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V o o r de t o t s t a n d k o m i n g v a n d i t p r o e f s c h r i f t h e b i k e e n b e r o e p m o e t e n d o e n o p d i v e r s e p e r s o n e n e n i n s t e l l i n g e n b i n n e n de T . H . D e l f t e n d a a r b u i t e n . I k d a n k h i e r v o o r a l l e n u i t de g r o n d v a n m i j n h a r t . S l e c h t s e n k e -l e n z a -l i k met name v e r m e -l d e n , v a n w e g e de u i t z o n d e r -l i j k e b i j d r a g e d i e z i j g e l e v e r d h e b b e n . I n de e e r s t e p l a a t s m i j n p r o m o t o r L e o de G a l a n . L e o b e d a n k t v o o r de p i t t i g e , m a a r v o o r a l o o k l e u k e d i s c u s s i e s e n v o o r j o u w e n t h o u s i a s t e e n s t i -m u l e r e n d e b e g e l e i d i n g . H e t h e e f t m i j v e e l g e n o e g e n g e d a a n , d a t i k e e n b i j d r a g e h e b k u n n e n l e v e r e n i n de o p l e i d i n g v a n e e n a a n z i e n l i j k g e d e e l t e v a n de v r o u w e l i j k e , D e l f t s e i n g e n i e u r s . L i e s b e t h , j e v i n d t j o u w w e r k o n g e t w i j f e l d t e r u g i n d i t p r o e f -s c h r i f t . B e d a n k t , G e r a r d K l e i j e r , v o o r h e t m a k e n v a n de z o s u c c e s v o l l e i n s t r u m e n -t a -t i e . G r o -t e w a a r d e r i n g g a a -t o o k u i -t n a a r F r a n s B o l m a n , d i e m i j n w a r r i g e v e r h a l e n s t e e d s k o n v e r t a l e n i n z e e r h e l d e r e p l a a t j e s . De n . v . P h i l i p s ' G l o e i l a m p e n f a b r i e k e n b e n i k e r k e n t e l i j k v o o r h e t t e r b e s c h i k k i n g s t e l l e n v a n e e n R F - g e n e r a t o r . V a n d e z e f i r m a w i l i k met name de h e e r J . W . de R u i t e r noemen v a n w e g e z i j n n u t t i g e i n b r e n g . T e n s l o t t e i s A d K l o k h e t v e r m e l d e n w a a r d , a l i s h e t a l l e e n a l omdat h i j h e t e n k e l e j a r e n met m i j o p é é n k a m e r h e e f t u i t g e h o u d e n . 3CONTENTS
V o o r w o o r d 3 C o n t e n t s * C h a p t e r I : G e n e r a l I n t r o d u c t i o n 7 C h a p t e r I I : A s a m p l e i n t r o d u c t i o n s y s t e m f o r an i n d u c t i v e l y c o u p l e d p l a s m a o p e r a t i n g o n a n a r g o n c a r r i e r g a s f l o w o f 0 . 1 1 / m i n1^ 2 . 1 I n t r o d u c t i o n 15 2 . 2 E x p e r i m e n t a l 2 . 2 . 1 B a b i n g t o n n e b u l i z e r 16 2 . 2 . 2 S a m p l e i n t r o d u c t i o n 16 2 . 2 . 3 N e b u l i z e r c h a m b e r 17 2 . 2 . 4 M e a s u r e m e n t s 17 2 . 3 R e s u l t s a n d D i s c u s s i o n 18 2 . 4 Summary 2 0 2 ) C h a p t e r I I I : A n i n d u c t i v e l y c o u p l e d p l a s m a u s i n g 1 1 / m i n o f a r g o n 3 . 1 I n t r o d u c t i o n 2 1 3 . 2 E x p e r i m e n t a l 3 . 2 . 1 Work c o i l 22 3 . 2 . 2 T o r c h 2 2 3 . 3 A n a l y t i c a l p e r f o r m a n c e 2 3 3 . 4 C o n c l u s i o n s 26 C h a p t e r I V : E m p i r i c a l p o w e r b a l a n c e s f o r c o n v e n t i o n a l a n d e x t e r n a l l y 3 ) c o o l e d i n d u c t i v e l y c o u p l e d a r g o n p l a s m a s 4 . 1 I n t r o d u c t i o n 27 4 . 2 T h e o r y 4 . 2 . 1 P l a s m a m o d e l 2 8 44 . 2 . 2 I n c i d e n t p o w e r 29 4 . 2 . 3 C o n v e c t i o n 30 4 . 2 . 4 C o n d u c t i o n 31 4 . 2 . 5 P l a s m a r a d i a t i o n 32 4 . 3 E x p e r i m e n t a l 3 2 4 . 4 R e s u l t s a n d D i s c u s s i o n 4 . 4 . 1 I n c i d e n t p o w e r 34 4 . 4 . 2 C o n v e c t i o n 37 4 . 4 . 3 C o n d u c t i o n 38 4 . 4 . 4 P l a s m a r a d i a t i o n 4 2 4 . 5 C o n c l u s i o n s 4 2 A d d i t i o n a l n o t e 4 4 A p p e n d i x 4 5 C h a p t e r V : A n a l y t i c a l p e r f o r m a n c e o f i n d u c t i v e l y c o u p l e d a r g o n 4 ) p l a s m a s w i t h e x t e r n a l c o o l i n g 5 . 1 I n t r o d u c t i o n 5 . 2 E x p e r i m e n t a l 5 . 3 I n f l u e n c e o f o p e r a t i n g c o n d i t i o n s u p o n s i g n a l -t o - b a c k g r o u n d r a -t i o 5 . 3 . 1 G e n e r a l r e m a r k s 5 . 3 . 2 C o o l a n t f l o w r a t e s 5 . 3 . 3 O u t e r t o r c h t u b e 5 . 3 . 4 S a m p l e i n t r o d u c t i o n 5 . 3 . 5 P l a s m a a r g o n f l o w r a t e 5 . 3 . 6 I n c i d e n t p o w e r 5 . 3 . 7 O b s e r v a t i o n h e i g h t 5 . 3 . 8 C o n c l u s i o n 5 . 4 O p e r a t i o n a l c h a r a c t e r i s t i c s 5 . 4 . 1 I g n i t i o n 5 . 4 . 2 W o r k i n g r a n g e o f o p e r a t i n g c o n d i t i o n s 5 . 4 . 3 C o n t i n u o u s o p e r a t i o n 5 . 4 . 4 Maximum p e r m i s s i b l e s a l t u p t a k e , o r g a n i c s o l v e n t s 5 . 4 . 5 C o n c l u s i o n 5 . 5 A n a l y t i c a l p e r f o r m a n c e 5 47 48 50 52 53 53 56 57 58 60 61 6 1 6 3 6 3 64
5 . 5 A n a l y t i c a l p e r f o r m a n c e 5 . 5 . 1 D e t e c t i o n l i m i t s 6 5 5 . 5 . 2 P r e c i s i o n a n d S t a b i l i t y 67 5 . 5 . 3 D y n a m i c r a n g e o f c a l i b r a t i o n c u r v e s 6 9 5 . 5 . 4 M a t r i x e f f e c t s a n d A c c u r a c y 7 0 5 . 5 . 5 C o n c l u s i o n 7 5 5 . 6 D i s c u s s i o n 7 5 Summary 79 S a m e n v a t t i n g 81 1 ) R e p r i n t v a n S p e c t r o c h i m . A c t a 3 6 B , 71 ( 1 9 8 1 ) 2 ) R e p r i n t v a n S p e c t r o c h i m . A c t a 3 7 B , 7 3 3 ( 1 9 8 2 ) 3) R e p r i n t v a n S p e c t r o c h i m . A c t a 3 8 B , 7 0 7 ( 1 9 8 3 ) 4 ) S u b m i t t e d t o A n a l . C h e m . 6
CHAPTER I
GENERAL INTRODUCTION T h e d e t e r m i n a t i o n o f m e t a l l i c e l e m e n t s i n a v a r i e t y o f s a m p l e s i s i m -p o r t a n t f o r many -p u r -p o s e s , w h i c h c a n b e f o u n d i n a f i e l d , r a n g i n g f r o m m i n i n g a n d h e a v y m e t a l i n d u s t r y v i a e n v i r o n m e n t a l a n d t o x i c o l o g i c a l l a b o -r a t o -r i e s ( h e a v y m e t a l s ) t o m i c -r o - e l e c t -r o n i c a l i n d u s t -r y ( s e m i - c o n d u c t o -r s ) . T h e s a m p l e s c a n b e i n t h e s o l i d ( o r e s , s t e e l ) , l i q u i d ( w a s t e w a t e r , b o d y f l u i d s ) o r e v e n i n t h e g a s e o u s s t a t e ( a i r . e x h a u s t ) a n d s o m e t i m e s t h e s a m p l e s t a t e i s n o t c l e a r l y d e f i n e d ( s o i l , f o o d ) . A p a r t f r o m t h e w i d e l y d i f f e r e n t s a m p l e m a t r i x , t h e c o n c e n t r a t i o n l e v e l c a n v a r y f r o m m a j o r t o u l t r a t r a c e a n d a n a l y t i c a l demands c a n c h a n g e f r o m o n e p r o b l e m t o a n o t h e r . F o r m e t a l m a n u f a c t u r e r s , f o r i n s t a n c e , i t i s e s s e n t i a l t o k n o w a c c u r a t e l y a n d p r e c i s e l y t h e b u l k c o n c e n t r a t i o n o f c o m p o n e n t s i n o r e s , i n o r d e r t o a g r e e o n a f a i r p r i c e w i t h t h e d e l i v e r e r . By c o n t r a s t , i n e n v i r o n m e n t a l a n d t o x i c o l o g i c a l p r o b l e m s o f t e n u l t r a t r a c e l e v e l s o f h e a v y m e t a l s m u s t b e d e t e r m i n e d , b u t a c c u r a c y a n d p r e c i s i o n a r e u s u a l l y n o t s o i m p o r t a n t . F o r p r o p e r t i e s l i k e c o r r o s i o n r e s i s t a n c e a n d c o n d u c t i v i t y a c c u r a t e k n o w -l e d g e o f s u r f a c e , r a t h e r t h a n b u -l k c o n c e n t r a t i o n s i s n e c e s s a r y . O b v i o u s l y , a l l t h e s e p r o b l e m f i e l d s c a n n o t b e c o v e r e d w i t h o n e t e c h -n i q u e o r p r o c e d u r e a -n d s e v e r a l i -n s t r u m e -n t a l m e t h o d s h a v e b e e -n d e v e l o p e d , a l l w i t h t h e i r own f i e l d o f a p p l i c a t i o n : n e u t r o n a c t i v a t i o n a n a l y s i s , X - r a y f l u o r e s c e n c e , X - r a y d i f f r a c t i o n , e l e c t r o n s c a t t e r i n g , i n o r g a n i c m a s s s p e c t r o m e t r y a n d a t o m i c s p e c t r o s c o p y . I n t h e l a t t e r t e c h n i q u e t h e c l a s s i c a l e m i s s i o n f o r m w i t h a r c a n d s p a r k d i s c h a r g e s h a s t o a l a r g e e x -t e n -t b e e n r e p l a c e d b y a -t o m i c a b s o r p -t i o n w i -t h e i -t h e r f l a m e [1] o r f u r n a c e [2] a n d b y a t o m i c e m i s s i o n w i t h a n i n d u c t i v e l y c o u p l e d p l a s m a [ 3 - 5 ] . T h e s e t e c h n i q u e s a r e u s u a l l y a p p l i e d t o l i q u i d s a m p l e s . F l a m e a t o m i c a b s o r p t i o n ( F A A S ) p r o v i d e s r a p i d s i n g l e e l e m e n t a n a l y s i s a t m o d e r a t e d e t e c t i o n p o w e r ( t y p i c a l l y 10 p g / l ) , b u t some p r e c a u t i o n s a g a i n s t m a t r i x i n t e r f e r e n c e s c a n b e n e c e s s a r y . F A A S i s e s p e c i a l l y s u i t a b l e 7f o r t h e d e t e r m i n a t i o n o n a r o u t i n e b a s i s o f a f e w e l e m e n t s i n l a r g e n u m b e r s o f s a m p l e s . G r a p h i t e f u r n a c e a t o m i c a b s o r p t i o n ( G F A A S ) p o s e s s e s e x c e l l e n t d e t e c t i o n p o w e r ( 0 . 0 1 0 . 1 0 ] J g / l ) , a g a i n f o r s i n g l e e l e m e n t a n a -l y s i s , b u t w i t h s i g n i f i c a n t m a t r i x i n t e r f e r e n c e s a n d a t a -l o w e r s p e e d . I t i s u s e f u l w h e n h i g h d e t e c t i o n p o w e r i s n e e d e d . I n d u c t i v e l y c o u p l e d p l a s m a a t o m i c e m i s s i o n s p e c t r o m e t r y ( I C P - A E S ) c o m b i n e s g o o d d e t e c t i o n p o w e r ( 1 y g / 1 ) w i t h s m a l l m a t r i x e f f e c t s a n d m u l t i - e l e m e n t c a p a b i l i t i e s . T h i s i s n o t o n l y f a c i l i t a t e d b y p r a c t i c a l r e a s o n s o f e q u i p m e n t ( e m i s s i o n v s . a b s o r p t i o n ) , b u t a l s o b e c a u s e o f t h e l a r g e d y n a m i c r a n g e o f c a l i b r a t i o n c u r v e s , w h i c h c o v e r s some f i v e d e c a d e s . T h e f a v o u r a b l e p r o p e r t i e s o f I C P ' s a r e t o a l a r g e e x t e n t c a u s e d b y t h e h i g h t e m p e r a t u r e s w h i c h c a n b e o b t a i n e d i n p l a s m a s . W h e r e a s f l a m e s h a v e a n u p p e r l i m i t o f s o m e 3 5 0 0 K a n d a r c s a n d s p a r k s o f 5 0 0 0 K , t e m p e r a t u r e s up t o 1 0 , 0 0 0 K h a v e b e e n r e p o r t e d i n p l a s m a s [ 6 , 7 ] . A n a p p r o p r i a t e p o w e r t r a n s f e r s y s t e m i s e s s e n t i a l i n o r d e r t o a c h i e v e t h e s e h i g h t e m p e r a t u r e s . T h e m o s t p r o m i s i n g s y s t e m s o f a r was d e v e l o p e d i n t h e 1 9 6 0 ' s b y
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[3] a n d i s b a s e d o n t h e p r i n c i p l e o f i n d u c t i v e c o u p l i n g : a n a l t e r n a t i n g c u r r e n t i n a p r i m a r y c o i l c a u s e s a c o r r e s p o n d i n g c u r r e n t i n a s e c o n d a r y c o i l . T h i s p r i n c i p l e i s e x t e n s i v e l y a p p l i e d i n a l l k i n d s o f e l e c t r i c a l c i r c u i t s , a s t r a n s f o r m e r s . I n o u r c a s e t h e s e c o n d a r y c o i l i s c r e a t e d i n a g a s f l o w i n g t h r o u g h a q u a r t z t u b e , w h i c h , i n t u r n , i s s i t u a t e d i n s i d e t h e p r i m a r y c o i l , i . e . t h e w o r k c o i l ( S e e f i g . 1 ) . I n t h e f l o w i n g p l a s m aft
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Fig. 1, Sahematioal drawing of an inductively coupled plasma, as used for
atomic emission spectrometry
g a s , F . , a r i n g s h a p e d h o t p l a s m a i s g e n e r a t e d b y means o f t h e i n d u -p l a s m a c e d c u r r e n t . T h e w o r k c o i l i s c o n n e c t e d t o a r a d i o f r e q u e n c y g e n e r a t o r ( 2 5 5 0 M H z ) a n d when c a r e f u l l y d i m e n s i o n a t e d , t h e s y s t e m i s q u i t e e f f i -c i e n t : 7 0 t o 8 0 % o f t h e g e n e r a t e d p o w e r i s t r a n s f e r r e d t o t h e p l a s m a . A l t h o u g h n i t r o g e n h a s b e e n u s e d a s a p l a s m a g a s [ 8 1 0 ] , a r g o n i s a n a l y t i -c a l l y m o r e u s e f u l a n d some 15 t o 2 0 1 / m i n i s n e -c e s s a r y t o s u s t a i n a n I C P . T h e s a m p l e i s u s u a l l y i n t r o d u c e d a s a n a e r o s o l i n a 1 1 / m i n a r g o n f l o w i n t o t h e c e n t r e o f t h e p l a s m a . O c c a s i o n a l l y , i f o r g a n i c s o l v e n t s a r e a s -p i r a t e d , a n i n t e r m e d i a t e a r g o n f l o w o f 0 . 5 1 / m i n i s a -p -p l i e d , m e r e l y t o l i f t t h e p l a s m a s l i g h t l y [ 1 1 ] . I n a n y c a s e t h e t o t a l c o n s u m p t i o n o f a n I C P i s i n t h e o r d e r o f 2 0 1 / m i n , c o n t r i b u t i n g t o t h e r u n n i n g c o s t b y H f l . 3 0 , 0 0 0 / y e a r . F u r t h e r m o r e , t o h e a t t h i s a m o u n t o f a r g o n , i n c i d e n t p o w e r l e v e l s b e t w e e n 1 . 0 a n d 1 . 5 kW a r e n e c e s s a r y , c o r r e s p o n d i n g t o g e n e r a t o r p o w e r s up t o 2 kW. T h e s e h i g h p o w e r l e v e l s a r e o n l y a c h i e v a b l e w i t h e x p e n s i v e a n d b u l k y R F - g e n e r a t o r s u s i n g v a c u u m t u b e s . T h e a i m o f t h e p r e s e n t s t u d y i s t o r e d u c e r u n n i n g c o s t s b y a s u b s t a n t i a l r e d u c t i o n i n a r g o n c o n s u m p t i o n w i t h o u t s a c r i f i c i n g a n a l y t i -c a l -c a p a b i l i t i e s . I f , f o r t h e s o - d e v e l o p e d p l a s m a g e n e r a t o r p o w e r l e v e l s b e l o w 0 . 5 kW a p p e a r t o b e a p p r o p r i a t e , s i m p l e s o l i d s t a t e R F - g e n e r a t o r s c a n be u s e d . S i n c e t h e l a r g e p l a s m a g a s f l o w r a t e i s n o t o n l y n e c e s s a r y t o b u i l d t h e p l a s m a b u t a l s o t o p r o t e c t t h e o u t e r q u a r t z t u b e , a d e c r e a s e i n t h e f l o w r a t e w i l l e v e n t u a l l y l e a d t o t o r c h d e f o r m a t i o n , i f a h i g h p l a s m a t e m p e r a t u r e i n t h e c e n t r e i s t o b e m a i n t a i n e d . T h e r e a r e e s s e n t i a l l y t w o d i f f e r e n t m e t h o d s t o a c h i e v e a r e d u c t i o n i n t h e a r g o n c o n s u m p t i o n , w h i l e a v o i d i n g o v e r h e a t i n g p r o b l e m s . Some i n v e s t i g a t o r s h a v e t r i e d t o i n f l u e n c e t h e i n t e r n a l c o o l i n g a c t i o n o f t h e p l a s m a a r g o n b y c h a n g e s i n t o r c h d i -m e n s i o n s [ 1 2 - 2 0 ] . A s s h o w n i n f i g . 2 a a n d 2b t h e a p p r o a c h o f Barnes a n d c o - w o r k e r s [ 1 2 - 1 4 ] a n d o f Hieftje a n d c o - w o r k e r s [ 1 5 - 1 9 ] i s s i m i l a r . T h e y b o t h make a r g o n c o o l i n g m o r e e f f i c i e n t b y a p p l y i n g a t u l i p s h a p e d i n t e r -m e d i a t e t u b e i n t h e i r t o r c h d e s i g n a n d b y -m i n i a t u r i z a t i o n o f t h e t o r c h . T o r c h e s w i t h d i a m e t e r s down t o 9 mm ( v s . 18 mm c o n v e n t i o n a l l y ) h a v e b e e n o p e r a t e d . T h e l o w e r p r a c t i c a l l i m i t o f a r g o n c o n s u m p t i o n w i t h t h i s a p p r o a c h
a p p e a r s t o b e some 5 - 8 1 / m i n . T h e same r e s u l t i s r e p o r t e d b y Zowe [ 2 0 ] ,
who i n f l u e n c e s t h e a r g o n c o o l i n g w i t h a n o p p o s i t e a p p r o a c h , n a m e l y b y
c r e a t i n g a r e l a t i v e l y l a r g e d i s t a n c e b e t w e e n i n t e r m e d i a t e a n d o u t e r t u b e
Fig. 2. Some approaches to achieve low argon consumption
( f i g . 2 c ) . H e r e a l s o an a d a p t i o n o f w o r k c o i l a n d o s c i l l a t o r c i r c u i t i s
n e c e s s a r y .
A m o r e d r a s t i c r e d u c t i o n o f t h e a r g o n c o n s u m p t i o n i s o n l y p o s s i b l e b y
r e p l a c i n g a t l e a s t p a r t o f t h e i n t e r n a l c o o l i n g a c t i o n o f a r g o n b y e x t e r
-n a l c o o l i -n g o f t h e t o r c h . T h i s m e t h o d was a p p l i e d f o r t h e f i r s t t i m e by
Britske et at. [ 2 1 ] , who r e p o r t e d a n a r g o n c o n s u m p t i o n o f 4 1 / m i n i n a r e l a t i v e l y l a r g e t o r c h ( 4 0 mm) b y u s i n g w a t e r a s a c o o l a n t m e d i u m ( f i g . 2 d ) . A w a t e r - c o o l e d t o r c h w i t h a c o n v e n t i o n a l d i a m e t e r ( 1 8 mm) was c o n s t r u c t e d b y Kornblwn et at. [ 2 2 ] , r e s u l t i n g i n an a r g o n c o n s u m p t i o n a s l o w a s 2 1 / m i n ( f i g . 2 e ) . U n f o r t u n a t e l y , a n a l y t i c a l r e s u l t s w i t h t h i s t o r c h w e r e u n s a t i s f a c t o r y , f o r r e a s o n s w h i c h w i l l be d i s c u s s e d b e l o w . T h e s y s t e m d e s -c r i b e d b y Kornblwn et at. -c a n be s e e n a s t h e s t a r t i n g p o i n t o f t h e p r e s e n t i n v e s t i g a t i o n . D u r i n g t h i s i n v e s t i g a t i o n Kawaguahi et at. [ 2 3 ] p r e s e n t e d a w a t e r - c o o l e d t o r c h w i t h p r o m i s i n g a n a l y t i c a l p r o p e r t i e s , b u t f o r t h e i r r e s u l t s a r g o n f l o w s o f 4 t o 5 1 / m i n w e r e n e c e s s a r y ( f i g . 2 f ) . T h e f i r s t r e a s o n f o r t h e u n s a t i s f a c t o r y a n a l y t i c a l r e s u l t s o f Kornblwn's t o r c h d e s i g n l i e s i n t h e f a c t , t h a t t h e l o w p l a s m a a r g o n f l o w r a t e a l s o e n t a i l s a l o w s a m p l e i n t r o d u c t i o n g a s f l o w r a t e . C o n v e n t i o n a l n e b u l i z e r s d o n o t f u n c t i o n o n f l o w r a t e s o f t y p i c a l l y 0 . 1 1 / m i n a n d Kornblwn et at. u s e d a s p l i t t i n g s y s t e m b e t w e e n n e b u l i z e r a n d p l a s m a . I n c h a p t e r 2 a n e -b u l i z e r -b a s e d o n t h e Ba-bington p r i n c i p l e [ 2 4 ] w i l l -b e d e s c r i -b e d , w h i c h h a s b e e n e s p e c i a l l y d e v e l o p e d f o r a f l o w r a t e o f 0 . 1 1 / m i n . A e r o s o l t r a n s -p o r t e f f i c i e n c y m e a s u r e m e n t s r e v e a l t h a t t h e d e s c r i b e d s y s t e m s h o u l d be a p p r o p r i a t e f o r l o w c o n s u m p t i o n I C P ' s . F r o m a d i s c u s s i o n o n t h e m e a s u r e m e n t
t e c h n i q u e ( s i l i c a a d s o r p t i o n ) w i t h Smith and Browner [ 2 5 , 2 6 ] i t b e c a m e
c l e a r t h a t t h e e f f i c i e n c y d a t a a r e s u f f i c i e n t l y a c c u r a t e f o r e v a l u a t i o n o f t h e 0 . 1 1 / m i n n e b u l i z e r . T h e s e c o n d , a n d m o s t i m p o r t a n t r e a s o n f o r t h e p o o r a n a l y t i c a l c h a r a c -t e r i s -t i c s o f -t h e w a -t e r - c o o l e d -t o r c h was -t h e f a c -t , -t h a -t -t h e w a -t e r - j a c k e -t t o o k s o m u c h s p a c e ( S e e f i g . 2 e ) , t h a t t h e c o u p l i n g e f f i c i e n c y d e t e r i o r a t e d a n d t h e p l a s m a was n o t p o w e r f u l e n o u g h f o r a d e q u a t e e x c i t a t i o n p e r -f o r m a n c e . I t i s i n d e e d e s s e n t i a l t h a t , i -f e x t e r n a l c o o l i n g i s a p p l i e d , t h e d i s t a n c e b e t w e e n p l a s m a a n d w o r k c o i l d o e s n o t become t o o l a r g e . I n c h a p -t e r 3 a n a i r - c o o l e d p l a s m a w i l l be p r e s e n -t e d , w h e r e v i r -t u a l l y a n y d i s -t a n c e b e t w e e n p l a s m a a n d w o r k c o i l c a n be s e l e c t e d . T h e ( p r e s s u r i z e d ) a i r i s b l o w n p e r p e n d i c u l a r l y a g a i n s t t h e o u t s i d e o f t h e t o r c h , w h i c h h a s a t w o -1 -1
t u b e r a t h e r t h a n t h e c o n v e n t i o n a l t h r e e t u b e a r r a n g e m e n t . T h e t o t a l a r g o n c o n s u m p t i o n o f t h i s p l a s m a i s t y p i c a l l y 1 1 / m i n a n d p r e l i m i n a r y a n a l y t i c a l m e a s u r e m e n t s i n d i c a t e p r o m i s i n g p e r f o r m a n c e . I n t h e n e x t c h a p t e r s t h i s a i r - c o o l e d p l a s m a w i l l be c o m p a r e d w i t h a w a t e r - c o o l e d p l a s m a u s i n g t h e c o o l i n g j a c k e t a f t e r Kawaguchi et at. ( f i g . 2 f , r e f . [ 2 3 ] ) , w h i c h i s a l s o c o n s t r u c t e d t o o c c u p y a m i n i m u m o f s p a c e . I n c h a p t e r 4 p o w e r b a l a n c e s r e v e a l , t h a t w a t e r i s a more e f f i c i e n t c o o l a n t m e d i u m t h a n a i r , b u t a i r c o o l e d p l a s m a s c a n be s u s t a i n e d o n c o n s i d e -r a b l y l e s s i n c i d e n t p o w e -r : t y p i c a l l y 3 0 0 v s . 8 0 0 W. T h e -r e f o -r e a i -r - c o o l i n g i s t o be f a v o u r e d f r o m t h e v i e w p o i n t o f o p e r a t i o n a l c o s t a n d i n s t r u m e n t a -t i o n , a l -t h o u g h some i m p r o v e m e n -t s i n -t h e a i r - c o o l i n g s y s -t e m a r e s -t i l l n e c e s s a r y t o e n h a n c e t h e e a s e o f o p e r a t i o n . The o v e r a l l a n a l y t i c a l p e r f o r m a n c e o f t h e two e x t e r n a l l y c o o l e d I C P ' s w i l l be p r e s e n t e d i n c h a p t e r 5 , f r o m w h i c h a l s o t h e i n f l u e n c e o f t h e m o s t i m p o r t a n t p a r a m e t e r s ( p l a s m a a r g o n , i n c i d e n t p o w e r , o b s e r v a t i o n h e i g h t ) c a n be j u d g e d . T h e p r o p e r t i e s o f t h e a i r - c o o l e d p l a s m a a r e c o m p a r a b l e t o t h o s e o f c o n v e n t i o n a l I C P ' s a n d t h e w a t e r - c o o l e d p l a s m a p e r f o r m s s l i g h t l y l e s s , a g a i n f a v o u r i n g a i r - c o o l i n g . I n c o n c l u s i o n , i t w i l l be d e m o n s t r a t e d t h a t : ( i ) a n a n a l y t i c a l l y u s e f u l I C P c a n b e made o p e r a t i n g o n o n l y 1 1 / m i n o f a r g o n b y means o f e x t e r n a l c o o l i n g w i t h p r e s s u r i z e d a i r . ( i i ) s u c h a n I C P c a n be s u s t a i n e d a n d o p e r a t e d u n d e r a n a l y t i c a l c o n d i t i o n s w i t h i n c i d e n t p o w e r l e v e l s b e t w e e n 2 5 0 a n d 4 0 0 W, c o r r e s p o n -d i n g t o g e n e r a t o r p o w e r s o f a t maximum 5 0 0 W. ( i i i ) some p r a c t i c a l p r o b l e m s c o n n e c t e d t o a i r - c o o l i n g c a p a c i t y a n d s a m p l e i n t r o d u c t i o n s y s t e m n e e d s t i l l t o be s o l v e d . R E F E R E N C E S [ 1 ] A . W a l s h , S p e c t r o c h i m . A c t a 7_, 108 ( 1 9 5 5 ) [ 2 ] B . V . L ' v o v , S p e c t r o c h i m . A c t a 17_, 761 ( 1 9 6 1 ) [3] T . B . R e e d , J . A p p l . P h y s . 32_, 821 ( 1 9 6 1 ) [4] S . G r e e n f i e l d , I . L I . J o n e s a n d C T . B e r r y , A n a l y s t 89_, 713 ( 1 9 6 4 ) [5] R . H . Wendt a n d V . A . F a s s e l , A n a l . Chem. 3 7 , 9 2 0 ( 1 9 6 5 ) 12
[6] R . C . M i l l e r a n d R . J . A y e n , J . A p p l . P h y s . 4 0 , 5 2 6 0 ( 1 9 6 9 ) [7] M . I . B o u l o s , I E E E T r a n s a c t i o n s o n P l a s m a S c i e n c e P S - 4 , 28 ( 1 9 7 6 ) [8] S . G r e e n f i e l d , I . L I . J o n e s , H . M c D . M c G e a c h i n a n d P . B . S m i t h , A n a l . C h i r a . A c t a 7 4 , 225 ( 1 9 7 5 ) [9] A . M o n t a s e r a n d J . M o r t a z a w i , A n a l . C h e m . 5_2, 255 ( 1 9 8 0 ) [ 1 0 ] R . M . B a r n e s a n d G . A . M e y e r , A n a l . Chem. 52_, 1 5 2 3 ( 1 9 8 0 ) [ 1 1 ] P . W . J . M . Boumans a n d M . C h . L u x - S t e i n e r , S p e c t r o c h i m . A c t a 5 7 B , 97 ( 1 9 8 2 ) [ 1 2 ] C D . A l l e m a n d a n d R . M . B a r n e s , A p p l . S p e c t r o s c . 3_1_, 4 3 4 ( 1 9 7 7 ) [ 1 3 ] J . L . G e n n a , R . M . B a r n e s a n d C D . A l l e m a n d , A n a l . C h e m . 49_, 1450 ( 1 9 7 7 ) [ 1 4 ] C D . A l l e m a n d , R . M . B a r n e s a n d C C W o h l e r s , A n a l . C h e m . 51_, 2 3 9 2 ( 1 9 7 9 ) [ 1 5 ] R . N . S a v a g e a n d C M . H i e f t j e , A n a l . C h e m . S l _ , 408 ( 1 9 7 9 ) [ 1 6 ] R . N . S a v a g e a n d C M . H i e f t j e , A n a l . C h e m . 52_, 1267 ( 1 9 8 0 ) [ 1 7 ] R . N . S a v a g e a n d C M . H i e f t j e , A n a l . C h i m . A c t a 1 2 3 , 319 ( 1 9 8 1 ) [ 1 8 ] A . D . W e i s s , R . N . S a v a g e a n d C M . H i e f t j e , A n a l . C h i m . A c t a 1 2 4 , 245 ( 1 9 8 1 ) [ 1 9 ] R . R e z a a i y a a n , C M . H i e f t j e , H . A n d e r s o n , H . K a i s e r a n d B . M e d d i n g s , A p p l . S p e c t r o s c . 3_C 6 2 7 ( 1 9 8 2 ) [ 2 0 ] M . D . L o w e , A p p l . S p e c t r o s c . 35_, 126 ( 1 9 8 1 ) [ 2 1 ] M . E . B r i t s k e , J . S . S u k a c h a n d L . N . F i l i m o n o v , Z h . P r i k l . S p e c t r o s c . 2_5, 5 ( 1 9 7 6 ) [ 2 2 ] G . R . K o r n b l u m , W. v a n d e r Waa a n d L . de G a l a n , A n a l . C h e m . 5 1 , 2378 ( 1 9 7 9 ) [ 2 3 ] H . K a w a g u c h i , T . I t o , S . R u b i a n d A . M i z u i k e , A n a l . C h e m . 5_2, 2 4 4 0 ( 1 9 8 0 ) [ 2 4 ] R . S . B a b i n g t o n , P o p u l a r S c i e n c e , May 1 9 7 3 , p g . 102 [ 2 5 ] P . A . M . R i p s o n a n d L . de G a l a n , A n a l . Chem. 55_, 372 ( 1 9 8 3 ) [ 2 6 ] R . F . B r o w n e r a n d D . D . S m i t h , A n a l . Chem. 55_, 373 ( 1 9 8 3 ) 13
CHAPTER II
A sample introduction system for an inductively coupled plasma
operating on an argon carrier gas flow of 0.11/min
1. INTRODUCTION
A N IMPORTANT drawback of present commercial inductively coupled plasmas (ICP) is
the need for a high coolant gas flow (ca. 20 1/min). A l t h o u g h a reduction to about 101/min may be achieved by redesigning the three concentric torch tubes [1, 2], a substantial decrease to 21/min has only been realized by KORNBLUM et al. [3], w h o proposed a water-cooled I C P torch. However, the tenfold reduction in total argon consumption required an equally strong reduction of the sample carrier gas flow to about 0.11/min. Currently available pneumatic nebulizers do not permit such low flow rates and KORNBLUM et al. had to resort to a 9 : 1 splitter behind their nebulizer, thereby wasting 9 0 % of the sample.
Recently MEDDINGS et al. have described a high precision crossflow pneumatic nebulizer operating at high pressure and 0.5 1/min argon [4]. This is achieved by careful positioning and further narrowing of the gas and liquid capillaries. Possibly, this design could be extended to even lower gas flows, but the problem of capillary blockage may become prohibitive. A n alternative solution is the use of ultrasonic nebulization, because here the gas and liquid flows are independent. Unfortunately, o u r experiences with ultrasonic nebulization without desolvation have not been very favourable.
A n o t h e r , more profitable nebulizer with independent gas and liquid flows is the Babington nebulizer [5]. A l t h o u g h this device has been advocated for I C P mainly because it allows the nebulization of viscous samples [6, 7], it should also be adaptable to minute gas flows.
B o t h the ultrasonic and the Babington nebulizer require the sample to be force-fed, for example with a peristaltic pump. W e share the opinion of others that force-feeding is also beneficial with pneumatic nebulizers used for I C P [8-10]. A disadvantage of systems proposed i n the literature [6-10] is the long time required for the sample to pass from the sample reservoir through the pump to the nebulizer, especially at low liquid flow rates. T h e time needed for changing samples may be shortened by introducing the sample into the feed line after the pump and by making the nebulizer chamber smaller. This note describes a complete sample introduction system for I C P incorporating these facilities.
[1] R . S A V A G E and G . HIEFTJE, Anal. Chem. 51, 408 (1979). [2] C. D. A L L E M A N D and R . M . BARNES, Anal. Chem. 51, 2392 (1979).
[3] G . R . KORNBLUM, W . V A N D E R W A A , and L . DE G A L A N , Anal. Chem. 51, 2378 (1979).
[4] B . MEDDINGS, H . KAISER, and H . ANDERSON, Proc. Int. Winter Conf. 1980: Developments in Atomic
Plasma Spectrochemical Analyses, San Juan, Puerto Rico (1980).
[5] Popular Science, May 1973, p. 102; and R . S. BABINGTON, U.S. Patents 3,421,692; 3,421,699; 3,425,058; 3,425,059; 3,504,859.
[6] R . F . SUDDENDORF and K . W . BOYER, Anal. Chem. 50, 1769 (1978). [7] R . C . F R Y and M . B . DENTON, App/. Spectry 33, 393 (1979).
[8] S. GREENFIELD, H . M . M C G E A C H I N and P. B . SMITH, Anal. Chim. Acta 84, 67 (1976). [9] R . L . DAHLQUIST and J . W . K N O L L App!. Spectry 3 3 , 1 (1978).
2. EXPERIMENTAL
2.1 Babington nebulizer
O f the different designs reported in the literature [6, 7, 11], we preferred the one described by WOLCOTT and B U T L E R [11]. T h e presence of a rectangular channel in the V - g r o o v e provides a uniform l i q u i d film flowing over the exit opening of the gas capillary. F o r conventional I C P s using a carrier gas flow of 11/min the inner diameter of the gas capillary funnel is about 2 0 0 /j-m [11]. Holes of that size can be drilled in test tubes after cutting the V - g r o o v e . H o w e v e r , if the gas flow is to be reduced to 0 . 1 1 / m i n , the bore of the Babington nebulizer must be reduced to 1 0 0 txm. W e have not been able to drill such a small hole in glass. A s an alternative we cemented a 2 mm long glass capillary (i.d. 7 0 /nm, o.d. 4 mm) inside a wider glass tube. Cutting the V - g r o o v e in the assembly spoiled many specimens and the few that retained a clear, unobstructed gas exit appeared to be very vulnerable when gas pressure was applied.
Fig. 1. Schematic drawing of the proposed sample introduction system consisting of an argon solvent drive system, a sample introduction loop, a Babington nebulizer and a nebulizer
chamber. Detail A shows the stainless steel nebulizer head constructed after [11].
T h e final design shown i n F i g . 1 is made from stainless steel. A l t h o u g h it takes about half a day, it is possible to drill a hole of 1 0 0 ixm over a length of 1 mm at exactly the right position. It is thus possible to machine the V - g r o o v e in the stainless steel r o d and drill the hole afterwards. T w o types were made. O n e with a bore of 2 0 0 /xm for use with a conventional I C P - t o r c h and the other with a bore of 1 0 0 (xm for use with the water-cooled torch running on 0.11/min of argon carrier gas.
2.2 Sample introduction
T h e desired liquid feed rates of 1-2 m l / m i n can be provided by a peristaltic pump. H o w e v e r , the time needed to transport the l i q u i d over 5 0 c m of tubing easily runs to a minute. In addition peristaltic pumps necessarily provide a pulsating liquid flow, which can lead to pulsating emission intensities. The latter problem could be overcome by utilizing a pulse damper or a pulse-free pump as used in high-pressure liquid chromatography. H o w e v e r , that is an expensive and, therefore, unattractive solution to a simple problem.
O u r approach is presented i n F i g . 1. A r g o n pressure is used to force pure solvent (water or any organic solvent) through a stainless steel sintered disc (filter) and a capillary tube to a syringe needle, that points to the V - g r o o v e of the Babington nebulizer. A i r or nitrogen should be avoided, because dissolved molecular gases will produce molecular bands in the I C P spectrum. Note, however, that little or no argon is actually consumed in the liquid drive system. T h e sample is introduced by means of a sample loop positioned close to the Babington nebulizer. Such sample loops are i n common use for liquid chromatography and accept volumes of a few u.1 up to several ml. They can be filled automatically with a syringe, successive samples being taken from a sample changer.
T h e proposed system is cheap and versatile. Its dead volume from sample introduc-tion to the nebulizer is about 0 . 1 m l . Because the liquid flow is determined by the capillary tubing before the sample loop, the flow rate is independent of the sample viscosity. If, for optimization studies an elongated period of sample introduction is desired, the solvent reservoir can be filled with sample solution permitting continuous introduction of the sample. W i t h appropriate three-way valves it is possible to empty and refill the solvent vessel under argon atmosphere. It should be noted that the two aspects of the present system (gas pressure feeding and sample loop introduction) are independent. If an expensive pump is used or if pulsed feeding is no problem, the gas pressure can be replaced by a pump and the sample loop retained for rapid sample interchange.
2.3 Nebulizer chamber
If the nebulizer chamber acts as an ideal mixing chamber, the time needed to transfer the aerosol to the plasma is some three times the volume of the chamber divided by the gas flow rate. F o r conventional I C P operating on 1 1/min and equipped with a 200 ml chamber it may already take half a minute for the signal to reach its final value. Reduction of the carrier gas flow to 0.11/min would lengthen the time to 5 min, unless the volume of the nebulizer chamber is simultaneously reduced. A miniature Scott chamber [12] of 30 ml proved unsuccessful: no aerosol was admitted into the plasma. Similar to N O V A K et al. [13] we therefore deleted the inner housing typical of the Scott chamber and further reduced the size of the chamber to 10 ml (Fig. 1). It is shaped in such a way, that the aerosol finds as little barriers as possible on its way to the plasma. A n important feature of such a small chamber is a smooth and continuous drain flow in order to avoid pressure fluctuations in the chamber, that become manifest as spikes in the analytical signal. This was realized by the shape and position of the wide drain (i.d. = 10 mm).
2.4 Measurements
Three sample introduction systems have been mutually compared:
—the system as shown in F i g . 1 with a small nebulizer chamber a n d a 100 /urn Babington nebulizer;
— a conventional I C P system using a concentric (Meinhard) nebulizer and a Scott chamber;
— a 200 /j.m Babington nebulizer connected to the Scott chamber.
Nebulization efficiencies were measured by connecting two adsorption columns in series to the plasma exit of the nebulizer chamber. Silica was used as adsorbent for water and active coal was used for organic solvents. In all cases the second adsorption column only slightly increased in weight, demonstrating the efficiency of the aerosol collection.
Spectroscopic data were taken with a conventional optical system consisting of a 1-m monochromator, photomultiplier, lock-in amplifier and recorder.
(12] R . H . SCOTT, V . A. FASSEL, R . N . KNISELEY and D . E . NIXON, Anal. Chem. 4 6 , 75 (1974). [13] J . N O V A K , D . E . LILLE, A. W . BOORN, and R . F . BROWNER, Anal. Chem. 5 2 , 5 7 6 (1980).
N e b u l i z a t i o n e f f i c i e n c y (%) 0.1 0.2 G a s f l o w ( t / m i n A r ) N e b u l i z a t i o n r a t e C m g / m i n )
t
G a s f l o w C l / m i n A r ) -0 5 10 1 I— A S . 0.1 0.2 G a s f l o w C l / m i n A r ) A * « A e r o s o l d e n s i t y C m g / 1 / i / f e r A r ) G a s f l o w ( L / m i n A r ) - l QJ 0.2 A-fr« 0.5 1.0 G a s f l o w C l / m i n A r ) -A e r o s o l d e n s i t yC m g / l cfftrtst A r )
G a s f l o w C l / m i n A r ) . . I O l Q.2 A-fr« 70 60 50 4 0 30 2 0 0.5 1.0 G a s f l o w U / m i n A r t-Fig. 2. Nebulizer performance for aqueous solutions as a function of the carrier gas flow for three types of nebulizers. x = Meinhard nebulizer with variable solvent uptake (1.6-2.6ml/min). 0 = Wide bore Babington with 3.4ml/min solvent supply. A = Narrow bore Babington with 1.0 ml/min solvent supply, it - Narrow bore Babington with 1.4 ml/min solvent supply. • —Narrow bore Babington with 1.7 ml/min solvent supply. Further discussion in the
text.
For the high consumption (22 l/min A r ) measurements a previously described torch assembly was used [14] while the low consumption (2 l/min A r ) data were obtained
with the water-cooled torch designed by KORNBLUM et al. [3]. 3. RESULTS AND DISCUSSION
T h e results of the aerosol adsorption measurements for water are collected in F i g . 2. In addition to the conventional efficiency (100 x m g aerosol per mg liquid supply), the nebulization rate (mg aerosol per min) and the aerosol density (mg aerosol per 1 argon) are also reported. W h e n the amount of sample available is critical, the efficiency is an important quantity. In agreement with others [15, 16] we also observe low efficiencies for I C P nebulization systems used with water. W i t h organic solvents efficiencies
[14] G . R . KORNBLUM and L . DE G A L A N , Speclrocfiim. Acta 3 2 B , 71 (1977). [15] P . SCHUTIJSER and E . JANSSENS, Spectrochim. Acta 3 4 B , 443 (1979). [16] K . W . OLSEN, W . J . H A A S and V . A . FASSEL, Anal. Chem. 4 9 , 6 3 2 (1977).
measured for the Babington nebulizer are appreciably higher: M I B K 3 . 5 % ; ethanol 5 % ; hexane 1 5 % . T h e efficiency shown i n F i g . 2(a) is a little lower for the narrow bore Babington, operating over 0.1-0.21/min of argon, than for the M e i n h a r d and the wide bore Babington, operating over 0.7-1.3 1/min. F o r the Babington nebulizers operating at constant liquid supply rate the efficiency increases linearly with the carrier gas flow rate, whereas the M e i n h a r d nebulizer shows an efficiency independent of the gas flow rate. However, the liquid flow to the M e i n h a r d nebulizer, not being force fed, increases nearly linearly with the gas flow rate.
A s a result the nebulization rate of all three nebulizers considered i n this study increases approximately linearly with the carrier gas flow rate [Fig. 2(b)]. If sample consumption is not critical this quantity is much more important than the efficiency, because i n combination with the gas flow it determines the aerosol density, which is directly proportional to the emission intensity. There are two extreme possibilities for the magnitude of the gas flow rate to be used i n calculating the aerosol density.
If, upon introduction into the plasma, the aerosol is distributed over the entire plasma gases the total gas flow rate must be used. The contribution of the carrier gas is then relatively small, so that the dependence of the aerosol density on the carrier gas flow rate is similar to that of the nebulization rate [Fig. 2(c)]. In this case the narrow bore Babington provides clearly higher aerosol densities in the low consumption plasma than either the M e i n h a r d or the wide bore Babington in the high consumption plasma.
If, o n the other hand, no mixing of carrier gas with the other plasma gases occurs, the aerosol density, being now the ratio of nebulization rate and carrier gas flow rate, will vary only little with the carrier gas flow rate. Figure 2(d) shows that the aerosol densities of the narrow bore Babington are again higher than of the other two nebulizers, though the effect is not as pronounced as in the former case. A c c o r d i n g to
BOULOS et al. [17, 18] the true situation is somewhere between and their calculations and
experiments show, that appreciable changes i n the plasma flow pattern can occur with changing gas flows and power input. It is therefore very difficult to make an exact comparison of the aerosol densities i n the low consumption and the high consumption plasmas. However, our experiments show, that the aerosol density i n a low consump-tion plasma i n combinaconsump-tion with a narrow bore Babington lies somewhere between 5 and 50 mg/1 A r , while that i n a high consumption plasma with one of the other nebulizers must be between 1 and 30 mg/1 A r . This means that the narrow bore Babington provides an appropriate sample introduction for a low consumption plasma. Obviously the narrow bore Babington is better than one of the high consumption nebulizers i n combination with a splitter ( 9 : 1 ) . In this situation the systems are used with the same plasma, which provides an unknown, but constant mixing factor. Hence, the nebulization rate is a good measure for the aerosol density. F o r the high consump-tion nebulizers the nebulizaconsump-tion rates as shown i n F i g . 2(b) must be divided by ten because of the splitting. This means a nebulization rate of about 3 mg/min for the high consumption nebulizers with splitter compared to one of about 10 mg/min for the narrow bore Babington. Furthermore the latter has a smaller sample uptake (1.5 vs 3 ml/min).
T a b l e 1 shows some data concerning the detection power obtainable with the described nebulizer systems in combination with a high and a low consumption plasma.
A s expected the M e i n h a r d and wide bore Babington nebulizers give comparable detection limits in the conventional plasma, whereas in the low consumption plasma the narrow bore Babington offers better detection power than the M e i n h a r d nebulizer with splitting system.
However, there is a difference of two orders of magnitude in the detection power between the low consumption systems and the high consumption systems. T h e observa-tions i n F i g . 2 have shown clearly that the sample introduction with the narrow bore
[17] M . I. BOULOS, [BEE Transactions on Plasma Science, P S - 4 , 28 (1976). [18] G . D U B E and M . I. BOULOS, Can. J. Specrry 2 2 , 68 (1977).
Table 1. Detection limits of Ca, Ba and Mg for a low consumption and a conventional, high consumption ICP using a Babington or a Meinhard nebulizer
Species Wavelength (nm) Detection limits (ng/ml) Species Wavelength (nm)
Low consumption plasma High consumption plasma
Species Wavelength (nm) Babington (100 u.m) Meinhard with Splitter Meinhard Babington (200 urn) Ca II 393.3 4 9 0.07 0.09 B a l l 455.5 7 30 0.07 0.08 M g l l 279.6 12 15 0.06 0.07
Babington is comparable with the high consumption systems, so that obviously the excitation characteristics of both plasmas are not the same. Consequently, further modification of the water-cooled torch itself is required to raise its detection power. Such a study is now in progress.
4. SUMMARY
A sample introduction system is described for use with a water-cooled I C P torch described previously [Anal. Chem. 5 1 , 2378 (1979)]. It consists of a narrow bore (100/xm) stainless steel Babington nebulizer operating o n 0.05 to 0.21/min argon inserted into a small (10 ml) nebulizer chamber. T h e solvent is force-fed continuously by gas pressure or with a peristaltic pump. L i q u i d samples can be supplied continuously or i n discrete quantities using a sample loop between the pump and the nebulizer. In the latter case only 25 s are required for sample change. T h e nebulization efficiency for water and organic solvents is comparable to that of conventional pneumatic nebulizers operating on 11/min argon.
CHATTER III
An inductively coupled plasma using 11/min of argon
1. INTRODUCTION
DURING THE past few years several attempts have been made to reduce the argon
consumption of an inductively coupled argon plasma ( I C A P ) , without sacrificing its analytical capabilities [1-9]. A l t h o u g h the approaches to this problem differ sub-stantially, they agree o n one point: additional c o o l i n g facilities are necessary to prevent melting o f the torch.
A L L E M A N D et al. [1], S A V A G E and HIEFTJE [2-4], and WEISS et al. [5] combine torch
miniaturization with the use of a tulip-shaped intermediate tube to make cooling b y argon more efficient. W i t h this arrangement the total argon consumption is reduced from its typical value o f 15 1/min for a conventional torch to 7 1/min, w h i c h seems to be the lower limit for the proposed design.
A very different type of torch is described by L O W E [6], who uses a thick outer tube at a fairly large distance from the (cylindrical) intermediate tube. Whereas the total argon consumption is only 51/min, the carrier gas takes 3 1/min, w h i c h is substantially more than i n a conventional I C A P (11/min). This might cause severe chemical interferences.
Still another approach is to add a water jacket to the I C A P torch. K O R N B L U M et al.
[7] described such a torch, for w h i c h the total argon consumption was 21/min, the
carrier gas flow being limited as low as 0.11/min. Unfortunately, the detection power achieved with this design was not very good, even w i t h the i m p r o v e d sample introduction system we reported previously [10].
P o s s i b l y , the poor detection power can be attributed to an inefficient coupling o f r.f. power into the plasma when the r.f. c o i l and the plasma torch are separated b y the water jacket. Indeed, a somewhat smaller separation and decidedly better analytical results were realized b y KAWAGUCHI et al. [8], who describe a water-cooled torch operating on 5 1/min o f total argon, o f w h i c h 0.8 1/min is used for sample introduction. F i n a l l y , BRITSKE et al. [9] constructed a big, water-cooled torch. The inner diameter of its outer tube is as large as 40 m m . The argon consumption of this design is 4 1 / m i n , but to get good analytical results " e n d - o n " registration of the spectra is necessary
[11].
F r o m this review it is clear that I C P torches that rely on the (inner) cooling action of argon require a m i n i m u m flow of 5 1/min. A further reduction is possible o n l y with the help o f external cooling. Water is an efficient cooling agent, but the space
[1] C . D . A L L E M A N D , R . M . BARNES and C . C . WOHLERS, Anal. Chem. 51, 2392 (1979). [2] R . N . S A V A G E and G . M . HIEFTJE, Anal. Chem. 51, 408 (1979).
[3] R . N . S A V A G E and G . M . HIEFTJE, Anal. Chem. 52, 1267 (1980).
[4] R. N. S A V A G E and G . M . HIEFTJE, Anal. Chim. Acta 123, 319 (1981).
[5] A. D. WEISS, R. N . S A V A G E and G . M . HIEFTJE, Anal. Chim. Acta 124, 245 (1981).
[6] M . D . L O W E , Appl. Spectrosc. 35, 126 (1981).
[7] G . R . K O R N B L U M , W . VAN DER W A A and L . DE G A L A N , Anal. Chem. 51, 2378 (1979). [8] H . KAWAGUCHI, T . ITO, S. RUBI and A. MIZUIKE, Anal. Chem. 52, 2440 (1980). [9] M . E . BRITSKE, J . S. S U K A C H and L . N . FILIMONOV, Zh. Prikl. Spectrosc. 25, 5 (1976). [10] P . A. M . RIPSON and L . DE G A L A N , Spectrochim. Acta 36B, 71 (1981).
o c c u p i e d by the c o o l i n g jacket reduces the ratio of the t o r c h diameter to the r.f. c o i l diameter and this, i n turn, deteriorates the c o u p l i n g efficiency of r.f. p o w e r into the plasma. T h e effect w o u l d be more apparent w i t h smaller torches. Indeed, our attempts to design miniaturized water-cooled torches resulted i n v e r y weak and c o o l plasmas (typically 3000 K ) . A t the smallest diameter tried (5.5 mm) the plasma becomes too small w i t h respect to the s k i n depth of 3 mm [12].
F u r t h e r attempts to construct water-cooled I C P torches were therefore dis-continued in favour of the design described in this N o t e w h i c h uses external c o o l i n g by pressurized air.
2. EXPERIMENTAL
2.1. The work coil
Figure 1 shows a schematical drawing of the work coil. It consists of two copper plates (dp = 4 mm), each
with a cylindrical bore (</> = 23 mm) and a saw cut (ds = 2 mm). The two saw cuts are displaced over a
distance of 4 mm from each other. With the aid of a little block of copper the plates are brazed together with a separation of 5 mm. Four pieces of copper tubing (O.D. = 4 mm) are brazed to the sides of the plates to enable water cooling of the coil. The top right side and the bottom left side of the coil are connected to the two poles of the r.f. generator, thus forcing the current to pass two complete circles around the torch. Air is blown against the torch through five inlets between the two copper plates (only two are shown in Fig. 1), in such a way, that the air is forced upwards to pass the gap between the coil and the torch.
The coil described above offers the advantage that external cooling with pressurized air is possible, while retaining a good coupling efficiency. In fact, because only some tenths of a millimeter are needed for the escaping air, the essential diameter ratio of plasma tube to r.f. coil can be very close to unity.
Some electrical parameters were measured to compare the proposed coil with the conventional standard two turn copper coil provided with the ICAP source unit used (Philips PV 8490).
From the results shown in Table 1 it is seen that the properties of both coils are similar. Perhaps slight changes of plate thickness and separation can improve this similarity even further, but the agreement is considered to be sufficient to warrant analytical experiments.
2.2. The torch
Torch dimensions greatly influence the shape and the stability of the plasma generated. Starting with the conventional three-tube arrangement, several configurations of the torch have been evaluated, using the detection limit for the Mg II line at 279.6 nm as the criterion. Because of the high sensitivity of this line, even poor plasmas can give information about the influence of certain parameters.
In all cases, except one, the relative position of the tubes was optimized and the carrier gas tube was held constant (diameter of orifice = 0.5 mm). The exception was the torch provided with the ICAP source unit
Fig. 1. Schematic drawing of the proposed work coil incorporating water-cooling of the coil and air-cooling of the torch. Points A and B are connected to the r.f. generator.
Table 1. Comparison of the characteristics of the novel and a conventional coil
Type of c o i l Inductanc C a p a c i t a ce Q u a l i t y fac t o r , * '"osiz b UlH) (pF) Q OnA) (MHz) Present study ( F i g . l ) 0.19 105 200 130 49.7 C o n v e n t i o n a l 0.17 60 250 180 47 i
aAnode current at a fixed voltage of 200 V.
bOscillator frequency of the (free running) generator circuit.
used (Philips PV 8490), which had a fixed tube arrangement, with a 1.5 mm orifice in the carrier gas tube. In this case, the position of the whole torch with respect to the coil was optimized.
From the results presented in Table 2 (data necessary to understand this table can be obtained from Fig. 2), the following conclusions can be drawn:
(i) There seems to be a trade off between coupling efficiency and plasma stability: a larger inner diameter of the outer tube does not necessarily provide the best plasma. Furthermore, wide diameter plasmas exhibit a fairly large and cool central channel.
(ii) The plasma yielding the lowest detection limit also operates with the lowest argon consumption. (iii) Apparently, a two-tube torch provides a more stable plasma than the conventional three-tube
arrangement.
(iv) Of the different configurations evaluated so far, optimum results were obtained with a two-tube torch with a fairly large separation between the outer tube and the r.f. coil. This configuration was used in further analytical studies.
3. A N A L Y T I C A L PERFORMANCE
Table 3 presents the instrumentation and operating conditions used to evaluate the proposed system. T h e resulting plasma emits a strong white light and, w h e n viewed from the side, it has a bullet-like shape w i t h a length of about 15 m m ( F i g . 3). T h e luminous white zone starts only 1 to 2 mm below the top of the c o i l . A t relatively low power a central dark spot can be o b s e r v e d , w h e n the plasma is viewed a x i a l l y , e v e n
Table 2. The influence of torch dimensions on the argon consumption, detection limit of Mg and plasma stability (see Fig. 2 for meaning of a and b and additional data)
T u b e d i a m e t e r A r g o n f l o w3 D e t . L i r a . R e m a r k s O u t e r ( a ) I n t e r m e d i a t e ( b ) Mg (mm) (mm) ( 1 / m i n ) y M g / 1 ) 1 8 . 4 1 7 . 8 1 7 . 8 1 7 . 8 1 5 ( t u l i p ) 1 6 . 5 1 5 . 7 13.5 5 4 2 . 5 2 0 . 7 3 0 0 6 0 20 0 . 6 p l a s m a t u b e p o s i t i o n c r i t i c a l t o o u n s t a b l e f o r s a m p l e i n t r o d u c t i o n u n s t a b l e , n o i s y c o o l c e n t r a l c h a n n e l s t a b l e
'In Fig. 2 this argon flow is written as "argon F".
This torch has a fixed tube arragnement. Its position with respect to the coil is optimized.
2 3
-Fig. 2. Schematic drawing of the torch coil assembly used for optimization of the torch configuration. The values of the tube diameters "a" and "b" and of the coolant flow
"Argon F" can be found from Table 2.
Table 3. Instrumentation and operating conditions for the low consumption ICAP system RF g e n e r a t o r - f r e q u e n c y ( M H z ) - o u t p u t p o w e r t o I C P (kW) C o i l T o r c h G a s f l o w s ( 1 / m i n ) S a m p l e i n t r o d u c t i o n M o n o c h r o m a t o r P h o t o m u l t i p l i e r L o c k - i n a m p l i f i e r P h i l i p s PV 8 4 9 0 ( f r e e r u n n i n g ) A / 5 0 c o n t r o l l a b l e b e t w e e n 0 . 7 a n d 1 . 9 a s d e s c r i b e d i n p a r a g r a p h 2 . 1 a s d e s c r i b e d i n p a r a g r a p h 2 . 2 ( i v ) o u t e r A r : 0 . 7 5 c a r r i e r A r : 0 . 1 5 a i r : 50 n a r r o w b o r e ( 0 . 1 mm) B a b i n g t o n n e b u l i z e r ( r e f . 1 0 ) , s o l v e n t d e l i v e r y r a t e : 1 . 5 m l / m i n J a r r e l l - A s h 0 . 5 m E b e r t g r a t i n g : 1 1 8 0 g r o o v e s / m m s l i t w i d t h s : 2 5 , M m EMI 6 2 5 6 S h i g h t e n s i o n : 8 0 0 V PAR m o d e l 1 2 0 c h o p p i n g f r e q u e n c y : 4 0 0 Hz t i m e c o n s t a n t : 0 . 3 s ( o p t i m i z a t i o n ) 1 5 s ( d e t e c t i o n l i m i t )
aThis range applies to a conventional torch and ICP. The actual power
output under the conditions of our experiments was not measured.
Fig. 3. Photograph of the plasma operating on 0.7 1/min of argon (no sample introduction).
without sample introduction. This indicates a clear toroidal shape, w h i c h is even more pronounced than in conventional plasmas.
W h e n a short plasma tube is used the tail flame can be observed unobstructed, but the background spectrum is raised due to entrained air. A l s o , ignition is difficult and the tail flame wavers. Consequently, it is advantageous to extend the plasma tube
20 mm above the c o i l . Ignition is now easy, the tail flame is stable and the background
spectrum intensity is reduced. The obvious disadvantage is, that the plasma must be observed through the quartz tube.
The plasma can be ignited with both coolant gas and carrier gas flowing, thus saving an elaborate and often tricky introduction procedure of carrier gas into a burning plasma. A t moderate power input levels (i.e. < 1 k W , if the power calibration for the conventional I C P is applied) the plasma can run for several hours, but above 1 k W the cooling by pressurized air is insufficient and the plasma tubes deform rapidly.
After optimization of the power setting, the observation height ( 5 - 1 5 mm above the coil with a 15 mm high observation w i n d o w ) and the position of the sample intro-duction tube with magnesium as test element, detection limits were measured for eleven elements (Table 4). Separate optimization for each element did not yield significant improvement.
The detection limits compare reasonably with the results reported for a c o n -ventional I C P [13], although there is an indication that the present plasma is some-what cooler than a conventional I C P . F i r s t the detection limit for the sodium atom line is rather low and second, the detection limits appear to worsen with increasing excitation energy of the analysis lines. H o w e v e r , a measurement of the two-line excitation temperature [14] using the intensity ratio of the two T i II lines at 322.28 and
322.42 n m , resulted i n a value of 5500 K . T h i s value differs little from the range of 5550 to 5 7 1 0 K reported for a conventional plasma by M E R M E T [15] with the same
spectral lines and gf-values.
This is confirmed by interference studies that show promising results ( F i g . 4). The signal of 1 mg/1 C a was studied with increasing amounts of three concomitants. A t the
1000 mg/1 level, A l yields a 15% enhancement, N a an about 5% enhancement and P 04 3~ a 12% depression. A g a i n these results are slightly inferior to those for a
conventional I C A P [16], but so far no major interferences have been found for i n t e r f è r e n t concentrations at the 1000 mg/1 level.
Table 4. Detection power of the proposed system
E l e m e n t W a v e l e n g t h E e x c i t a t i o n D e t . L i m . ( / • g / D (nm) ( e V ) p r e s e n t s t u d y r e f . 13 Co I I 2 3 8 . 8 5 6 0 7 0 6 B I 2 4 9 . 7 4 96 90 5 F e I I 2 5 9 . 9 4 78 6 0 6 Mg I I 2 7 9 . 6 4 43 0 . 6 0 . 1 5 V I I 3 0 9 . 3 4 40 11 5 Cu I 3 2 4 . 7 3 82 1 0 5 Co I 3 4 5 . 3 4 02 18
-C a I I 3 9 3 . 3 3 15 0 . 5 0 . 2 A l I 3 9 6 . 1 3 14 4 0 28 L a I I 4 0 8 . 6 3 03 5 10 Na I 5 8 8 . 9 2 10 2 29Detection limits were calculated using the 3<r-criterion. [13] R. K . WINGE, V . J . PETERSON and V . A . FASSEL, Appl. Speclrosc. 33, 206 (1979). [14] G . R. KORNBLUM and L . DE G A L A N , Speclrochim. Ada 32B, 71 (1977). [15] J . M . MERMET, Thesis, Université Claude Bernard, Lyon (1974). [16] G . E . LARSON and V . A . FASSEL, Anal. Chem. 48, 1161 (1976).
Relative intensity n g / L C a
0.5
I n t e r f è r e n t concentration, m g / L
0 10 100 1000
Fig. 4. Influence of Al, Na and P 04 3 on the signal of 1 mg/1 Ca. The measurements with Al
are corrected for spectral interference.
4. CONCLUSIONS
A n inductively coupled plasma with a total argon consumption of 0.85 1/min has been realized using a novel coil construction and a two tube plasma torch. E x t e r n a l cooling is provided by pressurized air, w h i c h is b l o w n against the exterior of the torch, thus realizing adequate cooling without losing high coupling efficiency.
The proposed system shows good detection power and no major interferences up to 1000 mg/1 of i n t e r f è r e n t . A l t h o u g h the results are still somewhat inferior to those of a conventional plasma, they are sufficiently encouraging to warrant further study.
F i r s t , the torch configuration deserves closer consideration. The inner diameter of the plasma tube may be varied and a side arm to the extended plasma tube can be included for observation purposes.
A l s o , air-cooling may be replaced by water-cooling after KAWAGUCHI et al. [8]. W i t h the present torch configuration there is just enough place for such a water jacket inside the c o i l .
The next step is a more elaborate optimization of the operating conditions including the observation w i n d o w and the orifice diameter of the carrier gas tube.
The final step is the measurement of p h y s i c a l characteristics (temperature, electron concentration) and analytical properties (detection limits, interferences, precision, accuracy etc.). These studies are currently in progress.
CHAPTER IV
Empirical power balances for conventional and externally cooled
inductively-coupled argon plasmas
1. INTRODUCTION
I N AN effort to improve the cost-benefit relation of the inductively-coupled plasma (ICP) some attempts have been described to develop an I C P with a largely reduced argon consumption but the same analytical performance as current, commercially available I C P s [1-10]. F r o m the experience gained so far three practical problems emerge: sample introduction, torch and coil construction and cooling facilities.
T o maintain analytical performance the carrier gas flow must be reduced in proportion to the total argon consumption. Consequently, sample introduction facilities must be designed that operate on argon flows as low as 0.1 1 min~ 1 [11]. The rf-coil and the plasma torch must
be optimized for an efficient transfer of power from the rf-generator to the plasma. This puts demands on the inductance of the rf-coil and on the relative dimensions o f the coil and the plasma torch [10].
The need for additional cooling facilities is recognized in all reports, since it is not possible in a conventional I C P to reduce the input power proportionally to the decrease in argon consumption. Even with a modification of the velocity profile of the argon through miniaturization and tulip-shaped torch tubes the lower limit o f the total argon flow is about 5 1 min " 1. Values lower than that can only be realized when the internal cooling with argon is
replaced by external cooling with water [7-9] or with air [10].
F r o m our previous experiments with an air-cooled I C P we obtained indications that such a plasma not only uses less argon, but also requires less power. This would contribute significantly to the cost-benefit performance of the I C P . In order to get more insight into the
[1] C. D. ALLEMAND. R. M. BARNES and C. C. WOHLERS, Anal. Chem. 51, 2392 (1979). [2] R. N . SAVAGE and G . M . HIEFTJE, Anal. Chem. 51, 408 (1979).
[3] R. N . SAVAGE and G . M . HIEFTJE. Anal. Chem. 52, 1267 (1980). [4] R. N . SAVAGE and G . M . HIEFTJE. Anal. Chim. Ada 123, 319 (1981).
[5] A. D . WEISS. R. N . SAVAGE and G . M . HIEFTJE. Anal. Chim. Ada 124, 245 (981).
[6] M . D . LOWE, Appl. Spec/rose. 35. 126 (1981).
[7] G . R. KORNBLUM. W. VAN DER WAA and L . DE G A L A N , Anal. Chem. 51, 2378 (1979). [8] H . KAWAGUCHI. T . ITO, S. RUBI and A . MIZUIKE, Anal. Chem. 52, 2440 (1980). [9] M . E . BRITSKE, J. S. SUBACH and L. N . FILIMONOV, Zh. Prikl. Spectrosk. 25, 5 (1976). [10] P. A. M . RIPSON, L . DE G A L A N and J. W. DE RUITER, Spearochim. Ada 37B, 733 (1982). [11] P. A . M . RIPSON and L . DE G A L A N . Spedmhim. Ada 36B. 71 (1981).
way in which power is transferred from different types of plasma to the environment, power balances are an essential tool. A n analysis o f the various terms in the power balance will reveal the influence of the cooling applied and show the direction for further improvement.
Power balances reported for the I C P fall into two categories. The first category is
exemplified by the work o f MILLER and A Y E N [12], BARNES and NUCDEL [13] and BOULOS
[14]. F r o m a theoretical analysis of the plasma processes these authors derive radial
temperature and gas velocity profiles. Apart from uncertain estimates o f plasma properties (e.g. skin depth and conductivity) their results are not suitable for our purposes, because they refer to plasmas very different from the ones we studied ( 6 k W , 3 M H z , N2) and assume
thermal equilibrium. As a result, no less than 20-30 % o f the input power is dissipated as (optical) radiation [12, 14] or the power transferred through the wall amounts to 4 k W [13]. Both figures cannot be expected for the plasmas we consider.
In the second category R E E D [15] and ARMSTRONG and RANZ[16] report experimental data for the power dissipated from single tube plasmas by convection, conduction and radiation, while GREENFIELD and M C G E A C H I N [17] report similar measurements for analytical plasmas. Some measurement techniques may be criticized for interference with the power balance studied. F o r example, i f a cooling jacket is applied to measure the conduction losses, or i f temperature sensitive paint is used to estimate the tube wall temperature, the relative contribution o f the power dissipation terms is certainly altered. Again, the plasmas considered here differ too much from our I C P s to permit direct utilization o f their results. Nevertheless, we also prefer an empirical approach to the study o f power balances i n analytical ICPs. The various terms on either side o f the power balance will be measured directly or derived from experimental data collected for this purpose or taken from the literature. This approach avoids the need for a detailed analysis of the basic processes inside the plasma and allows us to use well-known physical and technological relationships.
Special attention will be given to the measurement o f the incident power, because many publications simply state a value read from a meter on the rf-generator without consideration of its accuracy or relevance.
Power balances will be derived for three different plasmas: a conventional I C P cooled internally with 221 m i n "1 of argon, an air-cooled I C P described previously [10] and a
water-cooled I C P using the same coil-torch assembly but provided with a cooling jacket after
KAWAOUCHI el al. [8]. T o avoid lengthy repetitions in the text and to facilitate cross
referencing, Appendix 1 lists all parameters used in the various expressions together with their units and values.
2. THEORY
2.1. Plasma model
Throughout this paper the I C P will be considered as a black box to which power is supplied by the rf-coil and from which power is dissipated to the environment. The boundaries of the black box are first the physical boundary and the exit opening of the outer quartz tube and second the bottom of the plasma. What happens with the power inside the plasma is of no or at least minor importance. It will be assumed, that the power input leads to ionization and excitation, but that will only be visible in the amount of radiation and in the energy carried away by argon ions. W i t h this simplification the power balance in the stationary state can be written as:
Pin = Pconv + Pcond + Prad M
where the incident power, pm, is that part o f the power delivered by the rf generator that is
[12] R. C. MILLER and R. J. AYEN. J. Appt. Phys. 40, 5260 (1969). [13] R. M . BARNES and S. NUCDEL, J. Appl. Phys. 47, 3929 (1976). [14] M . I. BOULOS, IEEE Trans, on Plasma Sei. PS-4, 28 (1976). [15] T . B. REED, J. Appl. Phys. 32, 821 (1961).
[16] D . R. ARMSTRONG and W . E . RANZ, / & EC Process Design Devel. 7, 31 (1968). [17] S. GREENFIELD and H . McD. MCGEACHIN, Anal. Chim. Ada 100, 101 (1978).
