Inductively Coupled Plasma-Atomic Emission Spectroscopy with

4 Inductively Coupled Plasma-Atomic Emission Spectroscopy with Multichannel Array Detection FREDERICK GRABAU1 Ames Laboratory, U.S. Department of Energy, Ames, IA 50010 YAIR T A L M I Princeton Instruments, Inc., P.O. Box 2318, Princeton, N J 08540

Array detectors provide an attractive alternative to the polychrometers commonly used with ICP-AES, because they provide the capability of simultaneous detection of spectral lines and spectral background dispersed on hundreds of virtually independent optical channels. In this paper a critical evaluation of self-scanned photodiode arrays as radiation detectors for ICP-AES i s presented. Limited comparisons are also drawn for silicon intensified target vidicons (SIT) and intensified self-scanning photodiode array detectors (ISPD). The figures of merit of primary interest to analytical chemist that are discussed are: noise chara c t e r i s t i c s and powers of detection, l i n e a r i t y and dynamic range, spectral stripping and interpolative correction for variable underlying background, compromise between resolution and spectral coverage and application to nonideal " r e a l " samples.

I n a n a l y t i c a l a t o m i c e m i s s i o n s p e c t r o s c o p y , t h e most p o p u l a r approach t o s i m u l t a n e o u s m u l t i e l e m e n t d e t e r m i n a t i o n s has been b a s e d on t h e u s e o f m u l t i p l e d e t e c t o r s t h a t m o n i t o r d i s c r e t e wavelengths. The most common f o r m o f t h i s a p p r o a c h h a s i n v o l v e d f i x e d a r r a y s of narrow s l i t s prearranged along t h e e x i t f o c a l plane o f a s p e c t r o m e t e r , w i t h c o r r e s p o n d i n g a r r a y s o f p h o t o m u l t i p l i e r tube (PMT) d e t e c t o r s t o m o n i t o r t h e r a d i a t i o n t r a n s m i t t e d t h r o u g h t h e s e slits. These i n s t r u m e n t s a r e i d e a l l y s u i t e d f o r t h e r o u t i n e , s i m u l t a n e o u s m u l t i e l e m e n t d e t e r m i n a t i o n o f t h e same s e t o f e l e m e n t s , w h i c h may number up t o 30 o r more, i n s a m p l e s o f s i m i l a r t o t a l composition. F o r these p u r p o s e s , p o l y c h r o m a t o r s have p r o v i d e d t h e w i d e dynamic r a n g e , f a s t r e s p o n s e a n d e x c e l l e n t s i g n a l - t o - n o i s e (SNR) p e r f o r m a n c e c h a r a c t e r i s t i c o f PMT's, s p e c i f i c a l l y when 1Current address: St. Louis County Water Company, Chesterfield, MO 63017.

0097-6156/83/0236-0075S11.50/0 © 1983 American Chemical Society

MULTICHANNEL IMAGE DETECTORS

76

o p e r a t e d as e n e r g y ( i n t e g r a t o r s ) r a t h e r t h a n power ( c u r r e n t ) d e t ectors . When t h e a n a l y s t i s f a c e d w i t h t h e d e t e r m i n a t i o n of a b r o a d r a n g e of e l e m e n t s i n s a m p l e s of w i d e l y v a r y i n g c o m p o s i t i o n , t h e l i m i t e d s e t of l i n e s o b s e r v a b l e w i t h a p o l y c h r o m a t o r r e s t r i c t s t h e s c o p e of a p p l i c a t i o n and o p t i m i z a t i o n of t h e s e i n s t r u m e n t s . O p t o e l e c t r o n i c image d e v i c e s ( O I D ) ( 1 - 3 9 ) , p a r t i c u l a r l y s e l f s c a n n e d p h o t o d i o d e a r r a y s ( S P D ) , p r o v i d e an a t t r a c t i v e a l t e r n a t i v e t o the p o l y c h r o m a t o r a p p r o a c h b e c a u s e t h e y t o o p r o v i d e t h e c a p a b i l i t y o f s i m u l t a n e o u s d e t e c t i o n of d i s p e r s e d r a d i a t i o n . When these d e t e c t o r s are i n t e r f a c e d w i t h a p p r o p r i a t e o n - l i n e computer c o n t r o l and d a t a p r o c e s s i n g f a c i l i t i e s , the s i g n a l s f r o m h u n d r e d s of v i r t u a l l y i n d e p e n d e n t , o p t i c a l ( s p e c t r a l ) c h a n n e l s can be i n t e g r a t e d and d i g i t i z e d s i m u l t a n e o u s l y . I n a d d i t i o n , the c o n t r i b u t i o n of c o n t i n u u m s p e c t r a l b a c k g r o u n d o r u n d e s i r a b l e o v e r l a p p i n g s p e c t r a l f e a t u r e s p r e s e n t i n t h e " b l a n k " s p e c t r u m can be s u b t r a c t e d or " s t r i p p e d " from the t o t a l s i g n a l f o r a l l c h a n n e l s . I f d e s i r a b l e , random o r f a s t - a c c e s s t o t h e s p e c i f i c s p e c t r a l f e a t u r e s of i n t e r e s t may be employed t o c o n s e r v e c o m p u t e r memory s p a c e and t i m e ( 4 0 ) . O t h e r c a p a b i l i t i e s p r o v i d e d by t h e s e d e t e c t o r s i n c l u d e : a) F l e x i b i l i t y i n c h o i c e of mode o f s i g n a l i n t e g r a t i o n and p r o c e s s i n g ; r e a l - t i m e , in-memory i n t e g r a t i o n o r ont a r g e t i n t e g r a t i o n ; b) m o n i t o r i n g of u l t r a r a p i d s p e c t r o s c o p i c e v e n t s ; c) c o m p e n s a t i o n f o r s o u r c e f l u c t u a t i o n s and f o r s e n s i t i v i t y and s p e c t r a l r e s p o n s e v a r i a t i o n ; d) h i g h quantum e f f i c i e n c y ; and e) l i n e a r r e s p o n s e . These d e s i r a b l e f e a t u r e s o f O I D s a r e o f f s e t , a t l e a s t p a r t i a l l y , by a few u n d e s i r a b l e f e a t u r e s t h a t impose u l t i m a t e l i m i t s on t h e a t o m i c s p e c t r o m e t r i c p e r f o r m a n c e o f t h e s e d e t e c t o r s . A f o r e m o s t l i m i t a i t o n i s s e t by the number of d i o d e s i n c o m m e r c i a l l y a v a i l a b l e d i o d e a r r a y s , and t h e i r p h y s i c a l a r r a n g e m e n t and s p a t i a l r e s o l u t i o n . I n a d d i t i o n , t h e low a m p l i f i c a t i o n g a i n o f t h e SPD r e q u i r e s l o n g e r s i g n a l i n t e g r a t i o n t i m e s , and f i n a l l y , t h e dynamic r a n g e o f t h e SPD i s n a r r o w e r t h a n t h a t of t h e PMT. As shown i n t h e d i s c u s s i o n a b o v e , t h e r e a r e a m u l t i p l i c i t y of d e s i r a b l e and u n d e s i r a b l e f e a t u r e s o f OID's t h a t i m p a c t t h e i r g e n e r a l a p p l i c a t i o n as d e t e c t o r s i n a n a l y t i c a l a t o m i c e m i s s i o n spectrometry. I t i s t h e r e f o r e a p p r o p r i a t e t o compare, i n a c r i t i c a l and o b j e c t i v e s e n s e , t h e e x p e r i m e n t a l f i g u r e s o f m e r i t o f these d e v i c e s v i s - a - v i s the c l a s s i c a l p o l y c h r o m a t o r p h o t o m u l t i p l i e r a p p r o a c h . These c o m p a r i s o n s s h o u l d be p e r f o r m e d v i r t u a l l y on a c o n t i n u i n g b a s i s b e c a u s e of a d v a n c e s i n p e r f o r m a n c e s , n o t o n l y of the a r r a y d e t e c t o r s t h e m s e l v e s b u t a l s o i n t h e a s s o c i a t e d spectroscopic e x c i t a t i o n sources. An e v a l u a t i o n of t h e o v e r a l l p e r f o r m a n c e f i g u r e s of m e r i t of OID's when t h e y a r e employed i n c o n j u n c t i o n w i t h i n d u c t i o n - c o u p l e d p l a s m a e x c i t a t i o n i s of* p a r t i c u l a r c u r r e n t i n t e r e s t b e c a u s e of t h e p o p u l a r i t y t h a t t h i s source i s a t t a i n i n g f o r the simultaneous d e t e r m i n a t i o n of the e l ements a t a l l c o n c e n t r a t i o n l e v e l s . I n t h i s p a p e r we p r e s e n t s u c h an e v a l u a t i o n f o r s e l f - s c a n n e d , p h o t o d i o d e a r r a y d e t e c t o r s 1

4.

GRAB AU AND

TALMI

Coupled

Plasma-Atomic

Emission

Spectroscopy

11

(SPD) and draw c o m p a r i s o n s t o PMT a r r a y s as u s e d i n c l a s s i c a l p o l y c h r o m a t o r s . To a l e s s e r e x t e n t , some c o m p a r i s o n s a r e drawn w i t h s i l i c o n i n t e n s i f i e d t a r g e t v i d i c o n s ( S I T ) and i n t e n s i f i e d s e l f canning photodiode array detectors (ISPD). A similar evaluation of t h e SPD f o r s p e c t r o p h o t o m e t r y and s p e c t r o f l u o r o m e t r y was r e c e n t l y published elsewhere (41). A p p a r a t u s And

Procedure

The e x p e r i m e n t a l f a c i l i t i e s and o p e r a t i n g c o n d i t i o n s u t i l i z e d i n t h i s s t u d y a r e summarized i n T a b l e I . B o t h s p e c t r o m e t e r s were u s e d e i t h e r as m o n o c h r o m a t o r s , i n t h e s i n g l e w a v e l e n g t h d e t e c t i o n mode, o r as p o l y c h r o m a t o r s i n t h e m u l t i c h a n n e l d e t e c t i o n mode. The c o n v e r s i o n of t h e J a r r e l l - A s h s p e c t r o m e t e r t o a p o l y c h r o m a t o r was a c c o m p l i s h e d by r e p l a c i n g t h e camera a t t a c h m e n t of t h e s p e c t r o m e t e r w i t h an OID a r r a y d e t e c t o r . A r o t a t i o n a l l y a d j u s t a b l e mount a l l o w e d t h e a r r a y d e t e c t o r c h a n n e l s t o be a l i g n e d p a r a l l e l w i t h t h e s l i t image. S p e c t r a l l i n e s l i t images were f o c u s e d on the d e t e c t o r s u r f a c e by a d j u s t m e n t o f a 45 folding m i r r o r t h a t d e f l e c t e d t h e e x i t s p e c t r u m o n t o t h e camera mount. S e l e c t i o n of e i t h e r s i n g l e c h a n n e l of m u l t i c h a n n e l d e t e c t i o n modes was made by r o t a t i o n o f t h e 45 m i r r o r t o one of two p o s i t i o n s , e i t h e r a l l o w i n g t h e e x i t s p e c t r u m t o r e a c h t h e e x i t s l i t or d e f l e c t i n g i t to the a r r a y d e t e c t o r . The s l i t - h e i g h t s e t t i n g f o r t h e SPD d e t e c t o r was 2.5 mm, e q u a l t o t h e h e i g h t of e a c h i n d i v i dual diode. The 1% HNO^ r e f e r e n c e s o l u t i o n s were p r e p a r e d f r o m d i s t i l l e d HNO^ and d e i o n i z e d d i s t i l l e d w a t e r . A n a l y s e s o f t h e US G e o l o g i c a l S u r v e y r e f e r e n c e s a m p l e s were p e r f o r m e d a f t e r d i s s o l u t i o n by sodium h y d r o x i d e f u s i o n ( 4 2 ) . R e f e r e n c e s o l u t i o n s n e c e s s a r y f o r t h e a n a l y s i s of t h e g l a s s s a m p l e s were p r e p a r e d i n a 2°L N a C l - H C l m a t r i x t o match t h e t o t a l a c i d and s a l t c o n t e n t o f t h e g l a s s samples. D e t e c t i o n l i m i t s , d e f i n e d as t h e c o n c e n t r a t i o n r e q u i r e d t o p r o v i d e n e t a n a l y t e s i g n a l s t h a t were t h r e e t i m e s g r e a t e r t h a n t h e s t a n d a r d d e v i a t i o n o f t h e b a c k g r o u n d s c a t t e r , were d e t e r m i n e d f r o m t e n t e n - s e c o n d i n t e g r a t i o n s of t h e b l a n k and d i l u t e r e f e r e n c e s o l u t i o n s f o r t h e PMT s y s t e m . S t a n d a r d d e v i a t i o n ( n o i s e ) o f t h e b a c k g r o u n d was c a l c u l a t e d f r o m the t e n b l a n k i n t e g r a t e d s i g n a l s . The m u l t i c h a n n e l n a t u r e o f t h e SPD and SIT d e t e c t o r s was u t i l i z e d to d e t e r m i n e d e t e c t i o n l i m i t s w i t h t h e s e s y s t e m s . P a i r s o f b l a n k s p e c t r a were s u b t r a c t e d f r o m e a c h o t h e r t o o b t a i n n e t d i f f e r e n c e spectra. These d i f f e r e n c e s p e c t r a r e p r e s e n t e d t h e n o i s e and t h e d r i f t between m e a s u r e m e n t s . The s t a n d a r d d e v i a t i o n of t e n a d j a c e n t d i o d e s i n t h e r e g i o n of the a n a l y t e w a v e l e n g t h was determined f o r e a c h of t h e n e t d i f f e r e n c e s p e c t r a . The s t a n d a r d d e v i a t i o n o f f i v e d i f f e r e n t s p e c t r a were t h e n a v e r a g e d t o p r o d u c e a representative noise l e v e l . To measure t h e b a c k g r o u n d i r r a d i a n c e of t h e ICP i n t h e 180440 nm r e g i o n , a p r e c a l i b r a t e d d e u t e r i u m a r c lamp was u s e d . The

78


Table I .

I n s t r u m e n t Setup and O p e r a t i n g C o n d i t i o n s

Plasma

Power S u p p l y :

Plasma Therm, I n c . , M o d e l NFS-2500 D w i t h M o d e l AMN-3000 Ε impedance match ing network.

Plasma

Torch:

Ames L a b o r a t o r y c o n s t r u c t i o n ( 4 3 ) 20 L/min p l a s m a A r f l o w ; 1.0 L/min a e r o sol c a r r i e r Ar flow.

Forward

Power:

1100W

Ultrasonic Nebulizer:

Ames L a b o r a t o r y c o n s t r u c t i o n ( 4 4 ) 1.4 MHz, w a t e r c o o l e d d e s o l v a t i o n , 2.6 m l / m i n sample i n t r o d u c t i o n r a t e .

Spectrometers :

1) J a r r e l l - A s h , M o d e l 78-466, 1 m e t e r f / 8 . 7 , C z e r n y - T u r n e r mount w i t h a 1180 g/mm g r a t i n g b l a z e d a t 250 nm, r e c i p r o c a l l i n e a r d i s p e r s i o n o f 0.825 nm/mm (0.0206 n m / d i o d e ) . Entrance s l i t wid t h 20 urn, h e i g h t 2 mm. 2) M c P h e r s o n , M o d e l 2 0 5 1 , 1 m e t e r , f / 8 . 7 , C z e r n y - T u r n e r mount w i t h a 1800 g/mm h o l o g r a p h i c g r a t i n g b l a z e d a t 200-700 nm r e c i p r o c a l l i n e a r d i s p e r s i o n o f 0.0138 n m / p h o t o d i o d e , e n t r a n c e s l i t 20 urn, h e i g h t 2 mm.

Lens :

Observation

Irradiance

S p h e r i c a l , 4 cm d i a m e t e r , 125 mm f o c a l l e n g t h 30 cm f r o m p l a s m a and e n t r a n c e slit. Region:

3 cm segment c e n t e r e d 15 mm above t h e load c o i l .

Reference:

O p t r o n i c L a b o r a t o r i e s , I n c . , M o d e l UV40 U l t r a - v i o l e t I r r a d i a n c e S t a n d a r d . C a l i b r a t e d a t 10 nm i n t e r v a l s f r o m 180 nm t o 400 nm.

4.

GRABAU AND TALMI

Coupled

Table I.

D e t e c t i o n System:

Plasma-Atomic

Emission

Spectroscopy

79

Continued

Multichannel: 1) EG&G PARC M o d e l 1412 SPD ( 2 . 5 mm h i g h p e r t u r e X 25 urn c e n t e r - t o - c e n t e r , 1024 d i o d e a r r a y , R e t i c o n RL/1024S) o p e r a t e d by d e t e c t o r c o n t r o l l e r EG&G PARC, M o d e l 1218 and d a t a - m a n i p u l a t i o n by t h e 0MA-2 c o n s o l e M o d e l 1215. SPD was m a i n t a i n e d a t -22° + 0.002°C. The s p e c t r a l window w i t h M o d e l 78-466 s p e c t r o m e t e r was 21.094 nm. 2) EG&G PARC, M o d e l 1254 s i l i c o n i n t e n s i f i e d v i d i c o n (SIT) detector with u l t r a v i o l e t t o v i s i b l e c o n v e r t e r , ope r a t e d by d e t e c t o r c o n t r o l l e r M o d e l 1216 v i a 0MA-2 c o n s o l e M o d e l 1215. Number o f c h a n n e l s 5 0 0 . S p e c t r a l w i n dow was 10.31 nm. 3) EG&G PARC, M o d e l 1420, m i c r o c h a n n e l p l a t e (MCP) i n t e n s i f i e d SPD w i t h 700 c h a n n e l s ( I S P D ) . Temperature: - 20°C. O p e r a t e d by d e t e c t o r c o n t r o l l e r M o d e l 1412. S i n g l e Channel ( S i g n a l

Integration):

PMT s i g n a l c o u n t e d 10 t i m e s f o r p e r i o d s o f 10 s e c o n d s e a c h u s i n g A n a l o g Technology C o r p o r a t i o n Model 151, wide range c u r r e n t - t o - f r e q u e n c y c o n v e r t e r . S a m p l i n g t i m e s were 100 ms. PMT; EMI M o d e l 978AQ, o p e r a t e d a t 700 V.


80

d e u t e r i u m lamp was p l a c e d a t t h e same o p t i c a l p o s i t i o n n o r m a l l y o c c u p i e d by t h e p l a s m a , i . e . , 60 cm away f r o m t h e e n t r a n c e s l i t of t h e p o l y c h r o m a t o r . The PMT r e s p o n s e t o t h e c a l i b r a t e d a r c lamp was m e a s u r e d o v e r t h e 180-440 nm r e g i o n , i n w a v e l e n g t h i n t e r v a l s of 10 nm. These measurements were t h e n r e p e a t e d f o r t h e p l a s m a f e d w i t h a 1% HNO^ b l a n j ^ s o l u t i o n . The i r r a d i a n c e o f t h e p l a s m a ( I p ^ ) , i n m i c r o w a t t s / c m /nm, was t h e n d e t e r m i n e d a c c o r d i n g t o :

R

I , = p l X I ( a t 30 n

cm)

where R ^ and a r e t h e PMT r e s p o n s e c u r v e s (180-440 nm) f o r t h e p l a s m a End d e u t e r i u m lamp, r e s p e c t i v e l y , and I r e p r e s e n t s t h e c a l i b r a t e d i r r a d i a n c e of t h e d e u t e r i u m lamp, a t a d i s t a n c e of 30 cm away f r o m t h e e n t r a n c e s l i t . Because a l l o p t i c a l parameters r e m a i n e d unchanged f o r t h e c a l i b r a t e d lamp and ICP m e a s u r e m e n t s , p r i o r knowledge of the s p e c t r a l t r a n s f e r c h a r a c t e r i s t i c s of the spectrometer ( p a r t i c u l a r l y the g r a t i n g ) or the s p e c t r a l response of t h e photDcathode was n o t n e c e s s a r y . The m e a s u r e d I represent e d t h e b a c k g r o u n d i r r a d i a n c e a t a d i s t a n c e o f 30 cm^rrom t h e plasma. 1

R e s u l t s And

Discussion

L i n e a r i t y And Dynamic Range. T h e r e i s e x t e n s i v e e v i d e n c e t h a t e m i s s i o n i n t e n s i t i e s o f many a t o m i c l i n e s e x c i t e d i n an ICP source are l i n e a r l y r e l a t e d to c o n c e n t r a t i o n of the c o r r e s p o n d i n g a n a l y t e s o v e r a r a n g e o f a t l e a s t one m i l l i o n . T r a c e and m a j o r c o n s t i t u e n t s a r e t h e r e f o r e d e t e r m i n a b l e w i t h o u t changes i n t h e o p e r a t i n g c o n d i t i o n of t h e p l a s m a . Ideally, the d e t e c t i o n system s h o u l d have a c o m p a r a b l e l i n e a r dynamic r a n g e p e r f o r m a n c e . As p r e v i o u s l y d i s c u s s e d ( 4 5 ) , t h e r e a r e s e v e r a l d e f i n i t i o n s of dynamic r a n g e t h a t a r e a p p l i c a b l e t o t h e SPD d e t e c t i o n s y s t e m . F i r s t , t h e r e i s t h e s i n g l e d i o d e dynamic r a n g e f o r a f i x e d i n t e g r a t i o n t i m e , d e f i n e d h e r e as t h e a n a l y t e c o n c e n t r a t i o n r a n g e t h a t p r o d u c e s a l i n e a r s i g n a l r e s p o n s e . T h i s f o r m o f dynamic r a n g e c a n be s e v e r e l y r e d u c e d by t h e r m a l l y g e n e r a t e d d a r k c h a r g e , b e c a u s e i t r e d u c e s t h e e f f e c t i v e A/D c o n v e r t e r r a n g e a v a i l a b l e f o r r e a d o u t of p h o t o n g e n e r a t e d c h a r g e s ( s i g n a l ) . To r e d u c e t h e e f f e c t o f d a r k c h a r g e b u i l d up by a p p r o x i m a t e l y a f a c t o r of 100, t h e SPD a r r a y was c o o l e d t o -20°C. The c o o l i n g was done t h e r m o e l e c t r i c a l l y and t h e t e m p e r a t u r e was k e p t c o n s t a n t t o w i t h i n a few m i l l i d e g r e e s C e l s i u s . Under t h e s e c o n d i t i o n s a s i n g l e d i o d e dynamic r a n g e o f a p p r o x i m a t e l y 16,000:1 was r o u t i n e l y a c h i e v e d ( l i m i t e d o n l y by t h e 14 b i t s A/D c o n v e r t o r u s e d ) . To enhance t h e o v e r a l l dynamic r a n g e o f t h e SPD, t h e v a r i a b l e i n t e g r a t i o n t i m e ( V I T ) mode of o p e r a t i o n c a n be u s e d ( 4 1 ) . I n t h i s a p p r o a c h t h e s i g n a l s f r o m each s p e c t r a l window c a n be

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c o n s e c u t i v e l y o b t a i n e d a f t e r i n t e g r a t i o n o f 16 ms, 160 ms and 16 seconds. I n t h i s manner, t h e o r e t i c a l dynamic r a n g e s o f 1 t o 16,000, 10 t o 160,000 and 1,000 t o 16,000,000 c a n be a c h i e v e d w i t h an o v e r a l l l i n e a r dynamic r a n g e o f 1.6 X 10^. In r e a l i t y , t h e p h o t o n n o i s e and t h e combined d e t e c t o r d a r k c h a r g e and p l a s m a b a c k g r o u n d s i g n a l r e d u c e t h e u s e f u l VIT r a n g e of t h e SPD. The o v e r a l l r e a d o u t n o i s e a s s o c i a t e d w i t h a t y p i c a l 16 s e c o n d , on t a r g e t , ICP s i g n a l i n t e g r a t i o n was s i x c o u n t s rms ( m o s t l y r a d i o f r e q u e n c y i n t e r f e r e n c e as d i s c u s s e d b e l o w ) . A t t h e h i g h end, t h e maximum r e a d a b l e s i g n a l i s a l w a y s l i m i t e d by A/D c o n v e r t e r range. The l i m i t o f q u a n t i t a t i v e d e t e r m i n a t i o n (LQD) i s d e f i n e d as the c o n c e n t r a t i o n r e q u i r e d to y i e l d a net i n t e n s i t y t e n times g r e a t e r than the standard d e v i a t i o n of the readout s c a t t e r ( 4 0 ) . F o r i n s t a n c e , c o n s i d e r a 16 second s i g n a l i n t e g r a t i o n a t t h e Mn 257.6 nm l i n e where t h e combined d a r k c h a r g e and p l a s m a s p e c t r a l b a c k g r o u n d s i g n a l consumes a p p r o x i m a t e l y 20% o f t h e dynamic r a n g e of t h e A/D c o n v e r t e r (800 c o u n t s d a r k c h a r g e and 2600 c o u n t s p l a s m a b a c k g r o u n d ) . The r e s u l t a n t a n a l y t i c a l dynamic r a n g e ( s i n g l e d i o d e ) w o u l d be:

Maximum n e t s i g n a l _ 16,000 - 3400 _ Net s i g n a l a t LQD 10 X 6

2 1 Q

( w i t h p r o p o r t i o n a l l y low d a r k c h a r g e l e v e l s ) . Y e t , s h o r t e r ondetector integration times, usable at higher analyte concentra t i o n , w o u l d i n c r e a s e t h i s a n a l y t i c a l dynamic r a n g e . F o r t h e same Mn l i n e a t 257.6 nm c o n s e c u t i v e i n t e g r a t i o n s o f 16 msec, 160 msec and 16 sec w o u l d have an a c c u m u l a t e d ( V I T ) a n a l y t i c a l l i n e a r r a n g e o f a t l e a s t 2.1 Χ 1 0 . The e x c e l l e n t dynamic r a n g e a c h i e v e d by t h e VIT method i s r e p r e s e n t e d by t h e d a t a i n T a b l e I I and by t h e g r a p h i c a l p r e s e n t a t i o n i n F i g u r e 1. L i n e a r i t y was r e t a i n e d , w i t h i n t h e e x p e r i mental e r r o r of measurements, over a c o n c e n t r a t i o n range g r e a t e r t h a n 10^. I t s h o u l d be n o t e d , t h a t t h e l o w e s t p l o t t e d p o i n t s r e p r e s e n t LQD v a l u e s . }

5

D e t e c t o r and S p e c t r a l B a c k g r o u n d , S p e c t r a l I n t e r f e r e n c e s , and Spectral Stripping. For SPD s i t i s u s e f u l to d i s t i n g u i s h between t h e d e t e c t o r and s p e c t r a l b a c k g r o u n d u n d e r l y i n g most a n a l y t e lines. The m a j o r component o f t h e d e t e c t o r b a c k g r o u n d a r i s e s f r o m t h e r m a l l y g e n e r a t e d dark charge. There i s i n a d d i t i o n a c h a r a t e r i s t i c f i x e d p a t t e r n c a u s e d by s t r a y c a p a c i t i v e c o u p l i n g of t r a n s i e n t s a r i s i n g f r o m t h e c l o c k d r i v i n g s i g n a l s and by d i o d e t o diode dark charge v a r i a t i o n ( 4 5 ) . Radio frequency i n t e r f e r e n c e f r o m ICP s o u r c e s may a l s o p r o d u c e a t i m e v a r i a b l e p a t t e r n n o i s e s u p e r i m p o s e d on t h e d e t e c t o r b a c k g r o u n d . The s p e c t r a l b a c k g r o u n d u s u a l l y r e s u l t s from the c u m u l a t i v e c o n t r i b u t i o n s from sources 1

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Figure 2. Efficiency of plasma background subtraction. Key: a, emission spectrum of 1 μg/mL Be, 7% HN0 ; b, emission spectrum of 1% HN0 blank; and c, difference spectrum obtained by subtracting Spectrum b from a. Spectra were obtained after on-target signal integration for 8 s. 3

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Figure 3. Efficiency of plasma background subtraction. Spectra obtained after ondetector integration for 1.6 s. Key: a, spectrum of 10 μg|mL Ti; b, detector and spectral background; and c, net difference spectrum.

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windows may be i n a d e q u a t e l y n a r r o w f o r some a p p l i c a t i o n s ) . The a n a l y s t may t h e n use a v i d e o d i s p l a y f o r : a) c h a r a c t e r i z i n g t h e n a t u r e o f t h e u n d e r l y i n g b a c k g r o u n d and d e t e r m i n i n g t h e most s u i t able analyte l i n e s ; b) d e t e r m i n i n g w h i c h a n a l y t e l i n e s a r e l e a s t s u s c e p t i b l e t o s h i f t s i n t h e b a c k g r o u n d m a g n i t u d e due t o changes i n t h e c o n c e n t r a t i o n s of t h e m a j o r c o n c o m i t a n t s ; and c) s e l e c t i n g the b e s t wavelengths ( d i o d e s ) f o r a p p l y i n g base l i n e background c o r r e c t i o n s f o r each a n a l y t e l i n e . I f a p p r o p r i a t e , a computer a l g o r i t h m may be u s e d t o d e t e r m i n e b a c k g r o u n d l e v e l s f r o m s e v e r a l off analyte l i n e diodes. The e f f i c i e n c y of b a c k g r o u n d c o r r e c t i o n t e c h n i q u e s may be j u d g e d by c o m p a r i n g a s e r i e s o f r e f e r e n c e w o r k i n g c u r v e s c o n t a i n i n g i n c r e a s i n g c o n c e n t r a t i o n s of c o n c o m i t a n t s . Errors arising f r o m u n c o r r e c t e d b a c k g r o u n d s h i f t s w i l l r e d u c e t h e s l o p e s of t h e s e r e f e r e n c e w o r k i n g c u r v e s a t low a n a l y t e c o n c e n t r a t i o n s . W i t h PMT p o l y c h r o m a t o r s , b a c k g r o u n d c o r r e c t i o n has been p r o v e n e f f e c t i v e down t o t h e LQD l e v e l , even a t h i g h c o n c o m i t a n t c o n c e n t r a t i o n l e v e l s , e.g., 5000 ug/ml Ca, 5000 ug/ml Mg and 500 ug/ml A l ( 5 0 ) . F o r our e v a l u a t i o n s , t h e TI 190.8 nm and t h e As 193.7 nm l i n e s were c h o s e n t o d e t e r m i n e t h e e f f i c i e n c y of b a c k g r o u n d c o r r e c t i o n a c h i e v a b l e w i t h SPD d e t e c t o r s . F i g u r e 4 shows t h e s p e c t r a a t and a b o u t t h e 190.8 nm TI l i n e m e a s u r e d f o r 15 ug/ml TI s o l u t i o n s ( 1 % HNO3 a q u e o u s ) w i t h and w i t h o u t t h e a d d i t i o n of c o n c o m i t a n t s . In both cases the blank s u b t r a c t i o n ( d e t e c t o r p l u s blank s p e c t r a l b a c k g r o u n d ) was p e r f o r m e d . I t i s apparent t h a t the r e l a t i v e l y f l a t b a c k g r o u n d i s amenable t o l i n e i n t e r p o l a t i o n c o r r e c t i o n s , i . e . , the c h o i c e of wavelength ( d i o d e ) f o r background c o r r e c t i o n , i s r a t h e r non c r i t i c a l . I n t h e c a s e o f t h e As 193.7 nm l i n e , c o r r e c t i o n o f t h e b a c k g r o u n d i s r e n d e r e d more d i f f i c u l t b e c a u s e t h e a n a l y t e l i n e i s s u p e r i m p o s e d on t h e w i n g of an a d j a c e n t A l c o n c o m i t a n t l i n e , F i g u r e 5. I n t h i s c a s e , a t l e a s t two c o r r e c t i o n v a l u e s m e a s u r e d a t w a v e l e n g t h s l o c a t e d on each s i d e of t h e As l i n e a r e n e e d e d . A s t r a i g h t l i n e i n t e r p o l a t i o n between t h e s e two w a v e l e n g t h s i s t h e n u s e d t o e s t i m a t e t h e b a c k g r o u n d l e v e l a t t h e a n a l y t e w a v e l e n g t h . A n a l y t i c a l c a l i b r a t i o n c u r v e s of TI and As a t f o u r c o n c o m i t a n t c o n c e n t r a t i o n l e v e l s , o b t a i n e d a f t e r b l a n k b a c k g r o u n d s u b t r a c t i o n o n l y , a r e shown i n F i g u r e s 6 and 7. Clearl y , a t t h e h i g h e s t c o n c o m i t a n t c o n c e n t r a t i o n s , t h e LQD a n a l y t e l e v e l shows an o r d e r of m a g n i t u d e e r r o r i n c o n c e n t r a t i o n due t o u n c o r r e c t e d background s h i f t s . When t h e interpolâtive b a c k g r o u n d c o r r e c t i o n s were a p p l i e d t o t h e same d a t a , t h e d e t e r m i n a t i o n e r r o r s were d r a s t i c a l l y r e d u c e d , as shown by F i g u r e s 8 and 9. I n f a c t , b a c k g r o u n d c o r r e c t i o n s a c h i e v e d w i t h SPD, f o r b o t h t y p e s of u n d e r l y i n g s p e c t r a l backgrounds, a r e comparable to t h a t a c h i e v e d w i t h t h e a r r a y PMT p o l y c h r o m a t o r (50)· The s h i f t s i n t h e a n a l y t i c a l c u r v e s , v i s i b l e i n F i g u r e s 8 and 9 r e f l e c t t h e r e d u c t i o n i n the n e b u l i z e r e f f i c i e n c y a t the h i g h or concomitant concentrations. T h i s m a t r i x e f f e c t c o u l d be e l i m i n a t e d by u t i l i z i n g the i n t e r n a l r e f e r e n c e l i n e p r i n c i p l e to n o r m a l i z e v a r i a t i o n s i n the n e b u l i z e r e f f i c i e n c y . For t h i s n o r m a l i z a t i o n procedure

88


1905

WAVELENGTH (nm)

1*1.5

Figure 4. Net spectra after spectral stripping with 1% HN0 blank spectrum. Spectra obtained with Spectrometer 2, Table I. Key: a, 15 μg/mL TI with 5000 μg|mL Ca and Mg, and500 μg|mL Al; and b, 15 μg|mL 77. 3

ι

1955

194.0

WAVELENGTH (nm)

Figure 5. Net spectra after spectral stripping with 1% HN0 blank spectrum. Spectra obtained with Spectrometer 2, Table I. Key: a, 2.5 μg|mL As with 5000 μg|mL Ca and Mg, and500 μg|mL Al; and b, 2.5 μg|mL As. 3

GRABAU AND

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I0«J

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1 30

CONCENTRATION /uyml ) Figure 6. Analytical calibration for TI 190.8 nm after spectral stripping with 1% HN0 blank. Key: •, high concomitants level = Ca and Mg 5000 μg|mL, Al 500 μg|mL; A, medium concomitants level - Ca and Mg 750 μg|mL, A175 μ^\mL; o, low concomitants level - Ca and Mg 100 μ^ΙτηΕ, Al 10 ^glmL; and *, no concomitants. 3

blank. Concomitant

concentrations

are the same as in Figure 6.


Figure 9. Analytical calibration for As 193.7 nm after spectral stripping with 1% HN0 blank and interpolative background correction. Concomitant concentrations are the same as in Figure 6.

3

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to be s u c c e s s f u l , t h e i n t e r n a l r e f e r e n c e l i n e s h o u l d f a l l i n t h e same SPD s p e c t r a l window. Although the b a s e l i n e , i n t e r p o l a t i v e background-correction method i s v e r y e f f e c t i v e i t s l i m i t a t i o n must be u n d e r s t o o d . Whereas s p e c t r a l i n t e r f e r e n c e s c a u s e d by u n i f o r m b a s e l i n e s h i f t s and by b r o a d s m o o t h l y s l o p i n g l i n e w i n g s ( f o r s p a t i a l f r e q u e n c y f e a t u r e s a t t h e SPD f o c a l p l a n e ) can be f a i r l y a c c u r a t e l y c o r r e c t e d , t h o s e c a u s e d by s p e c t r a l o v e r l a p p i n g w i t h c o n c o m i t a n t l i n e s c a n n o t be c o r r e c t e d by t h i s s i m p l e b a s e l i n e , i n t e r p o l a t i v e p r o c e d u r e . M o r e o v e r , r e g a r d l e s s of w h e t h e r a PMT o r an SPD d e t e c t o r i s used, a c c u r a c y of t h i s i n t e r p o l a t i v e background c o r r e c t i o n method depends on t h e s t r a y r a d i a n t e n e r g y (SRE) c h a r a c t e r i s t i c s o f the s p e c t r o m e t e r u s e d . T h i s e f f e c t i s demonst r a t e d by a c o m p a r i s o n b e t w e e n F i g u r e s 4 and 5 and F i g u r e s 10 and 11. The s p e c t r a shown i n F i g u r e s 4 and 5 were o b t a i n e d w i t h s p e c t r o m e t e r 2 ( T a b l e I ) e q u i p e d w i t h a low SRE h o l o g r a p h i c grating. Those shown i n F i g u r e s 10 and 11 were o b t a i n e d w i t h s p e c t r o m e t e r 1 ( T a b l e I ) e q u i p e d w i t h a m e c h a n i c a l l y r u l e d (and r a t h e r o l d ) g r a t i n g t h a t p r o d u c e d h i g h l y s t r u c t u r e d and i r r e g u l a r background s h i f t s . These s t r u c t u r e s , o f t e n o v e r l a p a n a lyte spectral lines. The e f f e c t of t h i s o v e r l a p was s i g n i f i c a n t even a t c o n c e n t r a t i o n l e v e l s w e l l above t h e l i m i t of q u a n t i t a t i v e determination. As shown i n F i g u r e 12, s i m i l a r l y s t r u c t u r e d b a c k g r o u n d s were o b s e r v e d f o r t h e SPD and t h e PMT, t h u s r u l i n g out SPD d i o d e - t o - d i o d e r e s p o n s e v a r i a t i o n s as the c a u s e f o r t h i s phenomenon. H i g h l y s t r u c t u r e d SRE b a c k g r o u n d s have a l s o been observed w i t h other spectrometers u t i l i z i n g r u l e d g r a t i n g s (46). A l t h o u g h most s t a t e of t h e a r t r u l e d g r a t i n g s do n o t p r o d u c e s t r u c t u r e d SRE f e a t u r e s , a l a r g e number of g r a t i n g s s t i l l i n s e r v i c e may be u n s u i t a b l e f o r a c c u r a t e b a c k g r o u n d s h i f t c o r r e c tion. When i t i s i m p o s s i b l e t o s e l e c t a n a l y t e l i n e s t h a t a r e comp l e t e l y f r e e f r o m s p e c t r a l o v e r l a p by c o n c o m i t a n t s p e c t r a l l i n e s , a r r a y d e t e c t o r s may be u s e d t o r e d u c e t h e a d v e r s e e f f e c t of s u c h s p e c t r a l i n t e r f e r e n c e s . To a c c o m p l i s h t h i s t a s k , t h e s p e c t r u m of t h e known c o n c o m i t a n t i s f i r s t m e a s u r e d and s t o r e d . One or more c o n c o m i t a n t l i n e s t h a t a r e known t o be l o c a t e d i n an i n t e r f e r e n c e f r e e zone w i t h i n t h e m e a s u r e d s p e c t r a l window a r e t h e n u s e d as i n t e r n a l r e f e r e n c e l i n e s . The r e l a t i v e i n t e n s i t i e s of t h e s e l i n e s i n t h e sample s p e c t r u m a r e t h e n u s e d t o e s t a b l i s h a c o r r e c t i o n f a c t o r by w h i c h the s t o r e d c o n c o m i t a n t s p e c t r u m i s m u l t i p l i e d and s u b t r a c t e d f r o m the sample s p e c t r u m . T h i s c o r r e c t i o n p r o c e d u r e can be p e r f o r m e d f o r e a c h known c o n c o m i t a n t p r e s e n t i n t h e s a m p l e . A p r i o r k n o w l e d g e of t h e c o n c e n t r a t i o n of each c o n c o m i t a n t i s n o t r e q u i r e d . A s i m p l i f i e d v e r s i o n of t h i s s p e c t r a l s t r i p p i n g technique was d e m o n s t r a t e d w i t h the SPD f o r the V d o u b l e t ( 3 0 9 . 2 7 , 309.31 nm), w h i c h i s u n r e s o l v e d f r o m t h e A l d o u b l e t ( 3 0 9 . 2 7 , 209.28 nm). The s p e c t r a of p u r e A l , p u r e V ( a s shown i n F i g u r e 1 3 ) , and v a r i o u s m i x t u r e s o l u t i o n s of t h e two were f i r s t measured and s t o r e d .


20000 Γ -

15000

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C m m 6 5000

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Figure 10. Net spectra as in Figure 4, but obtained with Spectrometer

191.4

1, Table I.

I IOOOOI—

5000

195.1

19 WAVELENGTH (nm)

Figure 11. Net spectra as in Figure 5, but obtained with Spectrometer

1, Table I.

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Figure 13. Spectra ofl% HN0 aqueous solutions of pure V (top) and AI (bottom)ata concentration of400 μg|mL. Signal integration time = 100 ms. 3

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The A l 309.3 / A l 308.2 i n t e n s i t y r a t i o was m e a s u r e d on t h e s p e c trum o f p u r e A l , and i n t e n s i t y o f t h e A l 308.22 nm r e f e r e n c e l i n e was measured i n each o f t h e A l , V m i x t u r e s p e c t r a and m u l t i p l i e d by t h i s c o r r e c t i o n r a t i o t o c a l c u l a t e t h e i n t e n s i t y c o n t r i b u t i o n o f A l t o t h e 309.3 nm u n r e s o l v e d A l , V d o u b l e t . The a c c u r a c y o f d e t e r m i n i n g t h e a c t u a l i n t e n s i t y o f V a t t h e 309.3 nm l i n e , i n t h i s f a s h i o n , T a b l e I I I , d e c r e a s e d as t h e A l / V c o n c e n t r a t i o n r a t i o i n c r e s e d . T h i s i s e x p e c t e d b e c a u s e t h e combined s i g n a l o f t h e r e s o l v e d V, A l d o u b l e t was m a i n t a i n e d a t a r e l a t i v e l y c o n s t a n t l e v e l and t h e r f o r e , a l s o i t s a s s o c i a t e d n o i s e i . e . , m o s t l y due t o p h o t o n n o i s e and s u p e r i m p o s e d RF i n t e r f e r e n c e . A t t h e same t i m e , t h e f r a c t i o n o f t h e V s i g n a l ( w i t h i n t h e combined s i g n a l ) was r e d u c e d , r e s u l t i n g i n a c o r r e s p o n d i n g r e d u c e d p r e c i s i o n and a c c u r a c y . When a n a l y z i n g " r e a l " s a m p l e s w i t h c o m p l e x m a t r i c e s , t h e a c c u r a c y o f s p e c t r a l s t r i p p i n g becomes v e r y d e p e n d e n t on t h e s e l e c t i o n o f a p p r o p r i a t e c o n c o m i t a n t r e f e r e n c e l i n e s . G e n e r a l l y , u n d e r such c i r c u m s t a n c e s , t h e a c c u r a c y o f s p e c t r a l s t r i p p i n g c a n be s i g n i f i c a n t l y i m p r o v e d by r e s o r t i n g t o m a t r i x f o r m , l e a s t - s q u a r e s d e c o n v o l u t i o n t e c h n i q u e s (5_1) t h a t r e l y on a much w i d e r s t a t i s t i c a l d a t a b a s e , i . e . , t h e e n t i r e d i g i t i z e d spectrum (1024 p o i n t s ) . F i n a l l y , i t i s i m p o r t a n t t o c l a r i f y a few common m i s c o n c e p t i o n s r e g a r d i n g these s p e c t r a l s t r i p p i n g techniques. First, e r r o r s may be c a u s e d by u n s t a b l e g e o m e t r i c r e g i s t r a t i o n , i . e . , wavelength-to-diode c a l i b r a t i o n , r e s u l t i n g from thermal expans i o n o r c o n t r a c t i o n of t h e polychromator system. Second, the s t a t i s t i c a l n a t u r e o f t h e s e t e c h n i q u e s s e t s a l i m i t on t h e i r a c c u r a c y , e.g., i t d e t e r i o r a t e s as t h e s u b t r a c t e d i n t e r f e r r i n g l i n e s become more i n t e n s e and t h e s o u g h t a n a l y t e l i n e s l e s s i n tense. D e t e c t i o n L i m i t s , N o i s e C h a r a c t e r i s t i c s and S/N C o n s i d e r a t i o n . The d e t e c t i o n l i m i t s m e a s u r e d w i t h t h e u n i n t e n s i f i e d SPD d e t e c t i o n s y s t e m o p e r a t e d i n t h e o n - t a r g e t i n t e g r a t i o n r e a d o u t mode a r e compared t o t h o s e o b t a i n e d w i t h t h e ISPD ( i n t e n s i f i e d SPD) and t h e SIT ( b o t h h i g h g a i n m u l t i c h a n n e l d e t e c t o r s ) and t h e PMT, a h i g h g a i n s i n g l e channel d e t e c t o r , Table IV. I t i s important t o r e a l i z e t h a t t h e S I T u s e d i n t h i s s t u d y was u n c o o l e d . I t i spossible t o c o o l a S I T t o d r y - i c e t e m p e r a t u r e s and t h u s e f f e c t i v e l y a c h i eve l o n g s i g n a l i n t e g r a t i o n t i m e s (52^)· However, c o o l i n g a S I T i s m e c h a n i c a l l y d i f f i c u l t , and r e q u i r e s e l a b o r a t e s c a n n i n g and t a r g e t p r e p a r a t i o n t e c h n i q u e s , t o overcome t h e h i g h d i s c h a r g e l a g c h a r a c t e r i s t i c s of v i d i c o n d e v i c e s . I t i s , g e n e r a l l y , r a t h e r d i f f i c u l t t o m a i n t a i n a good l i n e a r i t y ( s i g n a l r e c i p r o c i t y ) w i t h cooled SIT's. U l t r a s o n i c sample n e b u l i z a t i o n was u s e d t h r o u g h o u t t h i s study. These c o m p a r a t i v e d e t e c t i o n l i m i t v a l u e s , h o w e v e r , must be c a u t i o u s l y e v a l u a t e d s i n c e t h e a b s o l u t e p e r f o r m a n c e o f a l l f o u r d e t e c t o r s c a n v a r y by a f a c t o r o f 2 - 4. T h i s i s p a r t i c u l a r l y t r u e f o r t h e S I T , f o r w h i c h s p a t i a l and s p e c t r a l v a r i a t i o n s i n photocathode response, s i g n a l a m p l i f i c a t i o n , e f f i c i e n c y of the

GRAB AU AND

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Ο

ο

ω

Ο 1

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u CO u •

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•

•

rH

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—/ Φ rH CO CM u r i n e , by 3.5 t i m e s i n 507> serum, and by 14

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11

Figure 19. Emission pulsefrom 100 μΖ,, 1.0 μg/mL Mn; 10 ng/mL Mn + 500 μg|mL Ca; and500 μg|mL Ca, observed with a PMT at 257.6 nm.

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t i m e s i n 1 0 % b l o o d compared t o v a l u e s r e p o r t e d f o r d i l u t e a c i d solutions. The n o r m a l c o n c e n t r a t i o n o f many t r a c e e l e m e n t s i n b i o l o g i c a l f l u i d s i s below the l i m i t of q u a n t i t a t i v e determinat i o n o f p u l s e d ( 1 0 0 u L ) ICP-AES. N e v e r t h e l e s s , c l i n i c a l l y s i g n i f i c a n t e l e m e n t s such a s Na, K, Mg, C a , P, F e , Cu and Zn c a n s t i l l be d e t e r m i n e d by t h i s t e c h n i q u e . S i g n i f i c a n t l y improved d e t e c t i o n l i m i t s , a l t h o u g h a t t h e expense of reduced s p e c t r a l r e s o l u t i o n , c o u l d be e x p e c t e d w i t h t h e ISPD d e t e c t o r .

T a b l e X I I . 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 n S I T f r o m 100 uL s a m p l e s compared t o c o n t i n u o u s n e b u l i z a t i o n and compared t o AAS detection limits.

Element Al As Be Cd Co Cr Cu Fe Mn Mo Ni Pb Se V Zn

WaveLength 308.2 193.7 313.0 226.5 228.6 205.5 324.7 261.1 257.6 386.3 351.5 283.3 196.0 311.0 213.8

nm

D e t e c t i o n L i m i t s ng/ml ICP-AES pulsed continuous u l t r a sonic n e b u l i z a t i o n nebulization SIT PMT SIT 0.2 4 2 200 40 0.08 0.02 0.003 1.2 0.07 1.0 0.2 3 0.08 3 1 0.4 0.06 4 0.09 0.3 0.07 0.01 25 0.3 0.2 6 70 1 14 350 1 65 2 0.06 0.3 1 0.1 0.2

AAS graphite furnace 0.02 0.2 0.03 0.001 0.4 0.003 0.01 0.05 0.002 0.05 0.04 0.02 0.1 0.02 0.001

F i g u r e 19 i s a l s o an example o f t h e b a c k g r o u n d s h i f t e r r o r s e n c o u n t e r e d w i t h ICP-AES. The t h r e e e m i s s i o n ( 1 0 0 u L ) p u l s e s were m e a s u r e d w i t h a PMT a t 257.6 nm. W i t h o u t t h e b e n e f i t o f b a c k g r o u n d c o r r e c t i o n t h e c a l c i u m c o n c o m i t a n t c a u s e d an e r r o r o f 52% i n t h e d e t e r m i n a t i o n o f t h e Mn i n t h e Ca i n t e r f e r e d s o l u t i o n . W i t h t h e S I T , where an e n t i r e s p e c t r a l window was s i m u l t a n e o u s l y r e c o r d e d f o r t h e Mn + Ca s o l u t i o n s , b a c k g r o u n d s u b t r a c t i o n was p o s s i b l e and r e s u l t e d i n a mere 37 e r r o r i n t h e Mn measurement. T h i s e r r o r i s w i t h i n t h e 1-3% s a m p l i n g p r e c i s i o n o b t a i n e d w i t h 100 uL s a m p l e s . Copper and z i n c were d e t e r m i n e d i n 100 uL s a m p l e s o f w h o l e b l o o d , h o r s e b l o o d serum and human u r i n e t o d e m o n s t r a t e t h e a p p l i c a b i l i t y o f t h i s t e c h n i q u e t o r e a l s a m p l e s . Sample p r e p a r a t i o n was k e p t t o a minimum. S t r a i g h t f o r e w a r d d i l u t i o n was t h e c

4.

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only preparatory step. I t was, h o w e v e r , f o u n d a d v a n t a g e o u s t o i n c o r p o r a t e an i n t e r n a l r e f e r e n c e , Y, i n t h e d i l u t e d s o l u t i o n i n o r d e r t o m o n i t o r t h e n e b u l i z a t i o n e f f i c i e n c y of e a c h s a m p l e . U l t r a s o n i c n e b u l i z a t i o n of d i l u t e b l o o d s o l u t i o n s was g r e a t l y i m p r o v e d by i n c r e a s i n g t h e power i n p u t t o t h e u l t r a s o n i c t r a n s d u c e r f r o m 100 w a t t s t o 150 w a t t s . The n e b u l i z a t i o n e f f i c i e n c y was m o n i t o r e d by o b s e r v i n g t h e n e t a n a l y t e i n t e n s i t i e s . The i n c r e a s e i n power i m p r o v e d t h e n e b u l i z a t i o n e f f i c i e n c y o f 1% H N O 3 s o l u t i o n by 2 5 % t h a t of 50% b l o o d serum, by 50% and t h a t o f 10% w h o l e b l o o d by 5007,· D e t e r m i n a t i o n o f c o p p e r and z i n c i n w h o l e b l o o d samples made f r o m a r e f e r e n c e c u r v e and by t h e method o f s t a n d a r d a d d i t i o n s a r e shown i n T a b l e X I I I . The z i n c 213.8 nm l i n e was s u p e r i m p o s e d as a s h o u l d e r on t h e i n t e n s e Ρ 213.6 nm l i n e and b a s e l i n e i n t e r p o l a t i o n was n e c e s s a r y t o compensate f o r Ρ s p e c t r a l interference. S i m i l a r l y a l l u s e f u l Cu l i n e s were s p e c t r a l l y o v e r l a p p e d by i r o n l i n e s . The c o r r e c t i o n s f o r t h e i r o n i n t e r f e r e n c e s were p e r f o r m e d t h r o u g h l i n e r a t i o c o m p a r i s o n s w i t h o t h e r n o n - i n t e r f e r e d i r o n l i n e s , s i m u l t a n e o u s l y measured. Without t h e s e c o r r e c t i o n p r o c e d u r e s , c o n c e n t r a t i o n e r r o r s f o r Cu and Zn w o u l d h a v e e x c e e d e d 1007>. I t seems, t h e r e f o r e , t h a t p u l s e d ICP-AES (100 uL) c o u l d be s u c c e s s f u l l y employed i n t h e d e t e r m i n a t i o n o f c l i n i c a l l y s i g n i f i c a n t e l e m e n t s s u c h as Na, K, Mg, Ca, P, F e , Cu and Zn i n w h o l e b l o o d , b l o o d serum, u r i n e and s p i n a l f l u i d s . The a r r a y d e t e c t o r s have a d e f i n i t e a d v a n t a g e i n c o m p e n s a t i n g f o r b a c k g r o u n d and specific spectral interferences. W i t h the d e t e c t o r s used i n t h i s s t u d y , a l l of the c l i n i c a l l y s i g n i f i c a n t e l e m e n t s c o u l d n o t have been o b s e r v e d s i m u l t a n e o u s l y i n a s i n g l e s p e c t r a l window. W i t h t h e SPD d e t e c t o r Mg, P, F e , Cu and Zn a r e d e t e r m i n a b l e i n a s i n g l e s i m u l t a n e o u s s p e c t r a l window, 202-225 nm. The r e m a i n i n g t h r e e may be d e t e r m i n e d i n a 40 nm window as shown by B u s c h (K 808.8 nm; Na 818.3 nm, s e c o n d o r d e r ; Ca 845.4 nm, s e c o n d o r d e r ) , s i n c e a l o w e r s p e c t r a l r e s o l u t i o n i s required in this region. Conclusion This study suggests that determinations w i t h accuracy s i m i l a r t o PMT s y s t e m s can be a c h i e v e d w i t h SPD d e t e c t o r s i f p o l y c h r o m a t e r s w i t h h i g h e r d i s p e r s i o n and h i g h q u a l i t y g r a t i n g s a r e u s e d . However, f o r s u c h s y s t e m s t o be p r a c t i c a l , t h e s p e c t r a l window ( r e d u c e d by t h e i n c r e a s e d d i s p e r s i o n ) must a l s o be i n creased. A new 4096 e l e m e n t d i o d e a r r a y i s now a v a i l a b l e f r o m R e t i c o n ( M o d e l RL-4096/20 w i t h a 15 um c e n t e r s and a 508 um d i o d e height). The a s p e c t r a t i o , h o w e v e r , i s o n l y 34:1 compared t o t h e 100:1 r a t i o o f t h e RL-10245 d e t e c t o r . Consequently, photon c o l l e c t i o n i s a l s o l e s s e f f i c i e n t , t h e o r e t i c a l l y r e s u l t i n g i n worse detection limits. T h i s l o s s i n p h o t o n c o l l e c t i o n c a n be g r e a t l y a l l e v i a t e d by ' c o n c e n t r a t i n g t h e s p e c t r a l s i g n a l w i t h a c y l i n d r i c a l l e n s , d i r e c t l y a t t a c h e d t o the a r r a y ( 5 4 ) . 1

114


The s i l u l t a n e o u s s p e c t r a l window o f t h i s new 4096 e l e m e n t d e t e c t o r , w i t h a P M T - l i k e s p e c t r a l r e s o l u t i o n w i l l be e x t e n d e d t o a t l e a s t 32 nm. B e c a u s e a n a l y t e s p e c t r a l l i n e s a r e r a n d o m l y d i s t r i b u t e d across the e n t i r e U V - v i s i b l e spectrum, the i n c r e a s e i n t h e p r o b a b i l i t y o f t h e i r i n c l u s i o n i n e a c h expanded s p e c t r a l w i n dow i s more t h a n j u s t f o u r f o l d . Most p r o b a b l y , f o r t h e m a j o r i t y of a n a l y t i c a l a p p l i c a t i o n s o n l y 3-5 s u c h expanded s p e c t r a l w i n dows w i l l s u f f i c e t o i n c l u d e a l l a n a l y t e l i n e s s o u g h t . A d i f f e r e n t s o l u t i o n t o t h i s s p e c t r a l window v s . s p e c t r a l r e s o l u t i o n dilemma m a i n t a i n s t h e 1024 d i o d e a r r a y b u t r e s o r t s t o t h e u s e o f a m u l t i p l e e n t r a n c e s l i t S P D / p l y c h r o m a t o r scheme ( 8 ) t h a t s h o u l d be a b l e t o p r o d u c e a r a p i d s u c c e s s i o n o f c o n s e c u t i v e l y i n t e r c h a n g i n g and s e l e c t i v e l y c h o s e n s p e c t r a l windows. W i t h t h i s t e c h n i q u e a r a n d o m l y a c c e s s e d s p e c t r a l window may be o b s e r ved w i t h o u t t h e r e q u i r e m e n t o f a h i g h t o l e r a n c e m e c h a n i c a l scanning device. F i n a l l y , t h e e f f e c t s o f s p e c t r a l i n t e r f e r e n c e s c o u l d be a t l e a s t p a r t i a l l y a l l e v i a t e d by more e f f e c t i v e l y e x p l o i t i n g t h e m u l t i c h a n n e l n a t u r e o f t h e SPD. S p e c t r a l i n t e r f e r e n c e s c o u l d be e a s i l y r e c o g n i z e d v i a l i n e i n t e n s i t y r a t i o s and a l t e r n a t i v e , n o n i n t e r f e r e d a n a l y t e l i n e s c o u l d be c h o s e n o r s p e c t r a l s t r i p p i n g p r o c e d u r e s may be u s e d , as p r e v i o u s l y d i s c u s s e d .

Acknowledgments We w o u l d l i k e t o t h a n k D r . V e l m e r F a s s e l , i n whose l a b o r a t o r y t h i s work was p e r f o r m e d , f o r h i s h o s p i t a l i t y and h i g h l y a p p r e c i a t ed a s s i s t a n c e i n r e v i e w i n g t h i s m a n u s c r i p t .

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Inductively Coupled Plasma-Atomic Emission Spectroscopy with

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