LASER-ENHANCED IONIZATION SPECTROMETRY FOR TRACE METAL ANALYSIS

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LASER-ENHANCED IONIZATION SPECTROMETRY FOR TRACE METAL ANALYSIS

G. Turk, J. Travis, J. Devoe

To cite this version:

G. Turk, J. Travis, J. Devoe. LASER-ENHANCED IONIZATION SPECTROMETRY FOR TRACE METAL ANALYSIS. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-301-C7-309.

�10.1051/jphyscol:1983727�. �jpa-00223284�

JOURNAL D€ PHYSIQUE

Colloque C7, supplCment. a u nO1l, Tome 44, novembre 1983 page C7-301

LASER-ENHANCED I O N I Z A T I O N SPECTROMETRY FOR TRACE M E T A L A N A L Y S I S

G.C. Turk, J.C. Travis and J.R. DeVoe

Center f ~ r Ancr Z y t i c a l Chemistry, U . S . !lJational Rurenu o f Standards, k s h i n y t o n , D.C. 2 0 2 3 4 , U.S.A.

La spectrom6trie laser par augmentation de l'ionisation est une application de la spectromEtrie optogalvanique pour l'analyse quantitative des 61Ements rn@tal.

liques?il'btat d e traces dans les flammes. Cette communication passe en revue la LittCrature scientifique B ce sujet et r&sume les performances de cette mEthode dans son Gtat de d6veloppement actuel.

Abstract

Laser-enhanced ionization spect rometry is an application of optogalvanic s p e c t r o s c o p y for q u a n t i t a t i v e a n a l y s i s of t r a c e c o n c e n t r a t i o n s of m e t a l l i c e l e m e n t s in flames. T h i s paper r e v i e w s t h e s c i e n t i f i c l i t e r a t u r e o n t h i s subject, and summarizes the performance of the method i n its present state of development.

I

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Development o f t h e Method

D u r i n g t h e past decade considerable e f f o r t has been devoted by researchers i n t h e f i e l d o f a n a l y t i c a l chemistry r e g a r d i n g t h e use o f t u n a b l e dye l a s e r s as a l i g h t s o u r c e f o r s p e c t r o c h e m i c a l a n a l y s i s . One o f t h e f r u i t s o f t h i s r e s e a r c h has been t h e development o f a t o t a l l y new and unique technique f o r spectrochemical a n a l y s i s c a l l e d l a s e r - e n h a n c e d 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 . I t i s an a p p l i c a t i o n o f o p t o g a l v a n i c s p e c t r o s c o p y f o r q u a n t i t a t i v e a n a l y s i S o f t r a c e c o n c e n t r a t i o n s o f m e t a l l i c elements i n flames. As w i t h t h e more t r a d i t i o n a l forms o f f l a m e analysis, s o l u t i o n s o f samples t o be a n a l y z e d a r e a s p i r a t e d i n t o t h e f l a m e , where t h e y a r e v a p o r i z e d and decomposed t o a t o m i c form. U s i n g a t u n a b l e dye l a s e r f o r d i s c r e t e p h o t o - e x c i t a t i o n o f t h e element of i n t e r e s t r e s u l t s i n a s i g n i f i c a n t p e r t u b a t i o n o f t h e t h e r m a l e n e r g y l e v e l d i s t r i b u t i o n o f t h o s e atoms. The enhanced r a t e o f c o l l i s i o n a l i o n i z a t i o n o f t h e r e s u l t i n g e x c i t e d s t a t e atoms r e l a t i v e t o t h e ground s t a t e i s t h e b a s i s o f t h e t e c h n i q u e , and t h e s o u r c e o f t h e name

--

l a s e r - e n h a n c e d i o n i z a t i o n . T h i s enhanced r a t e o f i o n i z a t i o n can be detected d i r e c t l y w i t h i n t h e f l a m e w i t h electrodes c o n f i g u r e d as an e l e c t r i c a l probe. The non-optical n a t u r e o f s i g n a l d e t e c t i o n renders

LE1

unique among t h e methods o f spectrochemical analysis.

P r o b l e m s a s s o c i a t e d w i t h o p t i c a l d e t e c t i o n methods, such as i n t e r f e r e n c e f r o m s c a t t e r e d l a s e r r a d i a t i o n , a r e avoided. Laser-enhanced i o n i z a t i o n i s a p r i m e example o f a method o f a n a l y s i s made p o s s i b l e by t h e advent o f t h e laser, and which i s w e l l s u i t e d t o t a k e f u l l advantage o f many d e s i r a b l e p r o p e r t i e s , o f t h e l a s e r as a s p e c t roscopic t o o l .

The f i r s t experiments on optogal vanic spectroscopy i n flames were performed i n 1976 by Green, K e l l e r , Schenck, T r a v i s , and L u t h e r , a t t h e U.S. N a t i o n a l Bureau o f S t a n d a r d s (NBS) (1). U s i n g a chopped c o n t i n u o u s wave dye l a s e r an o p t o g a l v a n i c s i g n a l was observed f o r Na i n an a i r - a c e t y l e n e f l a m e when t h e l a s e r was tuned t o t h e t h e Na D-lines. D e t e c t i o n o f sodium s o l u t i o n concentrations a t t h e ng/mL l e v e l was

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983727

JOURNAL DE PHYSIQUE

accomplished using a s i m p l e d e t e c t i o n system c o n s i s t i n g o f a p a i r o f biased tungsten rods w i t h i n t h e flame, a picoammeter, and a l o c k - i n a m p l i f i e r .

Turk, e t a1 (2,3,4) measured l i m i t s o f d e t e c t i o n f o r 18 elements by LE1 u s i n g a frequency doubled flashlamp pumped dye laser. I n many cases these d e t e c t i o n l i m i t s were o r d e r - o f - m a g n i t u d e i m p r o v e m e n t s o v e r what c o u l d be a c h i e v e d by most o t h e r methods o f analysis, p a r t i c u l a r l y f o r l o w e r i o n i z a t i o n p o t e n t i a l elements such as t h e a l k a l i and Group 111-A metals. D e t e c t i o n l i m i t s f o r h i g h (

>

7 eV) i o n i z a t i o n p o t e n t i a l elements were q u i t e poor. Mu1 t i p l ic a t iv e i n t e r f e r e n c e e f f e c t s were noted i n s i t u a t i o n s where t h e element o f i n t e r e s t was present i n s o l u t i o n s which contained a l k a l i m e t a l s , even a t t h e r e l a t i v e l y l o w c o n c e n t r a t i o n l e v e l o f 1 0 ~ g / m L . T h i s i n t e r f e r e n c e caused enhancement, depression o r complete l o s s o f t h e a n a l y t e s i g n a l depending on t h e c o n c e n t r a t i o n o f a l k a l i m e t a l p r e s e n t . I t was a t t r i b u t e d t o changes i n t h e e l e c t r i c f i e l d g r a d i e n t s between t h e electrodes caused by t h e normal c o l l i s i o n a l i o n i z a t i o n o f a l k a l i metals i n t h e a i r - a c e t y l e n e flame.

Because a l k a l i metals a r e a common c o n s t i t u e n t o f many o f t h e samples which one may wish t o analyze u s i n g LEI, i n t e r f e r e n c e s caused by t h e presence o f such metals i s o f concern. F o r t h i s r e a s o n much e f f o r t has been d e v o t e d t o t h e u n d e r s t a n d i n g and r e d u c t i o n o f t h i s inteference. Green and h i s research group a t t h e U n i v e r s i t y o f Arkansas i n p a r t i c u l a r have made d e t a i l e d s t u d i e s c h a r a c t e r i z i n g t h e e x t e n t and b e h a v i o r o f t h e a l k a l i m e t a l i n t e r f e r e n c e (5-7). Turk (8) has d e s c r i b e d an e l e c t r o d e d e s i g n u t i l i z i n g a w a t e r - c o o l e d c a t h o d e w i t h i n t h e f l a m e w h i c h a1 l o w s alignment o f t h e l a s e r beam c l o s e t o t h e cathode. T h i s design has reduced t h e a l k a l i i n t e r f e r e n c e t o t h e p o i n t where i t s h o u l d no l o n g e r be a m a j o r i m p e d i m e n t t o t h e development o f LE1 as an a n a l y t i c a l technique.

Fundamental s t u d i e s regarding t h e mechanisms o f p r o d u c t i o n and d e t e c t i o n o f LE1 have been an important aspect o f t h e development of LE1 analysis. T r a v i s

,

e t al., ( 9 ) d e v e l o p e d t h e model f o r LE1 i o n p r o d u c t i o n based upon o p t i c a l and c o l l i s i o n a l p r o c e s s e s w i t h i n t h e flame. T h i s model was used t o g e n e r a t e a s e m i - e m p i r i c a l e q u a t i o n w h i c h can b e used t o p r e d i c t LE1 s e n s i t i v i t y f o r an e l e m e n t u s i n g a p a r t i c u l a r e x c i t a t i o n wavelength f r o m a f l a s h l a m p pumped dye laser. Smyth, e t al., (10) have extended t h i s model t o evaluate expected LE1 s i g n a l s from h i g h - l y i n g (n=4- 12) d l e v e l s o f sodium i n an air-hydrogen flame. Van Dyke and Alkemade (11,12) have made comparisons between simultaneously measured l a s e r induced fluorescence and LE1 s i g n a l S o f sodi urn i n a hydrogen-oxygen-argon flame.

The d e t a i l s o f how LE1 i s detected by t h e probe e l e c t r o d e s and r e g i s t e r e d as a c u r r e n t i n an e x t e r n a l c i r c u i t depend on t h e t y p e o f l a s e r e x c i t a t i o n b e i n g u t i l ized. F o r c o n t i n u o u s wave l a s e r e x c i t a t i o n , Schenck, e t al., (13) have s t u d i e d t h e s p a t i a l and t e m p o r a l c h a r a c t e r i s t i c s o f t h e LE1 s i g n a l . S i g n i f i c a n t agreement between experimental r e s u l t s and t h e o r e t i c a l p r e d i c t i o n s based on a one- dimensional model f o r charge c o l l e c t io n was demonstrated.

F o r p u l s e d LEI, B e r t h o u d , e t al., (14) have s t u d i e d t h e e f f e c t o f l a s e r beam p o s i t i o n , a p p l i e d p r o b e p o t e n t i a l , and f l a m e c o m p o s i t i o n on t h e shape o f t h e LE1 pulse f o r sodium i n t h e a i r - a c e t y l e n e flame u s i n g a n i t r o g e n pumped dye laser. They describe s i g n a l d e t e c t i o n w i t h a model based on c a p a c i t i v e e f f e c t s and d i r e c t charge c o l l e c t i o n . S i m i l a r measurements have been made by H a v r i l l a , e t al., (15) f o r i r o n LE1 w i t h a Nd:YAG pumped dye laser. The authors compare experimental r e s u l t s w i t h p r e d i c t i o n s o f pulse shapes based on a model o f charge i n d u c t i o n from t h e motion of a p o i n t charge p a i r created i n t h e flame by LEI. I n a d i f f e r e n t work (16) comparison o f s i g n a l d e t e c t i o n c h a r a c t e r i s t i c s o f p u l s e d and c o n t i n u o u s wave LE1 was made.

E x p e r i m e n t s d e m o n s t r a t e d t h a t c o n t i n u o u s wave LE I i S l e s s s u c c e p t i b l e t o i n t e r f e r e n c e e f f e c t s re1 ated t o t h e s i g n a l d e t e c t i o n process.

The fundamental d e t e c t i o n l i m i t o f t h e LE1 technique i s discussed i n papers by Matveev, e t a1.(17), and by T r a v i s (18). A d e s c r i p t i o n o f various sources o f noise encountered f o r LE1 w i t h a f l a s h l a m p pumped dye l a s e r i s a v a i l a b l e i n r e f e r e n c e (4).

A v e r y i m p o r t a n t advance i n t h e development o f a n a l y t i c a l LE1 was t h e

i m p l e m e n t a t i o n o f s t e p w i s e e x c i t a t i o n . The use o f t w o t u n a b l e l a s e r s t o s e q u e n t i a l l y e x c i t e t h e atom o f i n t e r e s t t o h i g h e r e n e r g y l e v e l s y i e l d s g r e a t l y i m p r o v e d d e t e c t i o n l i m i t s , p a r t i c u l a r l y f o r e l e m e n t s w i t h h i g h i o n i z a t i o n p o t e n t i a l S. I n a d d i t i o n , t h e r e q u i remevt o f a doubl e-resonance renders t h e method extremely selective. The f i r s t experiments i n t h i s area were performed a t Moscow S t a t e U n i v e r s i t y by Gonchakov, e t a1.,(19) f o r sodium. Subsequent p u b l i c a t i o n s on t h e subject have appeared f r o m t h e Moscow group (20,21), and t h e NBS group (22,23).

E x c e l l e n t l i m i t o f d e t e c t i o n f o r s i x elements w i t h i o n i z a t i o n p o t e n t i a l s between 7 and 10 eV were achieved u s i n g dual dye l a s e r s pumped by a frequency doubled and/or t r i p l e d Nd:YAG l a s e r (23). A d e m o n s t r a t i o n o f t h e use o f t h e d o u b l e - r e s o n a n c e s e l e c t i v i t y advantage o f stepwise e x c i t a t i o n t o c o r r e c t f o r an atomic l i n e overlap i n t e r f e r e n c e i s presented i n reference (23).

E x p e r i m e n t s w h i c h u t i l i z e a n i t r o g e n l a s e r t o p h o t o i o n i z e r a t h e r t h a n c o l l i s i o n a l l y i o n i z e e x c i t e d s t a t e sodium ions produced by s i n g l e s t e p e x c i t a t i o n i n a f l a m e w i t h a dye l a s e r have been r e p o r t e d by Van D i j k , e t a1.,(24), and a l s o i n a g r a p h i t e f u r n a c e a t o m i z e r by Gonchakov, e t a1.(25). The l a t t e r p u b l i c a t i o n a l s o describes a method which u t i l i z e s t h e fundamental r a d i a t i o n o f a Nd:YAG l a s e r as a p h o t o i o n i z a t i o n source f o r sodium f o l l o w i n g stepwise t u n a b l e l a s e r e x c i t a t i o n .

F o r f u r t h e r background i n f o r m a t i o n on t h e s u b j e c t o f laser-enhanced i o n i z a t i o n spectrometry t h e reader i s r e f e r r e d t o several review a r t i c l e s which have appeared on t h e subject (26-28).

I 1

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Present Status o f t h e Method

Laser-enhanced i o n i z a t i o n spectrometry i s now being t e s t e d a t t h e NBS Center f o r A n a l y t i c a l C h e m i s t r y f o r use as a t e c h n i q u e f o r c e r t i f i c a t i o n o f S t a n d a r d R e f e r e n c e M a t e r i a l s (SRM's). Much i s b e i n g l e a r n e d as e x p e r i e n c e i s g a i n e d u s i n g LE1 f o r a n a l y s i s o f r e a l samples. F i g u r e 1 summarizes t h e i n s t r u m e n t a t i o n which a t t h e present t i m e has proven t o be best s u i t e d f o r a n a l y t i c a l LE1 spectrometry.

F i g u r e 1. Schematic o f experimental system f o r a n a l y t i c a l LE1 spectrometry.

Pulsed Flame High Voltage (-)

Dye Laser

I I I I I

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Pre-

I

Trigger Amp

Pulse Amplifier

L

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Active Filter v

Trig. Sig.

Chart Boxcar

Recorder Averager

C7-304 JOIJRNAI- DE PHYSIQUE

Stepwise l a s e r e x c i t a t i o n i s e s s e n t i a l f o r most a n a l y t i c a l work because o f i t s h i g h s e n s i t i v i t y and s e l e c t i v i t y . The p u l s e d l a s e r s y s t e m c o n s i s t s o f a s i n g l e Nd:YAG l a s e r pumping t w o independently t u n a b l e dye lasers. A pumping pulse energy o f 70 mJ o f second harmonic and 30 mJ o f t h i r d harmonic l i g h t has proven adequate f o r most s i t u a t i o n s , even a f t e r beam s p l i t t i n g t o t w o dye l a s e r s f o r s t e p w i s e e x c i t a t i o n . I n f a c t , t h e a n a l y t e s i g n a l response t o l a s e r power i s o f t e n saturated, and f u r t h e r increases i n l a s e r power o n l y a c t t o increase background s i g n a l s f r o m m u l t i - p h o t o n i o n i z a t i o n (MPI) o f f l a m e gases, o r o f f - r e s o n a n c e LE1 s i g n a l s f r o m sample species. S i t u a t i o n s have arisen, however, where more l a s e r pump power would be h e l p f u l i n o r d e r t o enhance t h e wavelength coverage and f l e x i b i l i t y o f t h e system by broadening t h e dye t u n i n g curves. The dye l a s e r s are transverse-pumped w i t h an o s c i l l a t o r and a s i n g l e stage a m p l i f i e r , capable o f 30 % conversion e f f i c i e n c y a t optimum wavelengths. The output o f one o f t h e dye l a s e r s i s frequency doubled and used for t h e f i r s t s t e p o f t h e stepwise e x c i t a t i o n scheme. Frequency doubling o f t h e o t h e r dye l a s e r has not been needed since, f o r a l l o f t h e elements s t u d i e d t h u s f a r , t h e r e q u i r e d second step l i n e has been i n t h e v i s i b l e r e g i o n o f t h e spectrum.

The t w o l a s e r beams are a l i g n e d c o l l i n e a r l y w i t h i n t h e flame, 1-2 mm below t h e s u r f a c e o f t h e w a t e r - c o o l e d c a t h o d e (8). A -1500 V p o t e n t i a l i s a p p l i e d t o t h e cathode. The burner head serves as t h e anode, and i s a t nominal ground p o t e n t i a l , except f o r t h e small p o t e n t i a l drop generated across t h e 10 k ohm r e s i s t o r which connects t h e burner t o ground as a path f o r dc flame i o n i z a t i o n current. Pulsed LE1 c u r r e n t i s coupled f r o m t h e anode t o t h e p r e a m p l i f i e r w i t h a 0.03 uF capacitor.

The f l a m e i s a commercially a v a i l a b l e u n i t , commonly used i n atomic absorption spectrometry, which u t i l i z e s a pneumatic nebul i z e r f o r s o l u t i o n i n t r o d u c t i o n i n t o t h e f l a m e gas mixture. The o p t i c a l path l e n g t h o f t h e f l a m e i s 5 cm. I n most cases a i r - a c e t y l e n e i s used as t h e o x i d a n t - f u e l mixture, which burns a t a temperature o f about 2500 K. Many elements form r e f r a c t o r y oxides i n t h e a i r - a c e t y l e n e flame, and r e q u i r e t h e h o t t e r (3000 K ) n i t r o u s o x i d e - a c e t y l e n e f l a m e t o a c h i e v e e f f i c i e n t a t o m i z a t i o n . E x p e r i e n c e u s i n g LE1 w i t h t h i s f l a m e i s l e s s e x t e n s i v e . Green (30) has r e c e n t l y measured several l i m i - t s o f d e t e c t i o n i n , t h e n i t r o u s o x i d e - a c e t y l e n e f l a m e and found values as low as 100 pg/mL.

Many years o f experience i n a n a l y s i s u s i n g atomic absorption and emission have demonstrated t h e Acetylene flame w i t h e i t h e r a i r o r n i t r o u s oxide as oxidant t o be t h e most p r a c t i c a l i n terms o f good atomization, s t a b i l i t y , and freedom from i n t e r - element i n t e r f e r e n c e effects. It i s o f t e n t e m p t i n g t o consider t h e use o f c o o l e r f l a m e s w i t h l o w e r i o n i z a t i o n backgrounds as a l t e r n a t i v e atom r e s e r v o i r s f o r LEI.

Such flames would have l o w e r backgound n o i s e levels, and probably l o w e r l i m i t s o f d e t e c t i o n . However, i n most cases, t h e i o n i z a t i o n background i n t r o d u c e d t o t h e f l a m e f r o m t h e sample exceeds t h a t o f t h e n a t u r a l f l a m e c o n s t i t u e n t gases. T h i s f a c t , and t h e i n t e r - e l ement i n t e r f e r e n c e s common t o c o o l e r f l a m e s makes t h e i r general a n a l y t i c a l use i m p r a c t i c a l .

The preampl i f i e r used f o r a n a l y t i c a l LE1 i s a 1V/pA c u r r e n t - t o - v o l t a g e c o n v e r t e r as d e s c r i b e d i n r e f e r e n c e (31). An u p p e r f r e q u e n c y c u t o f f o f approximately 1 MHz i s experienced, which i n t e g r a t e s t h e s i g n a l pulse, which has a n a t u r a l c o l l e c t i o n t i m e p r o f i l e o f approximately 100 ns. Faster, b u t l o w e r gain, p r e a m p l i f i e r s have been u t i l i z e d t o measure LE1 p u l s e shapes (14,15), b u t p o o r s i g n a l - t o - n o i s e r a t i o s degrade d e t e c t i o n l i m i t s . I n t e g r a t i o n o f t h e LE1 p u l s e by t h e slower, h i g h g a i n p r e a m p l i f i e r has proven t o be b e n e f i c i a l , s i n c e pulse shapes i n t h e 100 ns t i m e p e r i o d have been shown t o b e much more s e n s i t i v e t o changes i n a l k a l i metal m a t r i x c o n c e n t r a t i o n than t h e area o f t h e LE1 pulse (14,15).

F u r t h e r f i l t e r i n g o f l o w e r frequency f l a m e n o i s e a f t e r t h e p r e a m p l i f i e r w i t h a 10 kHz-l MHz a c t i v e f i l t e r i s b e n e f i c i a l , p a r t i c u l a r l y f o r samples which increase t h e dc f l a m e i o n i z a t i o n c u r r e n t . F i n a l s i g n a l p r o c e s s i n g i s w i t h a 1 us g a t e d i n t e g r a t o r . A 10 us t i m e constant, which gives a t i m e constant i n r e a l - t i m e o f 1 second a t t h e l a s e r p u l s e r e p e t i t i o n r a t e o f 10, Hz, i s t y p i c a l l y used.

T a b l e I c o n t a i n s a l i s t o f t h e b e s t l i m i t s o f d e t e c t i o n by LE1 p u b l i s h e d t o

d a t e f o r 3 1 elements. I n f o r m a t i o n r e g a r d i n g t h e flame, e x c i t a t i o n wavelength(s), and l a s e r u t i l i z e d i s a l s o included. Many o f these d e t e c t i o n l i m i t s , p a r t i c u l a r l y those done using stepwise e x c i t a t i o n , o r some o f t h e elements w i t h low i o n i z a t i o n p o t e n t i a l s , a r e order-of-magnitude o r g r e a t e r improvements over what can be achieved w i t h most o t h e r methods o f analysis. Considerable improvement can be expected f o r many o f t h e elements which have n o t y e t been done u s i n g stepwise e x c i t a t i o n .

TABLE I LE1 L i m i t s o f D e t e c t i o n

Element 1 s t o r S i n g l e 2nd Step

as er^ lame^ ^LODC

Reference Step Wavelength Wavelength

(nm) (nm) (ng/mL

a ~ = f l ash1 amp pumped dye l aser, Y=Nd:YAG pumped dye l aser, N=nitrogen pumped dye laser, K=krypton i o n pumped cw dye l a s e r . b ~ = a i r - a c e t y l ene, N=nitrous oxide-acetyne, H=ai r-hydrogen,

P=propane-butane-air

' l i m i t o f d e t e c t i o n based on t h r e e times t h e standard d e v i a t i o n o f a s i n g l e measurement f o r a s i g n a l e q u i v a l e n t t o t h e background.

G.C. Turk, new data

C7-306 JOURNAL DE PHYSIQUE

Experience using LE1 f o r t r a c e metal a n a l y s i s o f r e a l samples i s increasing, but remains small i n comparison t o t h e considerable experience gathered w i t h t h e m e t h o d s o f a n a l y s i s p r e s e n t l y r e l i e d u p o n b y i n d u s t r y i n a m u l t i t u d e o f a p p l i c a t i o n s . T a b l e I 1 summarizes p u b l i s h e d d a t a on LE1 a p p l i c a t i o n s . M o s t o f t h i s work i s w i t h a l l o y samples, i n which a l k a l i metal concentrations a r e l e s s t h a n 1 pg/mL. The double-resonance s e l e c t i v i t y advantage o f s t e p w i se e x c i t a t i o n LE1 can be v e r y h e l p f u l i n e n s u r i n g a c c u r a t e r e s u l t s f r o m t h e complex s p e c t r u m o f t e n encountered i n such samples. I n a d d i t i o n t o t h e measurements mentioned i n Table 11, unpublished r e a l analyses done a t NBS u s i n g LE1 i n c l u d e t h e d e t e r m i n a t i o n of: Mn i n r i v e r s e d i m e n t (SRM 1645); N i i n f l a s h o i l d i s t i l l a t e and f u e l o i l (SRM 1634a); N i i n stearates; CO i n i n c o l o y s and inconels; CO i n h i g h a l l o y s t e e l (SRM 1289); and Cd, CO, Cu, Mn, N i , and Pb i n s y n t h e t i c f r e s h .water (SRM 1643a).

TABLE I 1 Sample Analyses by LE1

SAMPLE ELEMENT REFERENCE

High Temperature Ni-based A1 l oy I n

Low A l l o y S t e e l s (SRM 361,362,363) Mn (3

Pb, Sn (3)

Unalloyed Copper (SRM 396) Sn (23

Semi conductor A1 l oy I n (23)

Pine Needles (SRM 1575) Rb (21

(16)

One problem f r e q u e n t l y encountered i n t h e use o f LE1 f o r a n a l y s i s i s s p e c t r a l interference. These i n t e r f e r e n c e s f a l l i n t o t h r e e categories: (1) MP1 o f f l a m e gas m o l e c u l e s ; ( 2 ) broadband LE1 o f u n d i s s o c i a t e d sample m o l e c u l e s , and; ( 3 ) o f f - resonance LE1 o f sample m a t r i x atoms. For stepwise e x c i t a t i o n , e i t h e r o r b o t h o f t h e l a s e r e x c i t a t i o n wavelengths may be t h e source o f an interference. Occasionally t h e r e i s an i n t e r a c t i o n between t h e t w o w a v e l e n g t h s w h i c h a f f e c t s t h e s p e c t r a l background. Unused l a s e r r a d i a t i o n , such as t h e fundamental l i g h t o f a frequency doubled laser, should be e l i m i n a t e d from t h e f l a m e i f possible.

M u l t i - p h o t o n i o n i z a t i o n o f NO (32,33) and PO ( 3 4 ) i n flames has been reported.

The process i s a f u n c t i o n o f l a s e r i r r a d i a n c e r a i s e d t o t h e power o f t h e number o f p h o t o n s r e q u i r e d f o r i o n i z a t i o n , and h i g h peak power, f o c u s s e d l a s e r s a r e used t o i n d u c e MPI. However, t h e u n f o c u s s e d beam o f t h e f r e q u e n c y d o u b l e d dye l a s e r described above i s capable o f i n d u c i n g a MP1 spectrum. T h i s has n o t y e t proven t o be a serious problem. Lowering o f l a s e r power w i l l increase t h e LE1 t o MP1 r a t i o . A d j u s t m e n t o f t h e f u e l t o o x i d a n t r a t i o has a l s o been used i n o u r l a b o r a t o r y t o reduce MP1 backgound by l o w e r i n g t h e equi l i b r i um c o n c e n t r a t i o n o f t h e r e s p o n s i b l e molecule. PO i n t h e f l a m e a r i s e s as a decomposition product o f phosphine, which i s a common c o n t a m i n a n t i n a c e t y l e n e gas tanks. The u s e of p h o s p h i n e - f r e e t a n k s of acetylene removes t h i S p o s s i b l e source o f interfernce.

M o l e c u l a r LE1 spectra f r o m metal oxides and hydroxides have been observed (35).

These s p e c i e s a r e a s o u r c e o f s p e c t r a l i n t e r f e r e n c e when samples b e i n g a n a l y z e d i n t r o d u c e such m o l e c u l e s i n t o t h e flame. The p r e s e n c e o f Ca i n t h e sample has proven troublesome i n t h i s regard. Calcium i s o n l y p a r t i a l l y atomized i n t h e a i r - acetylene flame, and some i s present as CaOH. A broadband LE1 molecular spectrum of CaOH h a s been r e c o r d e d i n t h e w a v e l e n g t h r e g i o n b e t w e e n 558 and 600 nm, w h i c h o v e r l a p s t h e second s t e p e x c i t a t i o n l i n e s used f o r t h e a n a l y s i s o f s e v e r a l elements. The CaOH background can be separated f r o m a n a l y t i c a l s i g n a l by scanning t h e l a s e r across t h e atomic a n a l y s i s l i n e , b u t t h e l i m i t o f d e t e c t i o n i s degraded by t h e n o i s e associated w i t h t h e CaOH signal.

The t h i r d v a r i e t y o f s p e c t r a l i n t e r f e r e n c e t h a t has been encountered i s t h a t f r o m LE1 s i g n a l s produced by m a t r i x elements a t wavelength: whic?, a r e considerably o f f resonance, on t h e p r e s s u r e broadened L o r e n t z i a n w i n g s o f a l i n e . The

a t t e n u a t i o n i n t r a n s i t i o n p r o b a b i l i t y several nanometers from t h e l i n e c e n t e r might t y p i c a l l y be 7 orders o f magnitude. However, a s t r o n g v i s i b l e resonance l i n e can be o p t i c a l l y s a t u r a t e d a t peak center by a f a c t o r o f 3 o r 4 orders o f magnitude using t h e l a s e r system described above, b u t n o t saturated f a r o f f resonance. This b r i n g s t h e peak-to-wing r a t i o down t o 3 o r 4 orders o f magnitude. A s i t u a t i o n i n which t h e a n a l y t e e l e m e n t i s p r e s e n t i n a sample t o g e t h e r w i t h a n o t h e r e l e m e n t w h i c h i s several nanometers from any o p t i c a l resonance, b u t several orders o f magnitude more c o n c e n t r a t e d , a n d / o r more s e n s i t i v e , i s q u i t e common, and l e a d s t o a s e r i o u s s p e c t r a l i n t e r f e r e n c e f o r LEI, as we1 l as f o r t h e c l a s s i c a l o p t i c a l a n a l y t i c a l methods. F o r stepwise e x c i t a t i o n LEI, o n l y t h e a l k a l i metals have been a problem as i n t e r f e r i n g elements. Because o f t h e l o w i o n i z a t i o n p o t e n t i a l o f these elements, s i n g l e photon LE1 s e n s i t i v i t y i s comparable t o t h a t o f stepwise e x c i t a t i o n f o r o t h e r elements. I n t e r f e r e n c e between h i g h i o n i z a t i o n p o t e n t i a l elements i s l e s s l i k e l y because o f t h e great d i f f e r e n c e i n s e n s i t i v i t y between stepwi se e x c i t a t i o n o f t h e analyte, and s i n g l e photon e x c i t a t i o n o f t h e i n t e r f e r i n g element.

An example o f an off-resonance LE1 s p e c t r a l i n t e r f e r e n c e i s shown i n f i g u r e 2.

The f i g u r e shows t w o LE1 s p e c t r a l scans a c r o s s t h e f i r s t s t e p N i l i n e a t 300.2 nm w i t h t h e second s t e p l a s e r f i x e d a t 561.5 nm. The s m a l l e r l i n e a t 300.36 nm i s another Ni l i n e , which i s not connected t o t h e second step by a common i n t e r m e d i a t e e n e r g y l e v e l . The t w o scans were t a k e n w h i l e a s p i r a t i n g 0.1 ug/mL N i s o l u t i o n s w i t h and w i t h o u t 10 ~ g / m L o f Na present. A background s i g n a l i s observed i n t h e

Wavelength (nm)

F i g u r e 2. Wavelength scans o v e r t h e 1 s t s t e p N i t r a n s i t i o n a t 300.2 nm w i t h 2nd step wavelength f i x e d a t 561.5 nm. Scan (a) was taken w h i l e a s p i r a t i n g a 100 ng/mL Ni s o l u t i o n

,

and scan (b)was taken w h i l e a s p i r a t i n g a s o l u t i o n c o n t a i n i n g 100 ng/mL o f N i t o g e t h e r w i t h 10 ~ g / m L o f Na.

JOURNAI- D€ PHYSIQUE

p r e s e n c e o f t h e Na. T h i s background i s i n d u c e d by t h e unscanned, s e c o n d - s t e p v i s i b l e wavelength

,

which i s 7 nm away from t h e 3p-4d Na t r a n s i t i o n s near 569 nm, w h i c h g i v e v e r y s t r o n g LE1 s i g n a l s . A p e a k - t o - b a c k g r o u n d measurement makes t h e necessary background c o r r e c t i o n f o r q u a n t i t a t i v e measurement, but t h e degradation i n d e t e c t i o n l i m i t caused by t h e increased background noise i s obvious. The source o f t h i s b a s e l i n e n o i s e i s t h e pulse-to-pulse l a s e r i n t e n s i t y f l u c t u a t i o n , and may be subject t o improvement by n o r m a l i z a t i o n o f t h e LE1 s i g n a l t o l a s e r power on a shot- by-shot bas i s.

These s p e c t r a l i n t e r f e r e n c e s a r e a s e r i o u s impediment t o t h e a p p l i c a t i o n o f LE1 t o t r a c e m e t a l a n a l y s i s , more s e r i o u s t h a n t h e m u l t i p1 i c a t i v e i o n i z a t i o n i n t e r f e r e n c e mentioned e a r l i e r . The troublesome i n t e r f e r i ng elements a r e confined t o t h e f i r s t t w o rows o f t h e p e r i o d i c table. We have found chemical s e p a r a t i o n o f t h e s e i o n s f r o m t h e sample s o l u t i o n t o be n e c e s s a r y i n some cases. A s e p a r a t i o n procedure u t i l i z i n g c h e l a t i n g r e s i n t o separate a l k a l i and a l k a l i n e e a r t h elements from t h e t r a c e elements (36) has proven t o be very e f f e c t i v e as a pre-treatment o f samples f o r LE1 analysis.

The f u t u r e o f LE1 spectrometry looks promising. Increased understanding o f t h e fundamental p h y s i c a l processes i n v o l v e d i n LE1

,

a l o n g w i t h i n c r e a s e d a n a l y t i c a l a p p l i c a t i o n e x p e r i e n c e w i l l c o n t i n u e t h e s t e a d y i m p r o v e m e n t o f t h e method. New advances i n l a s e r t e c h n o l o g y w i l l c e r t a i n l y c o n t i n u e t o occur, a g a i n l e a d i n g t o improved a n a l y t i c a l performance o f LEI. The authors b e l i e v e t h a t LE1 w i l l become an important method o f a n a l y s i s t o be used t o solve d i f f i c u l t problems i n t h e area o f t r a c e metal a n a l y s i s i n complex matrices.

-

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