e and t h e s i g n a l processing e l e c t r o n i c s were t r i g g e r e d by a photodiode which monitored t h e l a s e r pulse. The o u t p u t o f t h e boxcar averager was i n t e r f a c e d t o an a n a l o g - t o - d i g i t a l c o n v e r t e r , and t h e d i g i t a l v a l u e s were f u r t h e r processed by a microcomputer ( 4 ) . S i g n a l s were a l s o recorded on a s t r i p c h a r t recorder (Sargent-Welch, S k o k i e , I L ) i n some c a s e s .

Reagents: Aqueous standards were prepared from h i g h - p u r i t y indium o x i d e , copper powder, and aluminum powder ( S p e x I n d u s t r i e s Inc., Metuchen, NJ) according t o t h e procedure described i n R e f . 5. Reagent-grade sodium c h l o r i d e ( F i s h e r S c i e n t i f i c Co., Fairlawn, NJ) was used t o prepare a n a l y t e and m a t r i x s o l u t i o n s .

Procedure: A BASIC program ( 4 ) f o r t h e microcomputer was used t o a c q u i r e d a t a , t o determine v a l i d s t a t i s t i c a l means f o r b l a n k s , standards and samples, and t o calcu- l a t e s i g n a l - t o - n o i s e r a t i o s ( 6 ) f o r t h e a n a l y t e . As i n previous s t u d i e s ( 7 ) , t h e d e t e c t i o n l i m i t s were d e f i n e d as t h e c o n c e n t r a t i o n which gave a s i g n a l t h r e e t i m e s t h e blank n o i s e l e v e l . D e t e c t i o n l i m i t s were e x t r a p o l a t e d from average s i g n a l - t o - n o i s e measurements f o r pure, aqueous a n a l y t e s o l u t i o n s which were t e n t i m e s more

concentrated than t h e e s t i m a t e d d e t e c t i o n l i m i t . R e s u l t s and Discussion

The p o s s i b l e e l e c t r o d e c o n f i g u r a t i o n s f o r use w i t h t h e t o t a l consumption burner were compared u s i n g indium as t h e a n a l y t e . The s i x e l e c t r o d e c o n f i g u r a t i o n s which were evaluated w i t h t h e t o t a l consumption burner are i l l u s t r a t e d i n Figure l . The immersed cathodes and anodes were c o n s t r u c t e d by p a r t i a l l y f l a t t e n i n g l e n g t h s o f s t a i n l e s s s t e e l t u b i n g and grinding t h e e l e c t r o d e s u r f a c e smooth. The c i r c u l a r ,

water-cooled anode was a 3.18-mm t u b e which had been c o i l e d i n t o a 13-m i . d . r i n g . The immersed e l e c t r o d e c o n f i g u r a t i o n used w i t h t h e premixed burner i s d e s c r i b e d i n R e f . 3. The flashlamp-pumped dye l a s e r was used f o r t h e e l e c t r o d e s u r v e y .

Figure 1. Schematic diagrams o f t h e s i x e l e c t r o d e c o n f i g u r a t i o n s used w i t h t h e t o t a l consumption burner. The e l e c t r o d e s are d a r k . The l a s e r beam (*) i s l o c a t e d

10 cm above t h e burner n o z z l e .

I n i t i a l l y , t h e e x c i t a t i o n p o s i t i o n which gave t h e b e s t s i g n a l - t o - n o i s e r a t i o f o r 100 ~.lg/mL indium w i t h t h e t o t a l consumption burner was determined. To determine t h e optimum e x c i t a t i o n h e i g h t above t h e burner head, t h e r e l a t i o n s h i p o f t h e l a s e r beam was f i x e d w h i l e t h e burner was t r a n s l a t e d v e r t i c a l l y t o i d e n t i f y t h e l o c a t i o n o f t h e maximum s i g n a l - t o - n o i s e r a t i o . The e l e c t r o d e s were separated by 5 m which i s s l i g h t l y more than t h e l a s e r beam d i a m e t e r . The optimum e x c i t a t i o n h e i g h t i n t h e a c e t y l e n e l a i r flame was found t o be 9.5 cm above t h e burner n o z z l e , but 50% or more o f t h e LE1 s i g n a l could be recovered a t any e x c i t a t i o n h e i g h t between 7.5 and 11.0 cm. The optimum e x c i t a t i o n h e i g h t f o r t h e premixed burner has been reported a s 1 cm ( 2 ) . The burner was a l s o t r a n s l a t e d h o r i z o n t a l l y w i t h t h e e l e c t r o d e s ' p o s i t i o n f i x e d and t h e l a s e r beam c e n t e r e d between them. The LE1 s i g n a l - t o - n o i s e r a t i o was g r e a t e s t when t h e e l e c t r o d e s were h o r i z o n t a l l y c e n t e r e d i n t h e flame a l t h o u g h , as w i t h t h e h e i g h t measurements, t h e h i g h c o n c e n t r a t i o n zone was not w e l l d e f i n e d . The optimum l a s e r beam p o s i t i o n f o r e x c i t a t i o n was used throughout t h e remaining e x p e r i m e n t s .

Although t h e most w i d e l y used e l e c t r o d e c o n f i g u r a t i o n f o r t h e premixed burner employs t h e burner head as t h e anode, t h i s approach was not s a t i s f a c t o r y ( F i g u r e 1A and Table I ) .

C7-322 JOURNAL DE PHYSIQUE

T a b l e I. LE1 d e t e c t i o n l i m i t s f o r indium u s i n g a flashlamp-pumped dye l a s e r . The symbol "exp" r e f e r s t o e x p e r i m e n t a l d a t a w h i l e " a d j " i s t h e d a t a a d j u s t e d f o r a

10-cm u a t h l e n e t h . Burner

T o t a l Consumption

E l e c t r o d e C o n f i g u r a t i o n

Imme r s e d E l e c t r o d e * Imme r s e d E l e c t r o d e +

L i m i t s of D e t e c t i o n (ng/mL)

exp a d l

*

+ ~ e f . 1

Combustion was n o t o b s e r v e d w i t h t h e a c e t y l e n e / a i r m i x t u r e u n t i l a p p r o x i m a t e l y 2 cm above t h e b u r n e r n o z z l e due t o i n c o m p l e t e mixing of t h e f u e l and o x i d a n t . L i m i t e d c o n d u c t i o n a c r o s s t h e gap between t h e flame and t h e n o z z l e makes s i g n a l c o l l e c t i o n d i f f i c u l t a t b e s t , e s p e c i a l l y c o n s i d e r i n g t h e r e l a t i v e l y h i g h optimum e x c i t a t i o n h e i g h t . C o n f i g u r a t i o n s B-D produced r o u g h l y e q u i v a l e n t d e t e c t i o n l i m i t s . By r o t a t i n g t h e e l e c t r o d e s 90° t h e p a t h l e n g t h was i n c r e a s e d a p p r o x i m a t e l y two-and-a- h a l f t i m e s , and t h e d e t e c t i o n limits were lowered c o n s i d e r a b l y . The b e s t d e t e c t i o n l i m i t f o r indium w i t h t h e t o t a l consumption b u r n e r (120 pg/mL indium) was o b t a i n e d when two water-cooled immersed e l e c t r o d e s were used i n a h o r i z o n t a l p o s i t i o n ( F i g u r e 1E and T a b l e I ) . The e l e c t r o d e s were s e p a r a t e d by 5 m, a d i s t a n c e s l i g h t - l y g r e a t e r t h a n t h e d i a m e t e r of t h e l a s e r beam. G r e a t e r e l e c t r o d e s e p a r a t i o n s produced a d e c r e a s e i n t h e LE1 s i g n a l . The d i f f e r e n c e i n d e t e c t i o n l i m i t s between c o n f i g u r a t i o n s E and F may be a t t r i b u t e d mainly t o sample removal v i a c o n d e n s a t i o n o n t h e c i r c u l a r water-cooled anode a t t h e base of t h e flame. The n o i s e l e v e l remained t h e same f o r c o n f i g u r a t i o n s E and F because t h e t u r b u l e n c e produced by t h e i n t r o d u c t i o n of a t h i r d e l e c t r o d e o f f s e t any improvements due t o removal of t h e l a r g e u n v a p o r i z e d sample d r o p l e t s .

A s t h e f l a m e c o n c e n t r a t i o n p r o f i l e s s u g g e s t e d , t h e r e g i o n of t h e h i g h a n a l y t e con- c e n t r a t i o n i s n o t c o n f i n e d t o t h e l a s e r beam volume. T h e r e f o r e , a n e x p a n s i o n of t h e l a s e r beam may i n c r e a s e t h e amount of s i g n a l c o l l e c t e d . The flashlamp-pumped dye l a s e r beam was expanded a f a c t o r of f i v e w i t h a t e l e s c o p e . The two e l e c t r o d e c o n f i g u r a t i o n s which gave t h e l o w e s t l i m i t s of d e t e c t i o n were i n v e s t i g a t e d f u r t h e r w i t h t h e expanded beam. The e l e c t r o d e s were s e p a r a t e d by a d i s t a n c e s l i g h t l y g r e a t e r t h a n t h e beam d i a m e t e r . C o n s i d e r i n g t h e power l o s s due t o t h e t e l e s c o p e o p t i c s , no s i g n i f i c a n t d i f f e r e n c e i n t h e d e t e c t i o n l i m i t s f o r indium was o b s e r v e d . T h i s r e s u l t i n d i c a t e s t h a t t h e 303.9 nm indium t r a n s i t i o n p r o b a b l y was n o t

s a t u r a t e d w i t h t h e normal o r unexpanded flashlamp-pumped dye l a s e r beam. Expansion o f t h e e x c i t a t i o n beam a l s o r e q u i r e d a l a r g e r e l e c t r o d e s e p a r a t i o n which may have o f f s e t any g a i n s r e s u l t i n g from i r r a d i a t i n g a l a r g e r sample volume.

The s i z e of t h e t u b i n g f o r t h e e l e c t r o d e s was i n c r e a s e d from 6.35 m t o 12.70 m i n 3.18 mm i n c r e m e n t s . These changes c o r r e s p o n d e d t o s t a n d a r d t u b i n g d i a m e t e r s . The maximum s i g n a l - t o - n o i s e r a t i o was o b s e r v e d w i t h t h e flashlamp-pumped dye l a s e r f o r 9.53 m e l e c t r o d e s . Only a n a l y t e atoms w i t h i n t h e l a s e r beam volume a r e e x c i t e d and c o n t r i b u t e t o t h e s i g n a l w h i l e i n t e r f e r i n g i o n s a r e c o l l e c t e d o v e r t h e e n t i r e e l e c t r o d e s u r f a c e w i t h i n t h e flame. T h e r e f o r e , i n c r e a s i n g t h e e l e c t r o d e a r e a beyond a s p e c i f i c p o i n t does n o t produce a d d i t i o n a l s i g n a l b u t does c o n t r i b u t e t o i n c r e a s e d n o i s e .

It i s d i f f i c u l t t o compare v a r i o u s e l e c t r o d e c o n f i g u r a t i o n s on a n e q u a l b a s i s b e c a u s e t h e c o l l e c t i o n f i e l d s d i f f e r . S i n c e t h e g o a l o f t h e p r e s e n t r e s e a r c h was t o i d e n t i f y t h e b e s t e l e c t r o d e c o n f i g u r a t i o n f o r t h e t o t a l consumption b u r n e r and compare i t w i t h t h e best r e s u l t s f o r t h e premixed b u r n e r , d i r e c t c o m p a r i s o n s were m e a n i n g f u l . The d e t e c t i o n l i m i t s o b t a i n e d u s i n g t h e t o t a l consumption and p r e m i x b u r n e r s w i t h t h e flashlamp-pumped dye l a s e r a r e shown i n T a b l e I. The e x p e r i m e n t a l d e t e c t i o n l i m i t s o b t a i n e d w i t h t h e t o t a l c o n s u m p t i o n b u r n e r a r e a b o u t a f a c t o r of f i v e h i g h e r t h a n t h e d e t e c t i o n l i m i t s o b t a i n e d i n t h i s l a b o r a t o r y w i t h t h e premixed b u r n e r . The a d j u s t e d d e t e c t i o n l i m i t s shown i n T a b l e I t a k e i n t o a c c o u n t t h e p a t h - l e n g t h d i f f e r e n c e which l o w e r s t h e t o t a l c o n s u m p t i o n b u r n e r d e t e c t i o n l i m i t s by a f a c t o r of f o u r . The a d j u s t e d d e t e c t i o n limit f o r t h e t o t a l c o n s u m p t i o n b u r n e r a g r e e s a p p r o x i m a t e l y w i t h t h e r e s u l t s f o r t h e premixed b u r n e r w i t h a 10-cm s l o t head.

S i n c e c o m p a r a b l e d e t e c t i o n ' l i m i t s were d e t e r m i n e d f o r i n d i u m u s i n g t h e t o t a l con- s u m p t i o n and premixed b u r n e r s , a n o t h e r e l e m e n t was i n v e s t i g a t e d . F o r e l e m e n t s , s u c h a s i n d i u m , where e x t r e m e l y low LE1 d e t e c t i o n limits have been d e t e r m i n e d , f u r t h e r improvements i n s e n s i t i v i t y may be d i f f i c u l t p a r t i a l l y b e c a u s e t h e l i m i t i n g f a c t o r may be t h e atom f r a c t i o n i n t h e f l a m e ( 1 ) . Copper (324.8 nm) was c h o s e n a s t h e a n a l y t e b e c a u s e s i n g l e - w a v e l e n g t h LE1 d e t e c t i o n l i m i t s c a n be improved by s t e p - w i s e e x c i t a t i o n ( 7 ) . The best e l e c t r o d e c o n f i g u r a t i o n ( F i g u r e 1E) and t h e Nd:YAG l a s e r (YAG)-pumped d y e l a s e r were u s e d f o r t h i s s t u d y . Because t h e dye l a s e r ' s beam d i a m e t e r was a b o u t 1 mm l a r g e r t h a n t h e flashlamp-pumped d y e (FL) l a s e r beam, t h e e l e c t r o d e s i z e d e p e n d e n c e was r e - e v a l u a t e d . The s i g n a l - t o n o i s e r a t i o o b t a i n e d w i t h t h e 12.70 mm e l e c t r o d e s a d t h e YAG-pumped d y e l a s e r was b e t t e r t h a n when t h e FL laser was u s e d b u t i t was c o m p a r a b l e t o t h e 9.53 mm e l e c t r o d e s . The 9.53 tmn

e l e c t r o d e s were used f o r s u b s e q u e n t work. F o r p u r p o s e s of c o m p a r i s o n w i t h p u b l i s h e d d e t e c t i o n l i m i t s ( 7 ) , -1500 V was a p p l i e d t o t h e e l e c t r o d e s .

The d e t e c t i o n l i m i t s o b t a i n e d f o r c o p p e r u s i n g t h e t o t a l c o n s u m p t i o n a n d t h e p r e - mixed b u r n e r w i t h a 10-cm h e a d a r e shown i n T a b l e 11. R e s u l t s are g i v e n f o r b o t h t h e normal o r unexpanded a n d t h e expanded dye l a s e r beam. The unexpanded

YAG-pumped, f r e q u e n c y - d o u b l e d dye l a s e r beam was 4 mm i n d i a m e t e r . A 5 X e x p a n s i o n p r o d u c e d a 201nm d i a m e t e r beam. A m u l t i p h o t o n i o n i z a t i o n (MPI) background c u r r e n t was o b s e r v e d when u s i n g e i t h e r t h e t o t a l c o n s u m p t i o n o r p r e m i x e d b u r n e r w i t h t h e laser a t f u l l power w i t h b o t h t h e YAG o s c i l l a t o r and a m p l i f i e r o p e r a t i n g . The MP1 background may be compensated f o r by s u b t r a c t i o n b u t t h e a d d i t i o n a l n o i s e was d e t r i m e n t a l t o s e n s i t i v i t y . I n p r a c t i c e , t h e b e s t LE1 d e t e c t i o n l i m i t s were a c h i e v e d by r e d u c i n g t h e YAG power u n t i l t h e MP1 background d i s a p p e a r e d . When t h e beam was e x p a n d e d , t h e l a s e r c o u l d be o p e r a t e d a t maximum power w i t h t h e t o t a l con- s u m p t i o n b u r n e r . The l a s e r power s t i l l had t o be r e d u c e d t o e l i m i n a t e t h e MP1 background w i t h a n expanded beam a n d t h e premixed b u r n e r b u t n o t a s much. The beam e x p a n s i o n improved t h e t o t a l c o n s u m p t i o n b u r n e r d e t e c t i o n l i m i t s c o n s i d e r a b l y w h i l e t h e premixed b u r n e r r e s u l t s remained e s s e n t i a l l y t h e same. The c i r c u l a r geometry a n d mre d i f f u s e h i g h c o n c e n t r a t i o n zone i n t h e t o t a l c o n s u m p t i o n f l a m e b e n e f i t s f r o m t h e l a r g e r e x c i t a t i o n volume, e v e n t h o u g h t h e e l e c t r o d e s mst be s e p a r a t e d c o m m e n s u r a t e l y . When t h e d e t e c t i o n l i m i t s a r e a d j u s t e d f o r t h e p a t h l e n g t h d i f f e r - e n c e i n t h e two b u r n e r s , t h e t o t a l c o n s u m p t i o n b u r n e r d e t e c t i o n limits a r e a b o u t a f a c t o r of 5 b e t t e r t h a n t h o s e o b t a i n e d w i t h t h e premixed b u r n e r . S i n g l e - w a v e l e n g t h c o p p e r d e t e c t i o n l i m i t s o b t a i n e d w i t h a YAG ( o s c i l l a t o r only)-pumped d y e l a s e r ( 7 ) a r e a l s o g i v e n i n T a b l e I1 f o r comparison. A l l of t h e c o p p e r d e t e c t i o n l i m i t s o b t a i n e d i n t h e p r e s e n t s t u d y w i t h b o t h t h e t o t a l c o n s u m p t i o n a n d premixed b u r n e r a r e l o w e r t h a n have been p r e v i o u s l y r e p o r t e d f o r s i n g l e - w a v e l e n g t h e x c i t a t i o n ( 7 ) . The d e t e c t i o n l i m i t s r e p o r t e d h e r e f o r i n d i u m a n d c o p p e r u t i l i z e d t h e a c e t y l e n e l a i r f l a m e s u p p o r t e d on a Beckman k u r n e r . T h i s f l a m e was c h o s e n b e c a u s e o f i t s a n a l y t i - c a l u t i l i t y . A l s o p r e v i o u s l y - p u b l i s h e d d e t e c t i o n limits u s i n g a premixed b u r n e r w i t h a c e t y l e n e l a i r w e r e h e l p f u l i n e v a l u a t i n g t h e p e r f o r m a n c e o f t h e t o t a l consump- t i o n b u r n e r . L a r g e LE1 s i g n a l s w e r e a l s o o b s e r v e d f o r sodium a n d i n d i u m w i t h h y d r o g e n l a i r , h y d r o g e n l o x y g e n and a c e t y l e n e / o x y g e n f l a m e s .

C7-324 JOURNAIL DE PHYSIQUE

T a b l e 11. LE1 d e t e c t i o n l i m i t s f o r copper u s i n g a Nd:YAG laser-pumped dye l a s e r . The symbol "exp" r e f e r s t o e x p e r i m e n t a l d a t a w h i l e " a d j " i s t h e d a t a a d j u s t e d f o r a 10-cm p a t h l e n g t h . _-

E l e c t r o d e Beam L i m i t s of D e t e c t i o n (ng/mL)

Burner C o n f i g u r a t i o n Diameter exP ad j

T o t a l Consumption E

Premixed Immersed 4 mm 2.2

E l e c t r o d e * 20 m 2.3

Immersed

~ l e c t r o d e + 4 mm

*

+ Ref. 7.

The l i m i t of d e t e c t i o n f o r aluminum was d e t e r m i n e d t o be 3 2 ng/mL u s i n g a n a c e t y l e n e / o x y g e n f l a m e w i t h t h e t o t a l consumption b u r n e r . This v a l u e was much l a r g e r t h a n t h e 1 ng/mL v a l u e ( 1 ) r e p o r t e d f o r t h e premixed b u r n e r w i t h a n

a c e t y l e n e l n i t r o u s o x i d e flame. Flame t e m p e r a t u r e s ( 8 ) i n d i c a t e t h a t t h e e x t r e m e l y h o t oxygen-based f l a m e s s h o u l d d i s s o c i a t e r e f r a c t o r y o x i d e s t o a g r e a t e r e x t e n t t h a n a c e t y l e n e l n i t r o u s o x i d e f l a m e s . However, t h e a s p i r a t i o n of aqueous s o l u t i o n s i n t o t h e f l a m e l o w e r s t h e t e m p e r a t u r e f o r hydrogenloxygen and a c e t y l e n e l o x y g e n f l a m e s by 400" and 80OoC, r e s p e c t i v e l y , i n t h e Beckman a t o m i z e r b u r n e r ( 9 ) . The t e m p e r a t u r e of t h e a c e t y l e n e l o x y g e n and t h e hydrogenloxygen f l a m e s i n a t o t a l con- sumption b u r n e r w i t h w a t e r a s p i r a t e d a r e a p p r o x i m a t e l y e q u a l t o t h e t e m p e r a t u r e of a n a c e t y l e n e l a i r f l a m e , and a r e 600°C l o w e r t h a n t h e a c e t y l e n e l n i t r o u s o x i d e flame i n t h e premixed b u r n e r . With premixed b u r n e r s , f l a m e t e m p e r a t u r e s a r e a l s o reduced by aqueous s o l v e n t s , but not s i g n i f i c a n t l y s i n c e o n l y t e n p e r c e n t of t h e a s p i r a t e d s o l u t i o n r e a c h e s t h e flame. The u s e of a n o r g a n i c s o l v e n t was i n v e s t i g a t e d t o p r e - v e n t a d e c r e a s e i n flame t e m p e r a t u r e o b s e r v e d w i t h aqueous s o l u t i o n s . When e t h a n o l was u s e d , t h e b l a n k n o i s e was i n c r e a s e d by a f a c t o r of t h r e e . T h i s i n c r e a s e was due t o a d d i t i o n a l i n t e r f e r i n g i o n s produced by combustion of t h e s o l v e n t , t h u s n e g a t i n g t h e a d v a n t a g e of i n c r e a s e d flame t e m p e r a t u r e .

The e f f e c t of a sample m a t r i x on a LE1 s i g n a l was s t u d i e d f o r t h e two e l e c t r o d e c o n f i g u r a t i o n s ( F i g u r e 1E and IF) which gave t h e l o w e s t l i m i t s of d e t e c t i o n . Indium was used a s t h e a n a l y t e and sodium a s t h e m a t r i x f o r i n t e r f e r e n c e s t u d i e s . The LE1 s i g n a l f o r 100 ng1mL of indium, e x c i t e d a t 303.9 nm r e s o n a n c e l i n e , was m o n i t o r e d a s t h e m a t r i x c o n c e n t r a t i o n of sodium was i n c r e a s e d . Complete s i g n a l r e c o v e r y was o b s e r v e d w i t h a s nuch a s 1500 pg/mL of sodium p r e s e n t i n t h e s o l u t i o n . When t h e c o n c e n t r a t i o n exceeded 1500 pgImL, t h e h i g h i o n c o n c e n t r a t i o n i n t h e f l a m e s h o r t - c i r c u i t e d t h e e l e c t r o d e s and s h u t down t h e HV power s u p p l y . A l l of t h e LE1 s i g n a l f o r indium i n a 3000 pg/mL sodium s o l u t i o n could be r e c o v e r e d u s i n g a p r e - mixed b u r n e r ( 3 ) b e c a u s e o n l y t e n p e r c e n t of t h e a s p i r a t e d s o l u t i o n r e a c h e d t h e premixed b u r n e r flame. T h i s r e s u l t e d i n a much s m a l l e r f l a m e c o n c e n t r a t i o n of t h e sodium m a t r i x . The u s e of i o n removal e l e c t r o d e s ( 1 0 ) o r p r i o r chromatography ( 1 1 ) s h o u l d r e d u c e m a t r i x i n t e r f e r e n c e e f f e c t s and a l l o w a n e x t e n s i o n of t h e m a t r i x con- c e n t r a t i o n which w i l l be t o l e r a t e d by t h e e l e c t r o d e system.

F u r t h e r o p t i m i z a t i o n of t h e e l e c t r o d e s y s t e m u s e d f o r s i g n a l c o l l e c t i o n w i t h t h e t o f a l consumption b u r n e r i s underway i n t h i s l a b o r a t o r y and s h o u l d r e s u l t i n even l o w e r d e t e c t i o n l i m i t s . A s a r e s u l t of t h e p r e l i m i n a r y work d e s c r i b e d a b o v e , t h e f o l l o w i n g s t r a t e g i e s have been d e v e l o p e d f o r i m p r o v i n g d e t e c t i o n l i m i t s w i t h t h e t o t a l consumption b u r n e r : ( 1 ) r e d u c e n o i s e ; ( 2 ) i n v e s t i g a t e o t h e r e l e c t r o d e m a t e r i a l s ; ( 3 ) improve sample n e b u l i z a t i o n e f f i c i e n c y ; ( 4 ) d e s o l v a t e t h e a n a l y t e ; ( 5 ) c o n f i n e t h e sample t o a s m a l l e r volume; and (6) i n c r e a s e t h e p a t h l e n g t h . The b a l a n c e of t h i s p a p e r w i l l d e s c r i b e t h e i n i t i a l e x p e r i m e n t s t o implement t h e s e s t r a t e g i e s .

The e n t i r e e l e c t r o d e which i s exposed t o t h e flame c o l l e c t s background i o n s w h i l e a more l i m i t e d r e g i o n i n proximity t o t h e l a s e r beam c o l l e c t s s i g n a l i o n s . There- f o r e , i n s u l a t i o n o f t h e non-signal c o l l e c t i n g p o r t i o n o f t h e e l e c t r o d e s should reduce n o i s e and improve d e t e c t i o n l i m i t s . A h i g h temperature epoxy has been used t o i n s u l a t e t h e water cooled e l e c t r o d e s . D e t e c t i o n l i m i t s have been reduced somewhat as a r e s u l t but decomposition o f t h e i n s u l a t i o n i t s e l f may be c o n t r i b u t i n g t o background n o i s e . Further t e s t i n g a w a i t s t h e c o n s t r u c t i o n o f ceramic-insulated e l e c t r o d e s .

Other e l e c t r o d e m a t e r i a l s are being i n v e s t i g a t e d . Extremely low d e t e c t i o n l i m i t s have been achieved w i t h g r a p h i t e rod e l e c t r o t h e r m a l a t o m i z e r s p a r t i a l l y because t h e g r a p h i t e provides a reducing s u r f a c e . Graphite e l e c t r o d e s have been used f o r LE1 spectrometry i n t h i s l a b o r a t o r y but degradation i s a problem when t h e e l e c t r o d e i s immersed i n t h e flame. Oxygen-based f l a m e s which would be u s e f u l f o r r e f r a c t o r y e l e m e n t s a r e p a r t i c u l a r l y d e s t r u c t 1 ve o f g r a p h i t e s u r f a c e s . I f t h e g r a p h i t e e l e c t r o d e s are adequately water-cooled, degradation i s c o n s i d e r a b l y reduced. Work i s i n progress t o f u r t h e r e v a l u a t e g r a p h i t e e l e c t r o d e s f o r LE1 s p e c t r o m e t r y . I n t h e t o t a l consumption burner, some l a r g e s o l u t i o n d r o p l e t s do not v a p o r i z e i n t h e flame and, t h e r e f o r e , e f f e c t i v e l y remove sample from t h e atom r e s e r v o i r . Improved n e b u l i z a t i o n e f f i c i e n c y should y i e l d lower d e t e c t i o n l i m i t s . S e v e r a l approaches t o more e f f i c i e n t n e b u l i z a t i o n are being i n v e s t i g a t e d i n t h i s

l a b o r a t o r y . Sample s o l u t i o n s are being pneumatically and u l t r a s o n i c a l l y n e b u l i z e d i n t o t h e o x i d a n t stream o f t h e t u r b u l e n t f l o w burner. One o f t h e problems has been compensating f o r t h e h i g h back pressure produced by i n t r o d u c i n g t h e sample through t h e r e s t r i c t e d oxidant o r i f i c e . Several s o l u t i o n s t o t h i s problem are being i n v e s t i g a t e d . The sample i n t r o d u c t i o n s y s t e m may be pressured or a l a r g e r through- put burner may be n e c e s s a r y .

To e f f e c t i v e l y use t h e t o t a l consumption burner w i t h . h i g h temperature oxygen-based f l a m e s , an e f f i c i e n t d e s o l v a t i o n system i s necessary t o reduce c o o l i n g o f t h e f l a m e . Various d e s i g n s ( 1 2 ) are being e v a l u a t e d . A d d i t i o n a l work w i t h t h e t o t a l consumption burner w i l l i n c l u d e a d e t a i l e d s t u d y o f post-mixing o f t h e flame gases w i t h t h e sample. Experimental measurements w i t h t h e e l e c t r o d e c o n f i g u r a t i o n s i l l u s t r a t e d i n Figures l B , D , and F i n d i c a t e t h a t l a r g e sample d r o p l e t s may be removed by condensation a f t e r t h e f u e l , o x i d a n t , and sample have been mixed.

Unvaporized sample reduces t h e flame temperature and i n c r e a s e s t h e flame t u r b u - l e n c e . Post-mixing has been accomplished by i n s e r t i n g a nichrome wire mesh a t t h e base o f t h e flame i n t h e a i r gap. The w i r e mesh a l l o w s t h e l a r g e d r o p l e t s t o be removed w i t h o u t c o n t r i b u t i n g t o t h e flame t u r b u l e n c e , a s does t h e presence o f t h e c i r c u l a r anode ( F i g u r e l B , D , and F ) . Post-mixing w i l l a l l o w f l e x i b i l i t y o f f u e l and o x i d a n t combinations. The sample throughput advantage w i l l be negated somewhat, but n i n e t y percent sample l o s s i s u n l i k e l y .

Expansion o f t h e YAG-dye l a s e r beam has reduced l i m i t s o f d e t e c t i o n because t h e sample i s d i s t r i b u t e d over a l a r g e r e g i o n o f t h e t o t a l consumption burner flame.

Confinement o f t h e sample may a l s o produce improved r e s u l t s . Sample confinement ( r a t h e r t h a n beam e x p a n s i o n ) has t h e advantage t h a t t h e e l e c t r o d e s may be moved c l o s e r t o g e t h e r . The u s e o f a Vycor t u b e t o c o n f i n e t h e flame ( 1 2 ) i s c u r r e n t l y being i n v e s t i g a t e d . A 15-cm l o n g , 15-mm diameter Vycor t u b e i s positioned horizon- t a l l y . The t o t a l consumption burner i s t i p p e d a t an angle t o i n t r o d u c e t h e flame i n t o t h e t u b e w h i l e a l l o w i n g clearance f o r t h e l a s e r beam. S e m i c i r c u l a r g r a p h i t e o r m t a l e l e c t r o d e s i n a c o n c e n t r i c holder f i t i n s i d e t h e Vycor t u b e . A s i m i l a r e l e c t r o d e system which u t i l i z e s two 15-cm l o n g , water-cooled aluminum e l e c t r o d e s separated by g l a s s i n s u l a t o r s i s being e v a l u a t e d a l s o . Again, t h e flame and l a s e r beam w i l l be d i r e c t e d i n t o t h e r e g i o n between t h e e l e c t r o d e s . Both o f t h e s e approaches w i l l i n c r e a s e t h e sampling pathlength as w e l l . A 10 cm l e n g t h o f Vycor t u b i n g has been used as "chimney" over t h e t o t a l consumption burner. The flame i s s t a b i l i z e d and t h e c r o s s - s e c t i o n a l area i s reduced. The flame remains condensed w e l l above t h e t o p o f t h e Vycor t u b e . The LE1 e l e c t r o d e s are positioned above t h e t u b e . O f c o u r s e , t h e p a t h l e n g t h may be increased f o r many e l e c t r o d e c o n f i g u r a t i o n s by m u l t i p l e r e f l e c t i o n s o f t h e l a s e r beam.

C7-326 JOURNAL DE PHYSIQUE

LE1 s p e c t r o m e t r y u s i n g t h e t o t a l consumption b u r n e r , w i t h g r e a t e r sample throughput and a wider range of u s a b l e f u e l l o x i d a n t combinations, expands t h e p o s s i b i l i t i e s f o r development of a more s e n s i t i v e and v e r s a t i l e d e t e c t i o n system f o r atomic s p e c t r o s c o p y . I n a d d i t i o n t o f u r t h e r i n g t h e a n a l y t i c a l m t h o d o l o g y , t h e s e pro- m i s i n g p r e l i m i n a r y r e s u l t s d e m o n s t r a t e t h a t h i g h s e n s i t i v i t y LE1 measurements a r e p o s s i b l e i n a d v e r s e sample environments where t r a d i t i o n a l methods of o p t i c a l s p e c t r o m e t r y have proven i n a d e q u a t e .

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The a u t h o r s acknowledge t h e s u p p o r t of t h e N a t i o n a l Science Foundation under Grant CHE-8105000 and t h e EPSCOR program.